The National Human Activity Pattern Survey (NHAPS): A Resource for Assessing Exposure to Environmental Pollutants

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The National Human Activity Pattern Survey (NHAPS): a resource for
assessing exposure to environmental pollutants
NEIL E. KLEPEIS,
a,b
WILLIAM C. NELSON,
c
WAYNE R. OTT,
d
JOHN P. ROBINSON,
e
ANDY M. TSANG,
f
PAUL SWITZER,
d
JOSEPH V. BEHAR,
g
STEPHEN C. HERN
g
AND WILLIAM H. ENGELMANN
g
a
Environmental Health Sciences, School of Public Health, University of California at Berkeley, Berkeley, California 94720
b
Indoor Environment Department, Lawrence Berkeley National Laboratory, One Cyclotron Road, MS 90 - 3058, Berkeley, California 94720
c
National Exposure Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711
d
Department of Statistics, Stanford University, Stanford, California 94305
e
Survey Research Center, University of Maryland, College Park, Maryland 20742
f
Anteon Corporation, Las Vegas, Nevada 89119
g
National Exposure Research Laboratory, U.S. Environmental Protection Agency, Las Vegas, Nevada 89193
Because human activities impact the timing, location, and degree of pollutant exposure, they play a key role in explaining exposure variation. This fact has
motivated the collection of activity pattern data for their specific use in exposure assessments. The largest of these recent efforts is the National Human Activity
Pattern Survey (NHAPS ), a 2 - year probability - based telephone survey ( n = 9386 ) of exposure- related human activities in the United States ( U.S. ) sponsored
by the U.S. Environmental Protection Agency ( EPA ). The primary purpose of NHAPS was to provide comprehensive and current exposure information over
broad geographical and temporal scales, particularly for use in probabilistic population exposure models. NHAPS was conducted on a virtually daily basis from
late September 1992 through September 1994 by the University of Maryland's Survey Research Center using a computer - assisted telephone interview
instrument ( CATI ) to collect 24 - h retrospective diaries and answers to a number of personal and exposure - related questions from each respondent. The
resulting diary records contain beginning and ending times for each distinct combination of location and activity occurring on the diary day ( i.e., each
microenvironment). Between 340 and 1713 respondents of all ages were interviewed in each of the 10 EPA regions across the 48 contiguous states. Interviews
were completed in 63% of the households contacted. NHAPS respondents reported spending an average of 87% of their time in enclosed buildings and about
6% of their time in enclosed vehicles. These proportions are fairly constant across the various regions of the U.S. and Canada and for the California population
between the late 1980s, when the California Air Resources Board ( CARB ) sponsored a state - wide activity pattern study, and the mid - 1990s, when NHAPS
was conducted. However, the number of people exposed to environmental tobacco smoke ( ETS ) in California seems to have decreased over the same time
period, where exposure is determined by the reported time spent with a smoker. In both California and the entire nation, the most time spent exposed to ETS
was reported to take place in residential locations. Journal of Exposure Analysis and Environmental Epidemiology (2001 ) 11, 231 ± 252.
Keywords: environmental pollutants, environmental tobacco smoke, exposure assessment, exposure modeling, exposure survey, household pollutants,
human ± activity patterns, human exposure, population survey, time activity, time budget
.
Introduction
National-level exposure assessments are required for major
policy decisions mandated under regulations of the Clean
Air Act, as well as for other risk analyses and regulatory
judgments of the U.S. Environmental Protection Agency
(EPA ). Concern has been broadened to include not only
traditional industrial and mobile sources, but the consumer
products and building materials with which a person
1. Abbreviations: CARB, California Air Resources Board; C
6
H
6
, benzene;
CAPS, California Activity Pattern Surveys sponsored by CARB ( n = 1200 for
ages under 12; n = 1762 for ages 12 and over ); CATI, computer - assisted
telephone interview; CHAPS, Canadian Human Activity Pattern Survey
( n = 2381 ); CHCl
3
, chloroform; CO, carbon monoxide; doer, a sampled
individual who is in a specific microenvironment for non - zero time during a
specified time interval; ETS, environmental tobacco smoke; HAPEM,
Hazardous Air Pollutant Exposure Model; indirect approach, an approach to
modeling human exposure by weighting pollutant concentrations by the time
spent in different microenvironments; LBNL, Lawrence Berkeley National
Laboratory; MCTBRP, Multinational Comparative Time Budget Research
Project; microenvironment, the occurrence in a person's day of a unique
combination of location and activity, although originally defined by Duan
( 1982 ) as a location of homogeneous pollutant concentration; n, sample size;
NAAQS, National Ambient Air Quality Standards; NHAPS, National Human
Activity Pattern Survey ( n = 9386 ); NHAPS - CA, the NHAPS California sub-
sample ( n = 988 ); NO
2
, nitrogen dioxide; O
3
, ozone; PAH, polycyclic aromatic
hydrocarbons; pNEM, probabilistic NAAQS Exposure Model; PSU, primary
sampling unit; time budget, the original term for a person's time diary; RDD,
random digit dial; SERD, smoking - exposure - related duration; SRP, self -
reported proximity ( to a smoker ); TEAM, Total Exposure Assessment
Methodology; U.S., United States; EPA, U.S. Environmental Protection
Agency; VOCs, volatile organic compounds
1. Address all correspondence to: Neil E. Klepeis, Lawrence Berkeley National
Laboratory, One Cyclotron Road, MS 90 - 3058, Berkeley, CA 94720. E-mail:
neklepeis@lbl.gov
Received 30 September 1998; accepted 6 February 2001.
Journal of Exposure Analysis and Environmental Epidemiology (2001) 11, 231±252
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typically has frequent contact (Wallace, 1995; Ott and
Roberts, 1998). The importance of activity pattern data
has increased with the realization that many types of
exposure to environmental pollutants occur indoors and
stem, in large part, from indoor pollutant sources such as
cigarettes (see, e.g., Wallace, 1996 ). Exposure monitor-
ing studies have demonstrated how people's locations
and activities can explain the variation in exposure to
benzene, tetrachloroethylene, and other volatile organic
compounds (Wallace et al., 1989, 1991; Thomas et al.,
1991, 1993 ).
Human activity data are major inputs to human exposure
models, such as the probabilistic National Ambient Air
Quality Standards (NAAQS) Exposure Model (pNEM)
(Johnson et al., 1996a,b) and the Hazardous Air Pollutant
Exposure Model (HAPEM ) (Glen, 1994; Glen and
Shadwick, 1998 ), both of which require data on the
occurrences and time sequences of activities. Until recently,
the activity pattern information required as input to these
exposure models has been limited with regard to geographic
and temporal coverage. With the completion of the EPA-
sponsored National Human Activity Pattern Survey
(NHAPS ), however, comprehensive national activity pat-
tern information is now available ( see Nelson et al., 1994;
Robinson and Blair, 1995; and Klepeis et al., 1996 ). The
EPA's Consolidated Human Activity Pattern Database
(CHAD ) provides for convenient access to the data
collected as part of NHAPS and a number of other human
activity pattern studies ( see Glen et al., 1997; McCurdy et
al., 2000 ).
The first section of this paper provides a brief history of
human activity pattern study, starting from its genesis in
sociological research and ending with the use of activity
patterns in exposure models. The next two sections describe
the NHAPS data collection methodology, including the
NHAPS sampling design and sample characteristics. In the
next section, we summarize unpublished results from some
previous analyses ( Klepeis et al., 1996; Tsang and Klepeis,
1996) and contribute some new analyses, which compare
the time spent by NHAPS respondents to the time spent by
respondents of the California Activity Pattern Survey
(CAPS ) (Jenkins et al., 1992; Wiley et al., 1991a,b) and
the Canadian Human Activity Pattern Survey (CHAPS )
(Leech et al., 1996). The final section contains a summary
and conclusions.
Historical perspective
The Sociological Study of Human Activity
The long history of studies on human activities in the
sociological literature contains frequent use of the term
``time budget'' (also known as ``zeitbudget'' or ``budget de
temps'' ). A time budget is conceptually similar to a person's
money budget in that it summarizes the amount of time an
individual spends in each of many activities over some time
period ( e.g., a day or a week ). According to Michelson
(1973 ):
A time budget is a record, presented orally or on
paper, of what a person has done during the
course of a stated period of time. It usually
covers a 24- hour day or multiples thereof. The
record is taken down with precision and detail,
identifying what people have done with explicit
reference to exact amounts of time. It is usually
presented chronologically through the day,
beginning with the time that a person gets up
in the morning.
The information that is normally gathered in a
time budget consists of the time an activity
began, the time it ended, the nature of the
activity per se, the persons who were present
and active in the given activities, and, not the
least, the exact location where the activity took
place.
Early reviews of the historical development of time budget
research are provided by Szalai ( 1966), Converse (1968),
Ottensmann (1972), and Chapin (1974). This early
research forms the basis for today's human activity pattern
surveys ( see the review by Ott, 1989 ).
The earliest documented studies of human activity in
America are by Lundberg et al. ( 1934) and Sorokin and
Berger (1939 ), with several time budget studies conducted
in France during the 1940s (see Szalai, 1966). However,
the idea that time budget studies could be used to compare
cultural characteristics ( McCormick, 1939 ) did not come
to fruition until about 30 years later when the Multinational
Comparative Time Budget Research Project ( MCTBRP)
(Szalai, 1972 ) tabulated data on 25,000 people in 12
countries ( Belgium, Bulgaria, Czechoslovakia, France,
East Germany, West Germany, Hungary, Peru, Poland,
Union of Soviet Socialists Republics, United States, and
Yugoslavia). This study allowed comparisons of activity
patterns across many countries; but like most other activity
pattern studies in the social science literature, it did not
collect exposure-related information. Historically, time
budget studies by social scientists usually did not even
distinguish, specifically, whether a person was indoors or
outdoors.
In 1989, Ott ``reinterpreted'' the codes from the
MCTBRP activity pattern data for 44 U.S. cities (Robinson
et al., 1972 ) to estimate the amount of time that people
spend in -transit, outdoors, and indoors, and he concluded
that employed persons in the U.S. spend only about 2% of
their time outdoors, 6% of their time in transit, and 92% of
Klepeis et al. The National Human Activity Pattern Survey (NHAPS)
232 Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3)

their time indoors. For the 11 other countries, he estimated
that time spent in transit for employed men ranged from 1.5
h ( 6.2% of the day) in France and Belgium to 2.5 h (10.4% )
in Lima, Peru, while the time spent outdoors ranged from
0.4 h (1.7% ) in Torun, Poland, to 1.9 h (7.9% ) in West
Germany (based on 100 districts). Although Ott cautioned
that these sociological time budget studies were not
designed to estimate human exposure, his recoded estimates
showed surprisingly small proportions of time spent
outdoors by people in the 12 countries. He suggests that
the large amount of time spent indoors is a fundamental
characteristic of the human species, ``The finding that
emerges is that we are basically an indoor species.'' ``In a
modern society, total time outdoors is the most insignificant
part of the day, often so small that it barely shows up in the
total.''
Health and Human Activity
As alluded to above, the critical problem with activity
pattern studies found in the sociological literature is that
they do not include many aspects of daily life that are
important for environmental pollution exposure assessment,
such as storing chemicals in the home, driving an
automobile on crowded highways, living with a smoker,
using gas appliances, visiting a dry cleaner, using solvents
in the home, or filling a gas tank. Nor do they provide
sufficient detail on the locations that people visit.
Using methods similar to those of the social scientists,
researchers in the environmental health sciences in the
1980s began to collect activity pattern data as part of
exposure and health research. For example, the following
studies appeared in the literature of this period.
(1 ) Johnson (1983) and Akland et al. (1985 ) conducted
a probability-based personal exposure field study of 1200
persons in Denver and Washington, DC, in which
respondents carried personal monitors to measure their
personal exposure to carbon monoxide (CO) while keeping
diaries to record the activities and microenvironments they
visited over 24 h. Schwab (1988) analyzed the activity
patterns and CO exposures using the diary data from this
study.
(2 ) Quackenboss et al. ( 1986) used a recall question-
naire to gather information on the times people spent in
various locations, or microenvironments, in a study of
personal nitrogen dioxide (NO
2
) exposures and indoor and
outdoor concentrations for 350 individuals in Portage, WI.
(3 ) Adair and Spengler (1989) reported findings on the
activity patterns of over 1800 third and fourth grade children
in six U.S. cities between 1984 and 1988.
(4 ) Freeman et al. (1989) used a seven- page question-
naire to obtain activity pattern information from 14
respondents over 14 days in Phillipsburg, NJ.
(5 ) Lichtenstein et al. (1989) studied the time ±activity
patterns of 973 respondents in Cincinnati, OH, using 3 -day
diaries to evaluate how much the activities of asthmatics
differ from those of the general population.
(6 ) Schwab et al. ( 1989a, 1990) collected diary data on
activity patterns from approximately 700 respondents in 500
households in Los Angeles in connection with a study of
personal exposure to NO
2
.
(7 ) Schwab et al. (1989b, 1992 ) report on time ±activity
data collected from 91 children in Kanawa Valley, WV, as
part of a study of children's respiratory and sensory
responses to air pollution. Schwab et al. (1991) explored
the use of these diary data in linking exposure and dose by
analyzing the self -reported exercise levels of the children.
In parallel scientific efforts, environmental health
scientists began developing mathematical exposure models
based on human activity patterns. Fugas ( 1975) initially
suggested a modeling approach for computing personal
exposure to sulfur dioxide ( SO
2
), lead (Pb), and manganese
(Mn ) by summing the concentrations in the locations a
person visited (home, work, streets, countryside ), weighted
by the time the person spent in each location. Subsequently,
Duan (1982 ) suggested a formal mathematical approach to
compute personal exposure by summing the pollutant
concentrations in the ``microenvironments'' ( defined by
Duan as locations of homogeneous concentration) that each
person visited, weighted by the time they spent in each
microenvironment. Ott (1984) then developed a proto-
typical computerized exposure model based on the concepts
of Fugas and Duan, referred to as the ``indirect approach'' to
exposure assessment. A variety of mathematical models
based on this approach were subsequently developed (see
Quackenboss et al., 1986; Sexton and Ryan, 1988; Ott et al.,
1992, 1998; Behar et al., 1993; Klepeis et al., 1994;
MacIntosh et al., 1995; McCurdy, 1995, 1997; Johnson et
al., 1996a,b; Miller et al., 1998a,b; Klepeis, 1999).
Large- Scale Activity Pattern Studies
Although exposure models require diary data on activity
patterns, few large- scale population studies existed before
1990 to provide the necessary data. To help meet this need
for activity pattern diary data for exposure assessment and
modeling, the California Air Resources Board ( CARB)
conducted a probability -based diary study of the activity
patterns of residents of California that included 1762 adults
and adolescents from 1987 to 1988 and 1200 children from
1989 to 1990 (see Wiley et al., 1991a,b ). Referred to as the
CAPS in this paper
1
, these data have been used in a variety
of analyses:
o Phillips et al. (1990) examined appliance use and
ventilation practices in California;
1
We use the CAPS acronym to mean both the California survey of adults ±
youth and the survey of children under 12. Miller et al. ( 1998a ) use CAPS to
refer only to the study of children.
The National Human Activity Pattern Survey (NHAPS) Klepeis et al.
Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3) 233

o Phillips et al. (1991 ) and Jenkins et al. (1992) provided
summary statistics and investigated the proximity of
Californians to indoor pollutant sources including a
comparison of the times people spent in various
microenvironments and the percentage of people who
were engaged in various activities;
o Robinson (1991 ) and Robinson and Thomas (1991 )
compared California activities to national activities; and
o Robinson et al. (1993, 1994a,b, 1996), Ott et al. ( 1994),
Klepeis et al. (1994), and Miller et al. (1998a,b ) studied
the potential exposure of Californians to environmental
tobacco smoke (ETS ).
NHAPS was conducted as a follow-up to CAPS, and
was closely patterned after this landmark study. NHAPS is
the first U.S. study with national scope that was designed
to collect exposure -relevant information on human activity
patterns. EPA's main purpose for collecting the NHAPS
data was to provide diary records that could be used as
inputs for computer- based human exposure models. A
select panel of exposure scientists with diverse back-
grounds (air pollution, pesticides, drinking water, exposure
modeling) served as ``subject matter experts'' and helped
insure that the NHAPS diary and questionnaires gathered
the correct type of activity pattern data for use in
estimating pollutant exposures.
Since the completion of NHAPS, two other exposure-
related human activity surveys have emerged with data
collection instruments and geographical scales similar to
NHAPS. Both of the following studies make use of the
computer- assisted telephone interview (CATI) instrument
and, like NHAPS, collected daily diaries on the time
spent in locations, activities, and in the presence of
smokers:
o A national survey of 1200 Americans sponsored by the
Electric Power and Research Institute (EPRI) from 1994
to 1995 that was focused on human exposure to soil
(Robinson and Silvers, 2000 ); and
o The 9 -month CHAPS, which surveyed 2381 Cana-
dians from 1994 to 1995 with respondents in Toronto,
Vancouver, Edmonton, and Saint John, NB (Leech
et al., 1996, 1999 ).
Data collection
NHAPS was a 2- year national probability telephone
survey (n=9386 ) of the contiguous states conducted by
the University of Maryland Survey Research Center with
support from EPA. The telephone interviewing began in
late September 1992, ended on October 1, 1994, and was
divided into eight quarters with each quarter, except the
first, exactly 3 months in duration. Each quarter of the
study was composed of an independent random sample of
households.
While NHAPS utilized methods from previous time diary
studies, particularly CAPS (Wiley et al., 1991a,b; Jenkins et
al., 1992 ), it was augmented to obtain more precise estimates
of the time spent in microenvironments such as kitchens,
restaurants, bars, automobiles, and outdoor travel. Many
questions were also adapted from the comprehensive
Environmental Inventory Questionnaire (Lebowitz et al.,
1989) and from questionnaires used in the Total Exposure
Assessment Methodology (TEAM ) studies ( Akland et al.,
1985; Wallace et al., 1991) to help determine the population
segments most likely to experience microenvironments with
elevated pollutant concentrations. Supplemental questions
were developed for pollutant sources not treated in the
respondents' diary accounts such as solvents or gas
appliances. All interviews were conducted from the Survey
Research Center telephone interview facility in the College
Park campus in Maryland using the CATI technology, which
was developed by the Survey Research Center at the
University of California at Berkeley. The interviewers
averaged approximately 13 completed interviews for each
day of the year. Each interview took about 20 ±30 min to
complete, most of which were devoted to the diary but with
some time allotted for demographic ( e.g., age, gender, health
status, ethnicity, educational attainment, and housing type)
and supplemental (or ``follow -up'') exposure questions.
Selection of Subjects
The target population for NHAPS was all persons residing
in telephone -equipped households in the 48 contiguous
states. Telephone households were selected using a standard
two- stage random digit dial ( RDD) sample design. The
selection of telephone exchanges was stratified by the four
major U.S. census regions ( Northeast, Midwest, South, and
West). All potential primary sampling units (PSUs; area
code+ telephone exchange +first two digits of phone
number) were selected at the beginning of the study, but
they were not initially screened for residential status.
Immediately before the beginning of each quarter, the
primary numbers for that quarter were screened to select
PSUs for the second and final stages of selection.
In addition to the four census strata, the PSUs for each
quarter were randomly assigned to either a weekend or
weekday sample. Therefore, weekends and weekdays were
sampled independently within each quarter. Since the study
design required a person to recall the chronology of their
activities for the prior day, the weekend sample was called
only on Sundays and Mondays and consisted of either
Saturday or Sunday time diaries. The weekday sample was
called Tuesday through Saturday and consisted of Monday
through Friday time diaries.
In households consisting of only adults (i.e., respondents
18 years of age or older), one adult was selected at random.
Klepeis et al. The National Human Activity Pattern Survey (NHAPS)
234 Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3)

In households consisting of both adults and children
(respondents 17 years of age or younger ), a child was
selected at random 60% of the time from among all child
residents. The other 40% of the time an adult was selected at
random from among all adult residents. These different
probabilities of selection were used to control the ratio of
adults-to-children interviews. To increase the number of
children selected, the percentage of households in which
children were selected was increased from 60% to 70% in
quarters 6 through 8.
The ``next birthday'' selection method was used for
within- household respondent selection. In the next birthday
method, the interviewer asks to interview the adult ( or
child) residing in the household who will have the next
birthday. This method provides a random respondent
without having to ask intrusive questions about household
composition.
All data on adults were collected directly from the
selected respondent. For children under the age of 10, the
adult in the household most knowledgeable about the
child's activities completed a proxy interview for the child.
For children aged 10±17, an adult respondent answered
the general household and demographic questions. The 10-
to 17- year- olds then answered the time diary and post -
diary questions about their own activities.
Participation and Response Rates
A total of 9386 interviews were collected during the 2-year,
eight- quarter data collection period. If individuals did not
have telephones ( e.g., they were low- income or homeless ),
or if, when they were telephoned by an NHAPS interviewer,
they were on vacation or away from home for an extended
period of time, they were not included in the survey. These
individuals are not expected to be large in number, but their
omission could lead to some bias in survey statistics (e.g.,
calculations of time spent indoors).
For those Americans who were contacted by telephone,
the survey response numbers and rates are shown in Table 1.
The overall response rate is defined as the number of
completed interviews (n=9386 ) divided by the total
number of identified telephone households (14,908), which
is 63%. This figure is fairly high given the mean time to
complete each interview (25 min). When the number of
interviews successfully completed (9386) is divided by the
number of interviews attempted [completed interviews
(9386 )+refusals ( 2944) =12,330] , the resulting coopera-
tion rate is over 76%. This cooperation rate is relatively high
for a survey that did not utilize financial or other incentives
to increase participation.
The Questionnaire
Since the panel of expert reviewers for NHAPS concluded
that a single 25-min interview could not include all the
requirements for each topic area, it was decided to
emphasize only air quality and drinking water ( with a
greater emphasis on air quality). This decision was based on
the high priority given by EPA's Air Quality Office to
human exposure models that require activity pattern data
and the limited availability of such data. To accommodate
both the drinking water questions and the air quality
questions without making the interview unnecessarily
lengthy, two different questionnaire versions, A and B,
were developed and each was administered to one half of
the sample, selected at random. Versions A and B both
included demographic questions, a 24- h time diary, and a
set of supplementary exposure questions emphasizing
potential exposure to pollutants in either household air
(version A ) or water (version B ). A smaller number of
questions on each questionnaire version concerned expo-
sures to pollutants in soil and food (see Table 2 for a list of
background factors and question types).
Twenty -Four -Hour Diary The diary was the central
component of both questionnaire versions. In their diaries,
respondents reported all their activities for the previous
day. Although time±diary data have often been used to
measure the amount of time populations spent performing
certain activities, perhaps the more important question for
environmental pollutant exposure research is the pollutant
level in the location where the activity occurs (and the
Table 1. The NHAPS sample sizes and participation rates.
Number %
Sample released
a
26,263 ±
Non - households
b
11,076 ±
Status unknown
c
279 ±
Households
d
14,908 100
Interviews
e
9,386 63
Refusals
f
2,944 20
Non - contacts
g
1,870 12
Other
h
708 5
a
Sample phone numbers is the count of telephone numbers called for the
study.
b
Non - households include businesses, group homes such as nursing homes
and dormitories, group quarters, disconnected numbers, fax machines, etc.
c
Status unknown numbers were called at least 20 times but were never
contacted; therefore, the household status could not be ascertained.
d
Households include all telephone numbers that were determined to be a
household.
e
Interviews are all households where the selected respondent completed the
interview through a time diary.
f
Refusals are households that refused to complete the interview or
terminated the interview before or during the diary section.
g
Non - contacts include households in which only a home recorder or
answering machine could be reached and households in which the
respondent was identified but never reached for interview, even after at
least 20 call attempts.
h
Other are cases in which the respondent was unable to complete the
interview due to lack of comprehension of English or some physical
problem such as difficulty in hearing or speaking.
The National Human Activity Pattern Survey (NHAPS) Klepeis et al.
Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3) 235

length of time spent in that location). Thus, to address
environmental exposure issues, the time ±diary categories
(codes ) in NHAPS focused on the location in which
activities occurred. Exposure -related activity coding was
generally limited to activities of concern for their potential
to increase exposure to environmental pollutants; e.g.,
activities that require higher breathing rates, such as sports,
or activities that involve exposure to chemicals, such as
painting and auto repair. The only part of the diary that
concerned exposure-specific activity was the reported
presence of a smoker during each location and activity
combination (microenvironment).
When respondents were asked whether or not there was
someone else smoking during each of the microenviron-
ments they visited, one's own smoking was not included.
The question took the form: ``Was there someone ( else)
present who was smoking during that activity and in that
location?'' The reported time spent in the presence of a
smoker constitutes a measure of ``potential'' exposure (or a
marker of exposure ) to ETS. Previous investigators of the
CAPS database, which contains answers to the same
question on the presence of smokers as the NHAPS database,
refer to the potential exposure as self -reported proximity
(SRP ) ( Miller et al., 1998b ) or smoking -exposure- related
duration ( SERD) (Robinson et al., 1994b ).
There exists the possibility for bias in the NHAPS results
for the time spent with a smoker, since two respondents may
have reported the same amount of time with a smoker when
the intensity of smoke (e.g., the number of smokers or
number of cigarettes) was quite different. Actual exposure
to ETS depends on both the mass of tobacco smoke emitted
and building characteristics such as volume and air flow
rates. Respondents also may have misjudged whether or not
Table 3. Distribution of the NHAPS respondents by selected demographic
factors.
Factor Sample size NHAPS
(%)
U.S. census
(%)
Male 4,294 46 49
Female 5,088 54 51
Under 5 years old 499 5 8
5 ± 17 years old 1,292 14 19
18 ± 64 years old 6,059 65 61
Over 64 years old 1,349 14 13
White 7,591 81 83
Black 945 10 13
Asian 157 2 3
Of Hispanic origin 385 8 10
Postgraduate education 924 10 6
College graduate 1,247 13 20
High school graduate 2,612 28 32
There were 9,386 total respondents. Of the respondents, 187 ( 2% ) did not
report an age; 308 ( 3% ) reported being a race not listed or did not report a
race. For 1,968 ( 21% ) of the respondents, no educational - level data were
recorded. Census proportions are 1994 estimates from the U.S. Department
of Commerce ( 1995, 1996 ).
Table 2. Summary of factors and question types for versions A and B of the NHAPS questionnaire.
Factors Version A Version B
Biological ( age, race, gender ) Air Ð storage ( gas cans, lawnmower,
paints, mothballs, deodorizer, humidifier,
windows open, door open )
Air Ð storage ( gas, lawnmower,
paints, solvents )
Status ( employment, education )
Role ( children, other adults, work
hours, work evening, work outdoors)
Air Ð yesterday ( smoking Ð home / away,
others smoke, paints, open flame, glues,
solvents, pesticides, floor wax, gas - powered
equipment, cleaning agents, excessive dust,
stain removers, perfumes, nail polish, gas station,
gas stove, microwave, aerosol spray, heating,
heavy traffic, roadway, parking garage, walk to car )
Air Ð last 6 months ( renovations,
paint, floors, addition, carpets, glues,
sleep elsewhere, pesticides, vacuum floors,
humidifier, gas stove, heat sources )
Water ( shower /bath, dishwashing,
washing machine, drinking water Ð
bottle / tap, juices, soft drinks)
Water ( shower /bath, dishwasher, washing machine )
Geographic ( zipcodes Ð home,
zipcodes Ð work, housing, structure,
stories, rooms, carpet, basement, garage )
Ingestion ( children Ð soil ) Water Ð last month ( pool swimming )
Lifestyle ( health ) 24 - h diary ( activities, locations, smoker present,
hard breathing )
Ingestion ( children Ð soil, seafood,
blackened food )
24 - h diary ( activities, locations, smoker
present, hard breathing )
Source: Robinson and Blair ( 1995 ).
Versions A and B of the NHAPS questionnaire were given to different randomly selected samples, each spanning the contiguous U.S.
Klepeis et al. The National Human Activity Pattern Survey (NHAPS)
236 Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3)

a smoker was actually present and smoking. A smoker
might have been present for only a small portion of the
entire microenvironment (e.g., a smoker was present for
only 10 min during a 60 -min -long microenvironment), but
the potential exposure (SRP or SERD ) for that time period
would be the same as if a smoker was actually present the
entire time. In two out of the total 16 quarters of the NHAPS
study, the respondents were asked to specify for what
fraction of time in the microenvironment the smoker(s) was
(were ) present. This information may be useful in sorting
out any bias for the study as a whole.
In the Sample and Data Characteristics section, we
describe the structure of the NHAPS diary data including
the location and activity categories.
Supplemental Questions In this section, we summarize
some main features of the NHAPS supplemental exposure
questions. More complete descriptions of these questions,
including the results of data analysis, are given in Robinson
and Blair ( 1995), Klepeis et al. (1996), and Tsang and
Klepeis (1996 ).
The supplemental questions on both versions of the
NHAPS questionnaire concerned occasions of potential
exposure to specific pollutants such as particles, polycyclic
aromatic hydrocarbons ( PAHs), CO, ozone ( O
3
), NO
2
,
chloroform ( CHCl
3
), benzene (C
6
H
6
), and volatile organic
compounds (VOCs) in general. These questions were
included to supplement the respondents' diary accounts,
since respondents might not have remembered to report the
stop they made to buy gasoline while commuting to work or
the stop they made at a dry cleaner during lunch time. The
following exposure associations illustrate the basis for
including particular questions:
o Activities involving cigarette smoke or wood burning
may increase exposure to particles, PAH, CO, C
6
H
6
, and
other VOCs;
o Activities involving gasoline (e.g., pumping gasoline
into automobiles ) may increase exposure to C
6
H
6
and
other VOCs;
o Driving in traffic and activities in a parking garage may
increase exposure to C
6
H
6
, other VOCs, particles, PAH,
and CO;
Table 4. Example 24 - h recall diary containing beginning and ending times, activity, location, presence of a smoker, and time spent for 22 microenvironments
visited on the diary day.
Microenvironment
number
Starting
time
Ending
time
Summary Detailed
activity
Simplified
activity
Detailed
location
Simplified
location
Smoker?
(1=Yes)
Time spent
( min )
1 00:00 01:45 At night club 77 0 405 90 1 105
2 01:45 02:00 Traveled home after night club 79 0 301 30 0 15
3 02:00 11:00 Sleeping or napping 45 0 105 10 0 540
4 11:00 11:05 Brushed teeth 44 40 104 10 0 5
5 11:05 11:15 Preparing meals or snacks 10 10 101 10 0 10
6 11:15 11:25 Eating meals or snacks 43 70 102 10 0 10
7 11:25 11:30 Dressing or personal
grooming
47 0 102 10 0 5
8 11:30 11:37 Traveling to play football 89 0 306 40 0 7
9 11:37 13:37 Playing flag football 80 60 507 50 0 120
10 13:37 13:44 Traveling to home 79 0 306 40 0 7
11 13:44 13:54 Preparing meals or snacks 10 10 201 10 0 10
12 13:54 13:57 Traveling to bar 79 0 301 30 0 3
13 13:57 15:30 At bar 77 0 405 90 1 93
14 15:30 15:33 Traveling from bar 79 0 301 30 0 3
15 15:33 16:30 Watching TV 91 0 102 10 0 57
16 16:30 17:00 Bathing or showering 40 40 104 10 0 30
17 17:00 19:00 Watching TV 91 0 102 10 0 120
18 19:00 19:10 Traveling to shopping 39 0 301 30 0 10
19 19:10 19:25 Shopping for food 30 0 414 90 0 15
20 19:25 19:35 Travel related to shopping
for food
39 0 301 30 0 10
21 19:35 21:00 Watching TV 91 0 102 10 0 85
22 21:00 24:00 Studying 54 0 102 10 0 180
The respondent, whose diary is shown in this table, was a Hispanic male from Connecticut between the ages of 18 and 24 who was interviewed on a weekend
in the fall. See the Sample and Data characteristics section for a description of the simplified ( i.e., recoded ) locations and activities.
The National Human Activity Pattern Survey (NHAPS) Klepeis et al.
Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3) 237

o Activities involving hot water sources (e.g., hot showers,
baths, boiling water) may increase exposure to dis-
infection byproducts such as CHCl
3
;
o Activities involving gas stoves or ovens may increase
exposure to NO
2
and CO;
o Activities involving solvents and paints may increase
exposure to various VOCs; and
o Activities involving the use of dry -cleaned clothes may
increase exposure to tetrachloroethylene, 1,1,1-tri-
chloroethane, or aromatic solvents.
Version A ( emphasizing ``air'' questions) contains most
of the supplemental exposure questions on breathing rates
and locations with potentially degraded air quality (see
Table 2) including the presence of smokers. Additional
questions on version A examined exposures both at work
and at home to pollutants such as vapors from paints and
solvents. Potential exposure to C
6
H
6
was assessed by
questions concerning time spent in gasoline stations or
parking lots. Further questions were asked about respondent
activities in near proximity to: (1) gas stoves, gas furnaces,
and supplemental heating sources like wood or kerosene
stoves; ( 2) aerosol spray products; (3) hot showers or baths;
(4 ) room air fresheners, deodorizers, or mothballs; and (5 )
automobiles parked in attached garages.
The supplemental questions on version B (emphasizing
``water'' questions ) include questions on tap water contact
via drinking water and using tap water for such appliances
as dishwashers, washing machines, and humidifiers. Other
questions dealt with tap water contact through washing and
bathing Ð either by rinsing dishes, baths, or showers.
Separate questions were included about whether the door
was open while taking a bath or shower and the use of
exhaust fans. Another set of questions dealt with water
sources, either from wells, piped-in utilities, or purchased in
bottles.
Sample and data characteristics
Coverage and Representativeness
A comparison of the number of NHAPS respondents in
each state shows generally good agreement with the 1990
U.S. census ( U.S. Department of Commerce, 1992 ): the
``relative comparisons'' of most states are close to 1, where
Table 5. Results of selected exposure - related supplemental NHAPS
questions ( unweighted ).
Question Response
Did the respondent take a shower or bath
yesterday, and if they did, for how long? ( A )
91% yes
44% 0 ± 10 min
37% 10 ± 20 min
13% 20 ± 30 min
What type of fuel is used in the central
furnace? ( A)
61% gas
18% electric
14% oil
Is any room heated with a wood stove? ( B ) 6% yes
Is any room heated with a kerosene space
heater? ( B )
2% yes
Is any room heated with a fireplace? ( B ) 10% yes
Where is water obtained for household use? ( B ) 81% public
15% private well
Is bottled water used? ( B ) 43% yes
Did you smoke cigars yesterday? ( B ) 1% yes
How many glasses of tap water did the
respondent drink yesterday? ( B )
29% none
26% 1 ± 2
27% 3 ± 5
15% 6 ± 20 +
How many times did the respondent wash
their hands yesterday? ( B )
8% 2 or less
60% 3 ± 9
28% 10 ± 30 +
For how many hours did the respondent work
with soil in the last month? (B )
63% none
28% 0 ± 24
5% 24 ± 72
2% > 72
In what type of house does the respondent live?
( A and B )
21% apartment
68% detached
single house
5% townhouse
7% other
The NHAPS questionnaire version on which each question appears is
indicated in parentheses ( either A, B, or both A and B); results rounded to
the nearest percentage point. Some of the percentages not listed could
include missing, refused, or ``don't know'' responses.
Table 6. Geographical comparison of NHAPS minutes spent on the diary
day for California ( NHAPS - CA ) versus the entire nation.
Location n Overall
mean
( min )
Doer
%
Doer
n
Doer
mean
( min )
NHAPS - nation
In a residence 9196 990 99.4 9153 996
Office± factory 9196 78 20.0 1925 388
Bar ± restaurant 9196 27 23.7 2263 112
Other indoor 9196 158 59.1 5372 267
In an enclosed vehicle 9196 79 83.2 7596 95
Outdoors 9196 109 59.3 5339 184
NHAPS - CA
In a residence 930 979 99.6 927 983
Office± factory 930 73 19.3 187 379
Bar ± restaurant 930 34 26.4 260 128
Other indoor 930 166 61.9 542 269
In an enclosed vehicle 930 80 84.4 775 95
Outdoors 930 108 59.1 592 182
Means and percentages have been calculated using sample weights,
whereas the sample sizes n and Doer n are raw counts.
Klepeis et al. The National Human Activity Pattern Survey (NHAPS)
238 Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3)

a relative comparison of NHAPS and U.S. census
proportions is defined as the ratio of the percentages in
each state of the U.S. Census data to the NHAPS
percentages. The only state that was oversampled in
NHAPS with a relative comparison under 0.5 was
Montana. States that were undersampled at a relative
comparison over 1.5 were Vermont, Mississippi, North
Dakota, and Idaho. The 20 sampled states (including
Washington, DC) that did not have at least 100 NHAPS
respondents were Delaware, the District of Columbia
(DC ), Idaho, Iowa, Kansas, Kentucky, Maine, Mississippi,
Montana, Nebraska, Nevada, New Hampshire, New
Mexico, North Dakota, Rhode Island, South Dakota, Utah,
Vermont, West Virginia, and Wyoming. At 12± 18
respondents each, Vermont, Wyoming, North Dakota, and
Idaho had the fewest respondents of any sampled state.
Note that residents of Alaska and Hawaii were excluded in
the NHAPS sample design frame. The states that had more
than 500 NHAPS respondents were California, Florida,
New York, Pennsylvania, and Texas.
The percentage of NHAPS respondents sampled in each
of the 10 EPA Regions and each of the four census regions
is comparable to the population observed in the 1990 U.S.
census (U.S. Department of Commerce, 1992 ) with
relative comparisons near 1. There is a sufficient sample
size in each EPA Region to perform detailed statistical
analyses with a low of 340 NHAPS respondents in EPA
Region 8. Each of the four U.S. Census regions had
approximately 2000±3000 respondents.
The NHAPS sample proportions for gender, age, race,
and educational attainment match the estimated 1994
proportions (U.S. Department of Commerce, 1995, 1996)
reasonably well ( see Table 3). The worst agreement is for
the proportion of college graduates ( 13%/ 20%=0.65),
which may be due to the large number of missing data
values (20% of the respondents had missing values for their
educational attainment).
The number of respondents in each quarter of the
NHAPS study was fairly uniform (approximately 13% per
quarter), except for the first, when only 7.8% of the
respondents was interviewed. The proportion of respond-
ents interviewed during each season (winter, spring,
summer, fall ) ranged from 20% to 27%. Most of the
respondents were interviewed on a weekday (67%), which
is somewhat smaller than the ideal proportion ( 5/7= 71%)
since weekends were intentionally oversampled.
Sample Weights Weights are available for the NHAPS
database that correct the sample based on the increased
selection probability of households with multiple phones,
the different selection probabilities for adults and children,
seasonal quarter, and census region, and the oversampling
of weekends. Klepeis et al. ( 1996) have devised post-
stratification weights that incorporate the original weights,
OFFICE-FACTORY (5.4%)
TOTAL TIME SPENT
INDOORS (86.9%)
OTHER INDOOR LOCATION (11%)
BAR-RESTAURANT (1.8%)
OUTDOORS (7.6%)
IN A RESIDENCE (68.7%)
NHAPS - Nation, Percentage Time Spent
Total n = 9,196
IN A VEHICLE (5.5%)
Figure 1. Pie chart showing the mean percentage of time the NHAPS respondents spent in six different locations on the diary day ( weighted ). Time
spent indoors ( composed of time in a residence, in an office or factory, in a bar or restaurant, or in some other indoor location) is represented by
lightly shaded slices. The percentages in the figure are the mean percentages taken over individual percentages for people in the NHAPS sample.
Individual percentages were calculated from the time spent in each location over the total amount of time spent, which was equal to 24 h ( 1440
min ) for each individual ( see Table 6 for the number of doers for each location ).
The National Human Activity Pattern Survey (NHAPS) Klepeis et al.
Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3) 239

but also adjust the NHAPS sample to match population
proportions for age and gender. Gender and age data were
obtained from the 1990 U.S. Census (U.S. Department of
Commerce, 1992). The desired day- of- week and season
proportions are absolute quantities (i.e., 1 /4 for each
season and 1/7 for each day of the week ). The resulting
post- stratification weight assigned to each NHAPS
respondent can be used to calculate weighted statistics
across any combination of factors for age, sex, season,
census region, and day of week. Weights could not be
assigned to respondents with missing age or gender
variables, and these individuals were excluded from
weighted calculations (missing n =190 across the nation;
missing n =58 in California ). In this paper, we use the
post- stratification weights to calculate weighted means,
histograms, and proportions ( see Cochran, 1977 for a
good treatment of sampling methodology, including
formulae for calculating unbiased estimators ). The reader
should note that a comparison of weighted and un-
weighted results showed only a small discrepancy for
most calculated statistics.
Location and Activity Categories
Table 4 gives an example 24- h diary for a single
individual, a Hispanic male from Connecticut. Each diary
record contains the beginning and ending times for each
microenvironment the respondent visited, uniquely deter-
mined by a single combination of location and activity
codes. Each record also contains a code for whether or
not a smoker was present and if the respondent was
``breathing hard.''
The original 83 location codes that were used to encode
the NHAPS respondents' whereabouts are split into
categories for each respondent's own house, a friend's or
someone else's house, traveling, some other indoor
location, and some ``other'' outdoor location (see Klepeis
et al., 1996; Tsang and Klepeis, 1996). For the
calculations of time spent that we present in the Data
Analysis section, a reduced set of six locations was used:
residence, office±factory, bar±restaurant, other indoor
location, enclosed vehicle, and outdoors. In this grouping
scheme, residential locations at one's own home were not
differentiated from residential locations at someone else's
home (i.e., respondent locations were grouped into a
residential category even if the original NHAPS code
states that they were at someone else's house ). The vehicle
location includes travel inside cars, trucks, buses, trains,
airplanes, boats, and public transit. Travels outdoors via
motorcycle, bicycle, walking, or stroller, or waiting for
transit outdoors were all grouped into the outdoor location.
The other indoor grouping includes all the remaining
indoor locations such as malls, stores, schools, churches,
other public buildings, autorepair shops, health clubs,
laundromats, salons, and parking garages. Note that these
locations may be associated with very different, and
potentially very high, exposures. Locations were not
divided, specifically, according to work-related activities.
The only location category that can be associated with
work- related activities is office± factory. It is not possible
to determine Ð based on location alone Ð whether
work- related activities were occurring in any of the other
locations, since, e.g., respondents that are in stores,
restaurants, bars, or hospitals could be present either as
patrons or staff.
There are 91 distinct activity codes for the 24 -h recall
portion of the NHAPS database (see Klepeis et al., 1996;
Tsang and Klepeis, 1996 ). Although specific activities are
not analyzed in the current paper, Klepeis et al. present an
attempt to create broad exposure activity categories based
on the available data. The original NHAPS categories were
regrouped into eight categories each containing nearly 2000
episodic occurrences or more: cooking/food preparation;
laundry/ dishes /cleaning kitchen; housekeeping; bathing /
showering/washing /using bathroom; yardwork /garden-
ing/ car or house -maintenance; sports/exercise; eating/
drinking, and some ``other'' activity. The most frequent
activities in the other exposure activity category Ð into
which 73% of the microenvironments (distinct occurrences
Table 7. Geographical comparison of NHAPS minutes spent with a
smoker on the diary day for California ( NHAPS - CA ) versus the entire
nation.
Location with
a smoker
n Overall
mean
( min )
Doer
%
Doer
n
Doer
mean
( min )
NHAPS - nation (17% of respondents reported being cigarette
smokers; weighted )
All locations 9196 163 43.8 3949 372
In a residence 9196 78 25.6 2331 305
Office± factory 9196 16 4.3 394 363
Bar ± restaurant 9196 14 10.0 951 143
Other indoor 9196 19 7.6 725 247
In an enclosed vehicle 9196 11 14.5 1340 79
Outdoors 9196 24 11.4 1038 213
NHAPS - CA (14% of respondents reported being cigarette smokers;
weighted )
All locations 930 114 36.9 332 309
In a residence 930 45 16.5 164 270
Office± factory 930 9 3.4 26 280
Bar ± restaurant 930 13 8.0 82 168
Other indoor 930 19 7.4 58 252
In an enclosed vehicle 930 5 8.3 82 58
Outdoors 930 23 11.0 108 209
Means and percentages have been calculated using sample weights,
whereas the sample sizes n and Doer n are raw counts. The time spent with
a smoker does not include one's own smoking.
Klepeis et al. The National Human Activity Pattern Survey (NHAPS)
240 Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3)

in the diary database ) was grouped Ð were sleeping/
napping, watching television, and dressing.
Data analysis
Klepeis et al. (1996) and Tsang and Klepeis (1996 ) provide
detailed analyses of the time that NHAPS respondents
reported spending in locations and activities on the diary
day, as well as the results of the more than 150 supplemental
and demographic questions. These analyses include an
examination across categories such as gender, race, age,
years of education, employment status, weekday /weekend,
season, and region. Additional results of the auxiliary
questions and time use issues are discussed in Robinson and
Blair (1995 ). In this section, we present selected results to
provide a basis for making broad comparisons between
demographic groups within NHAPS and other activity
pattern studies.
Most of our results are based entirely on the NHAPS
diary data rather than answers to the supplemental
questions. We present broadly grouped statistics on the
time that NHAPS respondents spent in six different
locations ( residence, office±factory, bar ±restaurant, some
other indoor location, enclosed vehicle, and outdoors)
including the time that they spent with a smoker. We also
make comparisons to the CAPS of adults and youth over
age 12 (1987± 1988) and of children under age 12
(1989 ±1990) (see Wiley et al., 1991a,b; Jenkins et al.,
1992) and the 9-month CHAPS of Toronto, Vancouver,
Edmonton, and Saint John, NB (1994 ±1995 ) (see Leech
et al., 1996).
Although the minute -by-minute 24 -h recall diaries are
the main subject of the current analysis, the NHAPS
database also provides exposure assessors with a large
variety of yes-or- no and categorical questions on expo-
sure- related activities and household conditions. Table 5
presents results from a small selection of unweighted
results from the supplemental NHAPS questions that will
be useful to risk and exposure assessors, including policy
makers.
Calculation Methodology
The NHAPS statistics we present in this paper have been
weighted using the sample weights described above (unless
otherwise noted ); they were generated using the freely
available R system for data analysis and graphics (Ihaka and
Gentleman, 1996). The CAPS statistics were generated
using the TIMEWT set of sample weights included in the
CAPS databases.
Since the NHAPS diaries span a single 24- h period, most
of our calculations use this as the primary time interval (i.e.,
we present limited results for breakdowns by time of day).
The mean proportion of time spent in different locations is
NHAPS - Nation, Percentage Time Spent with a Smoker
w/ SMOKER INDOORS (76.6 %)
TOTAL TIME SPENT
IN A RESIDENCE (42.7%)
OFFFICE-FACTORY (7.2%)
BAR-RESTAURANT (14.6%)
OTHER INDOOR (12.1%)
OUTDOORS (14.7%)
IN A VEHICLE (8.7%)
doers = 3,949
Figure 2. Pie chart showing the mean percentage of time the NHAPS respondents spent with a smoker in six different locations on the diary day
( weighted ). Time spent indoors ( composed of time in residence, in an office or factory, in a bar or restaurant, or in some other indoor location ) is
represented by lightly shaded slices. The percentages in the figure are means taken over individual percentages for people in the NHAPS sample
that reported being with a smoker for at least 1 min on the diary day ( the doers ). Individual percentages were calculated as the time spent in the
presence of a smoker in each location divided by the total amount of time spent with a smoker (see Table 7 for the total number of doers and the
number of doers for each location ). ( Please see the text for a discussion of SRP-SERD biases inherent in the NHAPS database with respect to
the time respondents reported spending with a smoker.)
The National Human Activity Pattern Survey (NHAPS) Klepeis et al.
Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3) 241

calculated by taking the mean of the total number of minutes
each respondent spent in each location and dividing by 1440
min ( 24 h ).
The total time spent with a smoker on the diary day varies
from person to person; so that individual percentages of
time spent with a smoker in each location use a different
denominator for each person. The mean percentage of time
spent with a smoker was calculated by simply averaging
over the individual percentages.
Different numbers of respondents spent time in each
location (both with and without a smoker ) on the diary day
and also at different times of day. Those who spent at least 1
min in each location, either across the entire day or for any
particular time interval, are called the ``doers.'' In each
results table, we present the weighted proportions of daily
doers alongside overall means and doer means (i.e., means
taken across only the doers ).
NHAPS: The Nation
Of any location visited on the diary day, the lowest
percentage of doers was 20% for office±factory (see Table
6). Of the total time spent by all respondents on the diary
day, 69% was spent, on average, in a residence ( Figure 1).
Approximately 87% of the time was spent indoors and 5±
6% in a vehicle Ð with the remaining 7± 8% spent
outdoors. These results are comparable with U.S. time
budgets reported by Robinson and Thomas (1991 ) from a
1985 study and Canadian time budgets reported by Leech et
al. ( 1996). For both of these two studies, which span a
period of about 10 years, respondents reported spending
µ(doers) = 184 min
59 % doers
µ = 109 min (n = 9,196)
99.9 % doers
µ(doers) = 1253 min
µ = 1251 min (n = 9,196)
83 % doers
µ = 79 min (n = 9,196)
µ(doers) = 95 min
150010005000
NHAPS - Nation
µ = 990 min (n = 9,196)
99.4 % doers
µ(doers) = 996 min
140010006002000
0.000 0.004 0.008 0.012
0.0120.0080.0040.000 0.0120.0080.000 0.004
0.000 0.004 0.008 0.012
200 600 1000 14000 2000 600 1000 1400
Probability
Probability
Minutes SpentMinutes Spent
Probability
Probability
Minutes Spent Minutes Spent
IN A RESIDENCE
INDOORS OUTDOORS
IN A VEHICLE
Figure 3. True histograms calculated from the weighted number of minutes that NHAPS respondents spent indoors, outdoors, in an enclosed vehicle,
and in a residence. The time each individual spent in a residence is a subset of his total time spent indoors. While the histograms for the first three
locations are strongly skewed ( either right or left ) with low variability, the time spent in a residence is highly variable and has three distinct modes: a
small one for those who spent no time in a residence on the diary day; a middle one for those who spent much of their day away from home; and a
third mode for those who were at home for most or all of the diary day. The overall weighted mean number of minutes spent, , is provided on each
graph, which, like the histograms, includes individuals who spent zero time in each location. The weighted percentage of respondents,  (doers), who
spent at least 1 min in each location ( the doers ) is also provided along with the weighted mean number of minutes they spent.
Klepeis et al. The National Human Activity Pattern Survey (NHAPS)
242 Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3)

89% of the time spent indoors with 5% in a vehicle and 6%
outdoors.
There may be some negative bias in the NHAPS results
for time spent outdoors, since those who were away from a
home for extended periods (e.g., on vacation or homeless )
were not included in the survey. These individuals may be
more likely than those who were at home to spend large
quantities of time outdoors. On the other hand, there may
be positive bias due to neglecting institutionalized and /or
hospitalized individuals. In addition, the surprisingly small
amount of outdoor doers (59%; see Table 6) suggests that
the brief amounts of time that people might spend walking
to their car or taking out the garbage, for example, were not
included in the diaries. Questions in the supplemental
portion of the NHAPS diary may be useful in understanding
the magnitude of this missing time. It seems unlikely,
though not impossible, that this unaccounted time contrib-
utes an appreciable amount to the total time spent outdoors.
In the NHAPS sample, 56% of respondents was never
with a smoker ( the non- doers), and was therefore not
included in the calculation of percentages (see Table 7 for
the percentage of doers in each location). The average
percentage of time spent with a smoker in residences was
43%; it was 15% for bars and restaurants and 9% for an
enclosed vehicle ( Figure 2 ).
The shape of the distribution for time spent indoors is
extremely positively skewed (a high proportion of long
times), while time spent outdoors and in a vehicle is
extremely negatively skewed ( a high proportion of short
times) Ð resulting in low variability ( see Figure 3 ). In
contrast, the variability in the time spent in a residence is
very high; the distribution has three distinctly different
modes corresponding to those respondents spending no
time at home (less than 1%; see Table 6), those spending
more than half their day at home, and those spending the
entire diary day at home.
For some exposures, it is useful to determine the
precise times of the day that the respondents are in certain
locations or engaging in specific activities, since expo-
sures to some air pollutants can depend on temporal
trends. For example, the amount of time that a person
spends outdoors during the day will greatly affect his
exposure to ground- level ozone. As illustrated by Figure
4, the NHAPS database provides information on how the
proportion of persons in different locations changes by
time of day. Here, we see that over 90% of respondents is
in a residence from about 11 PM to 5 AM, and, as
expected, the largest proportions of respondents in
schools, public buildings, offices, and factories occur
between 7 AM and 5 PM.
Figure 4. Stacked plot showing the weighted percentage of NHAPS respondents in each of 10 different locations according to the time of day. The
original minute - by - minute diary data have been smoothed for clarification.
Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3) 243
The National Human Activity Pattern Survey (NHAPS) Klepeis et al.

NHAPS: California Versus the Nation
In Figure 5, we see that the mean percentages of time
spent in the six grouped locations and the mean times
spent with a smoker are very similar for the national
NHAPS sample and the California subsample ( NHAPS-
CA). The overall means of time spent for each location
(calculated over the entire sample, including those who
spent zero time in a particular location), the proportion
of doers ( those who spent at least 1 min in a particular
location on the diary day), and the mean time spent
by the doers are very close for the two samples
(Table 6).
The largest mean time spent in any location is nearly
1000 min (17 h) for the residential location for both the
nation and California by itself. For both geographic
groups, nearly 100% percent of the respondents reported
being in a residence at some time on the diary day. The
largest mean time spent with a smoker (Table 7 ) was for
offices and factories at 363 min/ day for the nation and
280 min for California, followed by the residential
location at 305 and 270 min, respectively. The lower
means for California in these locations account for the
somewhat lower mean time spent with smokers across all
locations (372 vs. 309 min). California also appears to
have a slightly lower number of persons spending time
with a smoker (44% vs. 37% across all locations ),
apparently driven by the lower number of persons
spending time with smokers in residences (26% vs.
17%) and, perhaps, the somewhat lower number of
reported cigarette smokers (17% in the nation vs. 14% in
California, all ages).
NHAPS Versus CAPS
A comparison between the NHAPS California subsam-
ple (NHAPS -CA) and CAPS allows us to observe the
trends in activity patterns over time (from the late
1980s to the early -to- mid- 1990s) and to evaluate the
consistency between these two studies, which have
fairly similar methodologies. The studies had the same
survey instrument ( i.e., CATI), but CAPS was a
stratified sample of California and NHAPS-CA was
not ( although the overall NHAPS sample was indeed
stratified; see the above discussion on the NHAPS data
collection methodology ).
As we observed in a comparison of the national NHAPS
sample and NHAPS -CA, there is little difference between
the mean percentage of time spent in each of the six
locations between NHAPS-CA and CAPS for both adults
and youth ( age 12 and over) and for children under age 12
(see Figure 6 and Table 8). However, there are sizable
In a Residence
Office-Factory
Bar-Restaurant
Other Indoor
In a Vehicle
Outdoors
0
0
10
10
20
20
30
30
40
40
50
50
60
60
70
70
80
80
90
90
100
100
NHAPS-CA-w/SMKR
(doers=332)
NHAPS-w/SMKR
(doers=3949)
NHAPS
(total n=9196)
NHAPS-CA
(total n=930)
Weighted Percentage of Time Spent with a Smoker
Weighted Percentage of Time Spent
Percentage of Time Spent (%)
Percentage of Time Spent (%)
Nation-California Comparison
Figure 5. Comparison of the weighted percentage of overall time spent and time spent with a smoker in each of six locations for all of the NHAPS
respondents ( the entire national sample ) and for the California - based NHAPS respondents ( NHAPS - CA) ( see Tables 6 and 7 for the total
number of doers in each location ). ( Please see the text for a discussion of SRP-SERD biases inherent in the NHAPS database with respect to the
time respondents reported spending with a smoker. )
244 Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3)
Klepeis et al. The National Human Activity Pattern Survey (NHAPS)

differences for the time spent with a smoker (i.e., for the
mean time spent and the percentage of doers; see Table 9).
In both surveys, children under 12 spent small amounts
of time in offices, factories, bars, and restaurants (overall
means of 2±7 min, doer means of 40 ±60 min, and
negligible percentages of time; see Figure 6 ). Our results
show that children in California under the age of 12 spend a
larger percentage of time indoors and outdoors and a lower
percentage in vehicles than do adults. These are the same
results as reported for Canada (Leech et al., 1996 ).
Since the adult/youth sample contributes the bulk of
NHAPS-CA respondents (n= 805 for adult /youth vs.
n= 125 for children), there were not enough California
children respondents in NHAPS to calculate reliable
statistics for the time spent with a smoker in different
locations. However, from the statistics for children across
all locations (see Table 9), we see that while the doer
mean across all locations matches the CAPS mean fairly
well ( 222 vs. 204 min ), the percentage of doers is much
lower for NHAPS -CA (20% vs. 38%). In the 1994±
1995 CHAPS study of four Canadian cities, 30% of
children reported being with a smoker (Leech et al.,
1999). According to the results from CAPS alone,
residences were (by far) the location where children had
the longest mean time spent with a smoker (314 vs. 174
min for outdoors, the next highest mean). For CHAPS,
children also experienced the most time with smokers in
the residence. Twenty-five percent of CAPS children
reported being with a smoker in a residence, whereas
less than 13% reported being with a smoker in any of
the other locations.
The adult/youth age group has ample sample size and
can, therefore, provide an opportunity to observe the
change in time spent with a smoker in each location
from the earlier CAPS study to the later NHAPS study
(see Table 9). As with the children, there appears to be a
large reduction in the time spent with smokers over the
period from the late 1980s (CAPS) to the early - to mid-
1990s. The fact that the two studies have similar data
collection instruments and the total time spent in each
location match so well suggests that the differences in
time spent with a smoker are due to real changes in
human activity over the 5-year period.
There is a 22% decrease in the total number of adult /
youth doers (persons exposed to second- hand smoke in
all locations ) from CAPS to NHAPS-CA (62% down to
40%). The percentage of doers in the residence and
office± factory Ð the locations with the largest doer
mean times spent Ð dropped from 26% to 17% and
13% to 4%, respectively, over the time period. The
number of doers in bars and restaurants fell by almost
half, going from 19% to 9%. However, the doer means
do not drop (as they do slightly for the overall means,
since there are fewer doers) and even increase
dramatically for some locations; the bar± restaurant doer
mean increases from 93 min in CAPS to 178 min in
CAPS-CH
NHAPS-CA-CH
CAPS - AD/YTH
NHAPS-CA-AD/YTH
0
0
10
10
20
20
30
30
40
40
50
50
60
60
70
70
80
80
90
90
100
100
In a Residence
Office-Factory
Bar-Restaurant
Other Indoor
In a Vehicle
Outdoors
(total n=1762)
(total n=805)
(total n=125)
(total n=1200)
NHAPS-CAPS Comparison
Percentage of Time Spent for Children (Under 12)
Percentage of Time Spent for Adults and Youth (12 and older)
Percentage of Time Spent (%)
Percentage of Time Spent (%)
Figure 6. Comparison of the weighted percentage of time spent and time spent with a smoker in each of six locations for adult ± youth and child
NHAPS respondents and for adult± youth and child CAPS respondents. The children are under age 12. Both samples cover the entire state of
California ( see Tables 8 and 9 for the total number of doers in each location). ( Please see the text for a discussion of SRP-SERD biases inherent in
the NHAPS database with respect to the time respondents reported spending with a smoker. )
Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3) 245
The National Human Activity Pattern Survey (NHAPS) Klepeis et al.

NHAPS-CA, the outdoor doer mean goes from 121 to
210 min, and the mean in other indoor locations ( e.g.,
public buildings, malls, and stores) rises from 160 to
254 min. Possible explanations are that smokers are
asked or required to smoke in circumscribed locations
where they contribute to longer exposure times for others or
that policies have reduced casual exposures but not
dominant ones.
The reduction in the number of reported cigarette
smokers (20% for CAPS adults/youth vs. 16% for
NHAPS-CA adults /youth ) may have contributed to some
of the changes in the number of doers and the time spent
with a smoker for Californians of all ages. The California
Department of Health Services ( 1998) reports a similar drop
in cigarette smoking prevalence (20% in 1990 down to 17%
in 1994). With the passage of a statewide California
ordinance (AB13; effective January 1, 1995
2
) that prohibits
smoking in enclosed workplaces, we might expect that, in
recent years, the total time spent with a smoker in California
has dropped even further. Miller et al. (1998a ) predict a
reduction of 25±40% in adult ETS exposure in California
between the late 1980s and the late 1990s. However,
smoking in the home and automobile may be less affected,
Table 8. Comparison of minutes spent on the diary day for NHAPS
California respondents ( NHAPS- CA ) versus CAPS.
Location n Overall mean
( min )
Doer
%
Doer
n
Doer mean
( min )
NHAPS - CA adults and youth ( 12 and over )
In a residence 805 961 99.6 802 966
Office± factory 805 85 22.0 182 388
Bar ± restaurant 805 38 28.9 243 133
Other indoor 805 162 62.4 478 260
In a vehicle 805 86 86.0 682 100
Outdoors 805 106 58.8 508 181
CAPS adults and youth ( 12 and over )
In a residence 1762 954 99.3 1755 961
Office± factory 1762 106 32.6 515 327
Bar ± restaurant 1762 36 37.0 624 97
Other indoor 1762 157 70.2 1225 223
In a vehicle 1762 98 87.2 1516 113
Outdoors 1762 86 61.7 1112 140
NHAPS - CA children ( under 12 )
In a residence 125 1081 100 125 1081
Office± factory 125 2 3.3 5 60
Bar ± restaurant 125 7 11.4 17 65
Other indoor 125 188 59.2 64 318
In a vehicle 125 46 75.0 93 62
Outdoors 125 115 61.1 84 188
CAPS children (under 12 )
In a residence 1200 1093 99.7 1196 1097
Office± factory 1200 2 4.3 48 42
Bar ± restaurant 1200 6 12.7 176 49
Other indoor 1200 128 59.4 700 216
In a vehicle 1200 61 76.0 887 80
Outdoors 1200 149 83.5 994 178
Means and percentages have been calculated using sample weights,
whereas the sample sizes n and Doer n are raw counts.
2
AB13 banned smoking in California workplaces on January 1, 1995 Ð with
an exception for bars, clubs, and casinos. That exception was extended until
January 1, 1998 when smoking was banned in all bar ± restaurants throughout
the state.
Table 9. Comparison of minutes spent with a smoker for NHAPS
California respondents ( NHAPS- CA ) versus CAPS.
Location with
a Smoker
n Overall
mean
( min )
Doer
%
Doer
n
Doer
mean
( min )
NHAPS - CA youth and adults ( 12 and over ) ( 16% of respondents
reported being cigarette smokers; weighted )
All locations 805 126 39.8 308 317
In a residence 805 46 17.1 147 271
Office± factory 805 11 4.0 26 280
Bar ± restaurant 805 15 8.6 79 178
Other indoor 805 22 8.5 57 254
In a vehicle 805 5 9.6 78 57
Outdoors 805 26 12.5 103 210
CAPS youth and adults ( 12 and over ) ( 20% of respondents reported
being cigarette smokers; weighted)
All locations 1762 176 61.6 1014 285
In a residence 1762 63 26.3 430 238
Office± factory 1762 35 13.2 187 268
Bar ± restaurant 1762 18 19.2 320 93
Other indoor 1762 32 20.3 338 160
In a vehicle 1762 11 11.3 206 94
Outdoors 1762 17 13.9 255 121
NHAPS - CA children ( under 12 )
a
All locations 125 44 19.8 24 222
CAPS children (under 12 )
All locations 1200 77 37.9 483 204
In a residence 1200 56 24.6 314 227
Office± factory 1200 0.024 0.26 3 9
Bar ± restaurant 1200 3 4.8 70 54
Other indoor 1200 3 4.4 66 59
In a vehicle 1200 5 8.9 120 52
Outdoors 1200 12 12.7 174 92
a
The NHAPS-CA children sample size is too small to calculate statistics
for each location.
Means and percentages have been calculated using sample weights,
whereas the sample sizes n and Doer n are raw counts. The time spent with
a smoker does not include one's own smoking.
246 Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3)
Klepeis et al. The National Human Activity Pattern Survey (NHAPS)

with residences and cars remaining the locations where
children spend substantial amounts of time with smokers.
Variation Across EPA Regions
Surprisingly, we do not see much difference in the mean
percentage of time spent in different locations across the 10
EPA regions. The percentage of time spent with a smoker is
also very consistent across these geographically and
climatically distinct areas. The similarities are illustrated
in Figure 7. The percentage of doers in each location and the
mean doer times spent are also very close across the EPA
regions (Table 10). Differences are larger for comparisons
of percentage doers and doer mean for the time spent with a
smoker ( Table 11 ), but the statistics are still very
comparable. The states that comprise each EPA region are
listed in Table 11.
One should keep in mind that the respondents were
interviewed during all four seasons, and the results we
present are averaged over individuals who provided diaries
throughout the year. Nevertheless, it is interesting to observe
that persons living in the upper mid-western area of the
country (EPA Region 5 ), with its cold winters and mild
In a Residence
Office-Factory
Bar-Restaurant
Other Indoor
In a Vehicle
Outdoors
10
10
9
9
8
8
7
7
6
6
5
5
4
4
3
3
2
2
1
1
0
0
10
10
20
20
30
30
40
40
50
50
60
60
70
70
80
80
90
90
100
100
NHAPS, Percentage of Time Spent
NHAPS, Percentage of Time Spent With a Smoker
EPA Region
EPA Region
Percentage of Time Spent (%)
Percentage of Time Spent (%)
NHAPS Comparison By EPA Region
Figure 7. Comparison of the weighted percentage of time spent and time spent with a smoker in each of six locations across the 10 U.S. EPA regions
( see Table 11 for the number of doers in each location and EPA region ). The states comprising each EPA region are listed in Tables 10 and 11.
( Please see the text for a discussion of SRP-SERD biases inherent in the NHAPS database with respect to the time respondents reported spending
with a smoker. )
Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3) 247
The National Human Activity Pattern Survey (NHAPS) Klepeis et al.

summers, spend nearly the same percentage of time
outdoors, on average, as most parts of the country, including
the southwestern area ( EPA Region 9) with its hot summers
and mild winters. These results are consistent with U.S.
versus Canada comparisons.
Summary and conclusions
It is clear from studies of personal exposure that human
activity patterns are crucial in identifying and determining
human exposure to environmental pollutants. Activity
pattern data, such as that in the NHAPS database, may be
used to estimate the prevalence and duration of population
exposure, especially for high-risk groups, to many environ-
mental pollutants (such as tobacco smoke). For example,
we can make the following general observations based on
activity pattern data alone:
o Americans spend 87% of their time indoors and 6% in an
enclosed vehicle (on average);
Table 10. NHAPS minutes spent on the diary day by EPA region.
Location n Overall
mean
Doer
%
Doer
n
Doer
mean
EPA Region 1
In a residence 572 1000 99.3 569 1006
Office± factory 572 100 23.2 127 430
bar ± restaurant 572 28 23.9 146 115
Other indoor 572 145 58.9 323 246
In an enclosed vehicle 572 85 83.9 465 101
Outdoors 572 83 56.1 319 148
EPA Region 2
In a residence 965 974 99.9 964 975
Office± factory 965 107 26.8 228 400
Bar ± restaurant 965 19 17.0 196 110
Other indoor 965 154 59.2 571 260
In an enclosed vehicle 965 82 81.1 768 101
Outdoors 965 104 61.9 594 168
EPA Region 3
In a residence 1089 978 98.9 1081 989
Office± factory 1089 94 22.7 235 413
Bar ± restaurant 1089 27 20.5 241 132
Other indoor 1089 153 57.5 634 267
In an enclosed vehicle 1089 76 82.4 880 92
Outdoors 1089 112 58.3 627 193
EPA Region 4
In a residence 1713 977 99.7 1709 980
office± factory 1713 85 20.5 335 416
bar ± restaurant 1713 26 22.3 399 115
Other indoor 1713 160 58.6 1008 273
In an enclosed vehicle 1713 82 86.2 1437 95
Outdoors 1713 111 55.1 920 201
EPA Region 5
In a residence 1651 977 99.0 1639 987
Office± factory 1651 98 24.1 376 408
Bar ± restaurant 1651 31 27.1 442 114
Other indoor 1651 150 57.5 948 260
In an enclosed vehicle 1651 86 85.1 1388 101
Outdoors 1651 98 53.9 888 181
EPA Region 6
In a residence 1019 983 99.5 1015 988
Office± factory 1019 88 22.1 214 398
Bar ± restaurant 1019 21 23.3 239 90
Other indoor 1019 159 59.4 584 267
In an enclosed vehicle 1019 86 86.2 853 100
Outdoors 1019 104 58.0 597 179
EPA Region 7
In a residence 418 980 99.1 415 989
Office± factory 418 43 12.5 60 344
Bar ± restaurant 418 28 26.1 111 108
Other indoor 418 190 63.3 257 301
In an enclosed vehicle 418 81 83.8 344 97
Outdoors 418 117 58.5 234 201
EPA Region 8
In a residence 340 981 98.8 338 992
Office± factory 340 89 23.6 68 376
Bar ± restaurant 340 29 24.4 80 117
Other indoor 340 151 59.2 204 256
In an enclosed vehicle 340 70 84.5 281 83
Outdoors 340 121 60.3 203 200
EPA Region 9
In a Residence 1239 985 99.7 1235 988
Office± factory 1239 74 19.9 257 371
Bar ± restaurant 1239 30 24.9 338 121
Other indoor 1239 158 59.3 723 266
In an enclosed vehicle 1239 81 83.6 1028 96
Outdoors 1239 112 63.8 789 176
EPA Region 10
In a residence 380 978 99.6 378 982
Office± factory 380 67 18.4 74 365
Bar ± restaurant 380 26 27.1 103 97
Other indoor 380 151 61.8 220 244
In an enclosed vehicle 380 72 80.7 298 90
Outdoors 380 146 69.1 261 211
Alaska (AK) and Hawaii (HI) were not sampled as part of NHAPS.
Means and percentages have been calculated using sample weights,
whereas the sample sizes n and Doer n are raw counts (see Table 11 for the
states comprising each EPA region).
248 Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3)
Klepeis et al. The National Human Activity Pattern Survey (NHAPS)

o The percentage of time spent indoors, outdoors, and in
vehicles is fairly invariant across people in different parts
of the U.S. ( on average );
o Americans and Canadians spend similar amounts of time
indoors, outdoors, and in vehicles (on average);
o From sociological studies, it appears that the time
Americans spend indoors has remained fairly uniform
over the past few decades;
o Forty-four percent of Americans spends time with a
smoker each day (ca. 1992 ±94);
o Of any location, Americans spend the largest percentage
of time with a smoker in residences ( 43%, calculated as
an average across individual respondent percentages ca.
1992± 94); and
o The number of people spending time with smokers in
California has decreased between the late 1980s and the
mid- 1990s ( when NHAPS was conducted ).
When combined with measurements and /or models of
pollutant emission, activity pattern data that possess high
time resolution can be used to provide estimates of actual
population exposures caused by a variety of different
pollutant sources. These population exposure models make
it possible to estimate, with increased precision, the
frequency distribution of exposure across a population, as
well as the likely change in the distribution when exposure
to a particular pollutant source is modified ( e.g., by a
change in human behavior).
In the future, investigators may want to consider a
number of improvements upon the NHAPS survey design.
For example: (1) The NHAPS survey was limited to a
single 24- h period for each respondent and, therefore, did
not consider any day- to-day variation in the behavior of
each respondent. To examine diurnal cycles in human
behavior, future studies should sample individuals on
multiple days. ( 2) The NHAPS results on the reported
presence of a smoker may be biased.
3
The diary question on
the presence of a smoker did not require all respondents to
specify the portion of time that a smoker was actually
present in each microenvironment. For example, a smoker
might have been present for only 10 min when the total time
spent in the microenvironment was an hour or more. In such
a case, the reported time spent exposed to a smoker would
be 1 h, a large positive bias. Future studies should collect
more precise information on the presence of smokers and /or
other pollutant sources.
Acknowledgments
The research described in this article has been funded,
wholly or in part, by the U.S. Environmental Protection
Agency under Cooperative Agreement CR816183 with the
University of Maryland, under contract 68-W5 -0011 to
Lockheed Martin Services Group, and as part of a the
Human Exposure and Dose Simulation University Partner-
ship (HEADSUP) among Lawrence Berkeley National
Laboratory (LBNL), Stanford University, and EPA (agree-
ment number DW89931890). It has been subjected to
Agency review and approved for publication. Mention of
trade names or commercial products does not constitute
endorsement or recommendation for use.
Table 11. NHAPS total minutes spent with a smoker on the diary day by EPA region.
EPA Region n Overall mean ( min) Doer % Doer n Doer mean ( min )
( 1 ) New England: CT, ME, MA, NH, RI, VT 572 172 47.8 269 360
( 2 ) North Atlantic: NJ, NY 965 170 47.5 437 357
( 3 ) Mid Atlantic: PA, DE, DC, MD, VA, WV 1089 179 43.8 453 409
( 4 ) South Atlantic: AL, FL, GA, KY, MS, NC, SC, TN 1713 198 46.3 759 429
( 5 ) Midwest: IL, IN, MI, MN, OH, WI 1651 179 47.0 751 380
( 6 ) South Central: AR, LA, OK, TX, NM 1019 170 44.6 438 380
( 7 ) Central: IA, KS, MO, NE 418 168 43.8 176 384
( 8 ) North Central: ND, SD, CO, MT, UT, WY 340 118 33.0 112 359
( 9 ) Pacific: AZ, CA ( HI)
a
, NV 1239 125 37.5 459 333
( 10 ) Mountain: ( AK )
a
, ID, OR, WA 380 162 39.5 151 409
Means and percentages have been calculated using sample weights, whereas the sample sizes n and Doer n are raw counts. The time spent with a smoker does
not include one's own smoking.
a
Alaska ( AK ) and Hawaii ( HI ) were not sampled as part of NHAPS.
3
There are also a number of other recognized biases which are expected to have
a small impact on average statistics. These other biases include the following:
( 1 ) the survey was limited to individuals residing in homes with telephones;
( 2 ) the survey did not include individuals who were on vacation, away from
home for extended periods, or homeless, and who may, therefore, spend more
time outdoors than those who were actually surveyed; ( 3 ) the survey did not
include people in institutions / hospitals who might spend less time outdoors;
and ( 4 ) the diaries may be missing brief periods of time that people spent
outdoors such as might occur when one walks to a car or store, or takes out the
garbage.
Journal of Exposure Analysis and Environmental Epidemiology (2001) 11(3) 249
The National Human Activity Pattern Survey (NHAPS) Klepeis et al.

The preparation of this manuscript Ð including the data
analyses Ð was also funded, in part, by the Tobacco -
Related Disease Research Program (TRDRP ) of California
(award no. 6RT- 0118).
The authors thank the University of Maryland's Survey
Research Center for designing NHAPS, conducting the
NHAPS data collection and data management activities,
and for assisting in the data analysis phase of the study.
The authors also thank W.W. Nazaroff for reading and
commenting on the manuscript, particularly in pointing
out important sample biases, A.B. Bodnar and R.
Maddalena of LBNL for reviewing the manuscript, and
the anonymous peer reviewers for their thoughtful
suggestions.
Finally, we thank the following distinguished group of
scientists who served on the NHAPS panels. Mel Kollander,
Stanley Presser, and Lance Wallace served on the survey
design panel; Steve Colome, Naihua Duan, Peggy Jenkins,
Paul Lioy, and Barry Ryan served as the peer review panel;
and the subject matter expert panel consisted of Julian
Andelman, Michael Firestone, Patrick Kennedy, Ted
Johnson, Thomas McCurdy, and James Repace.
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