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Effectiveness of Portable Indoor Air Cleaners: Sensory Testing Results

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Abstract and Figures

The objective of this study was to test the effectiveness of individual commercially available portable indoor air cleaning units in removing dust particulates, tobacco smoke particulate and vapor phase constituents (nicotine and vinyl pyridine), viable and total fungal spores, pollen, and gaseous contaminants (carbon monoxide[CO], nitrogen dioxide[NO2], and formaldehyde[HCHO]), in a clean air test chamber. The air cleaner chamber results presented here represent initial-use results. In general, High Efficiency Particulate Air (HEPA) and electrostatic precipitator systems demonstrated the highest efficiencies with respect to particulate, contaminants, followed closely by electret filter systems. Ionizers and ozone generators were least effective in particulate removal. Systems which included sufficient sorbent material (i.e. activated carbon or potassium permanganate) were marginally effective at gaseous contaminant removal. None of the systems tested were effective at carbon monoxide removal. Sensory testing was conducted to discern potential correlation between human perceptive response and measured air cleaner performance (with respect to tobacco smoke removal). An electret filter (EF) loaded with carbon sorbent received the best ratings with respect to odor strength, nasal irritation, eye irritation, and overall air acceptability.
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Indoor
Air
1994,
4:
379-188
Rinted
zn
Denmark
.
all
rights
reserved
Copyright
0
Munksgaard 1994
Indoor
Air
ISSN
0905-6947
Effectiveness
of
Portable Indoor
Air
Cleaners:
Sensory
Testing
Results
Richard
J.
Shaughnessy, Ph.D.
’,
Estelle
Levetin,
Ph.D.2, Jean Blocker, Ph.D.3,
Kerry
L.
Sublette, Ph.D.4
University
of
Tulsa,
OK,
USA5
Abstract
The objective
of
this study was
to
test the effectiveness
of
indi-
vidual commercially available portable indoor air cleaning
units in removing dust particulates, tobacco smoke particulate
and vapor phase constituents (nicotine and
vinyl
pydine),
viable and total fingal spores, pollen, and gaseous contami-
nants (carbon monoxide[CO], nitrogen dioxide/N@], and
formaldehydefHCHO]), in a clean air test chamber. The air
cleaner chamber results presented here represent initial-use
re-
sults.
In
general, High Efficiency Particulate Air (HEPA)
and electrostatic precipitator systems demonstrated the
highest efficiencies with respect
to
particulate contaminants,
followed
closely
by
electret
filter systems. Ionizers and ozone
generators were least
effective
in particulate removal. Systems
which included sufficient sorbent material
(i.
e.
activated
carbon or potassium permanganate) were marginally effective
at gaseous contaminant removal. None
of
the systems
tested
were effective at carbon monoxide removal.
Sensory testing
was
conducted
to
discern potential correlation
between human perceptive response and measured air
cleaner pe$ormance (with respect
to
tobacco smoke removd).
An
electret filter (EF) loaded with carbon sorbent received
the best ratings with respect
to
odor
strength, nasal irritation,
eye
irritation, and overall air acceptability.
KEYWORDS
Tobacco smoke, Dust,
Spores,
Pollen, Carbon monox-
ide, Nitrogen dioxide, Formaldehyde, Nicotine, Sen-
sory
testing, Air cleaners, Filters
Manuscript received:
5
July
1993
Accepted
for
publication:
10
March
1994
A preliminary version
of
this
paper was presented at the
Indoor
Air
’93
Conference in Helsinki in Tulv
1993
Center
for
Environmental Research and Technology
Department
of
Biology
Department
of
Sociology
Department of Chemical Engineering
600
South
College Ave, Tulsa,
OK
74104,
USA,
Fax
(918)
631-3268
Introduction
The fundamental air cleaner performance charac-
teristics critical to indoor air quality are:
(1)
particle-
size dependent removal efficiency;
(2)
behavior of
semi-volatile particles after collection;
(3)
secondary
emissions such as ozone and particulates;
(4)
change in performance of collectors with age;
(5)
control of volatile organics at low concentrations;
and
(6)
energy requirements (Ensor et al.,
1989).
This study focuses on the use of commercially
available portable devices designed for cleaning air
in a confined space. The four basic types of portable
air cleaners include: HEPA filter models, models
which use other types
of
filters such as “electric (or
often termed electret) filters”, electrostatic precipi-
tators (EP) and ionizers. HEPA filters are high ef-
ficiency particulate arresting filters made of densely
packed submicron-diameter fibers that are pleated
for extended surface area.
So
called “electric (or
electret) filters” are made of electrically polarized
polyester mesh to trap dust particulates.
In
electro-
static precipitators, particles are charged by passing
over a high voltage
thin
wire followed by collection
on electrically polarized metal plates. Ionizers elec-
trically charge particles which are then attracted to
surfaces at or near ground potential such as walls,
table tops, draperies, occupants, etc.
In recent years there has been
an
increased inter-
est in devices that are generically called ozone-gen-
erating machines. These air cleaners are designed to
emit ozone gas to reduce contaminant levels in the
space. Professional fire restoration companies fre-
quently use ozone generating devices to eliminate
smoke odor that remains in a building after a fire.
Recent concern has mounted due to aggressive
marketing of ozone generators as devices to “purify))
occupied indoor air environments.
A
general over-
view of health effects related to ozone exposure
180
Shaughnessy et
01.:
Effectiveness of Portable Indoor Air Cleaners: Sensory Testing Results
(Lippman, 1989) indicates that both acute and
chronic health affects may be caused by ozone ex-
posure. Ozone devices have in some instances been
shown to emit elevated concentrations of ozone that
would be considered inappropriate for an occupied
space (Shaughnessy and Oatman, 1991). Ozone-
generating devices were included in
this
study.
Some air cleaning devices have incorporated the
use of activated charcoal andor other chemically
impregnated sorbent materials to remove gaseous
contaminants from air. Past research has not pro-
vided a great deal of encouragement of practical
control
of
such contaminants using sorption
methods (Levin, 1988)
Clean
Air
Delivery Rates (CADRs) and air
cleaner efficiencies (based on modified
AHAM
standards
(AHAM,
1989) in a closed environmen-
tal chamber) of portable air cleaning systems for re-
moving dust particulate matter, tobacco smoke par-
ticulates and vapor phase constituents (nicotine and
vinyl pyridine), viable and total fungal spores, pol-
len, and gaseous contaminants
(CO,
NOz,
and
HCHO) were measured. Vinyl pyridine (3-ethenyl-
pyridine) is produced by the pyrolysis of nicotine
during smoking and is present in environmental to-
bacco smoke (ETS) at concentrations high enough
to be measured quantitatively by gas chromatogra-
phy. The 3-ethenylpyridine is a representative
marker compound having ETS as its major source
and behaves similarly to other volatile organic ETS
compounds of interest. The removal of
this
com-
pound by the air cleaners reflected representative
effectiveness with respect to removal of other similar
volatile organic compounds. HEPA systems,
“HEPA-me” systems, electrostatic precipitators,
electret filters, ionizer units, and ozone generators
were included in the study. Two of the filter techno-
logies (HEPA and EP) that proved to be most effec-
tive at removing tobacco smoke particulate in
chamber experiments were further tested by placing
the devices in typical home and office environments
to observe changes in air cleaner efficiency with ex-
tended operation.
Since odors, as perceived by room or building
occupants, play a role in ventilation requirements
(Leaderer and Cain, 1983), sensory testing was
performed for each of the air cleaner types. The
goal
of
sensory testing was to provide a compari-
son of subjective perceptions and objective meas-
urements
of
indoor air quality. Clean air, as as-
sessed by instrumentation, may not be perceived
as clean air by building or room occupants. Odor-
ous or irritating air is often perceived as a threat
to health, regardless of objectively measured air
quality. As Cain (1987) points out, the motivation
to control indoor air quality is likely to derive fi-om
sensory perception.
Description
of
Air Cleaners
Air
cleaners used in the tests (Table 1) were received
fi-om the manufacturers in new condition and main-
tained as such throughout the course of the experi-
ments. The only prior usage of the devices was for
checkout
and
for airflow rate measurements.
With the exception of the
022
air cleaner, all air
cleaner flow rates were measured using both a Shor-
tridge flow hood and a vane anemometer on the air
inlets to the devices. Agreement in the airflow rates
measured by the anemometer and flow hood was
within
15%.
With the exception of the EP1 air
cleaner, the measured
flow
rates were consistently
equal to or lower than the manufacturers’ reported
values. Daisey and Hodgson (1988) and Olander
(1
987) have reported similar discrepancies between
manufacturers’ reported flow rates and measured
rates in the laboratory. Table 1 lists the maximum
flow rates established for control settings used in
chamber experiments along with a basic description
of each air cleaner system.
Methods
Experiments were conducted in a modified Associ-
ation of Home Appliance Manufacturers AC-1
chamber
(AHAM,
1989). The chamber dimensions
are approximately 3.7 m (length)
X2.8
m (wide)
X2.4
m (height); a total volume of
24.8
m3 of free
air space exists in the chamber. The interior sur-
faces consist of type SCX wallboard with seamless
linoleum flooring according to the
AHAM
specifi-
cations. The chamber is equipped with a heat pump
for temperature control and a dehumidifier for hu-
midity. The chamber is flushed by means of a
single-pass ventilation system consisting of a
852
m3/h fan drawing air from the room. Inlet air passes
through a
60%
efficient coarse pre-filter, a carbon
glide-pack, and a HEPA filter in series to pretreat
the air for testing. The supply and return vents can
be shut
off
during the chamber experiments for test-
ing in a static mode (no forced ventilation). A ceil-
Shoughnessy et
01.:
Effectiveness of Portoble Indoor
Air
Cleaners: Sensory Testing Results
18
1
Table
1
A,r
c
eoner descriptions
~~
Code name Airflow rate Basic description
HEPA 1 8.216.1“ Pleated glass “HEPA” filter section rated at
99.9%
for
0.3
p
particles, and 0.64 cm prefilter pad
of non-woven polyester material coated with 114 g of fine activated carbon
Filter bed consisting of a prefilter, a
5.9
kg granulated carbon layer, and a
95%
efficient at
0.3
p
particle size “HEPA-type” filter
Filter bed consisting of a fiberglass prefilter, a 284 g granulated carbon filter, and a
95%
efficient
at
0.3
u
particle size “HEPA-me” filter
HEPA
2
5.1
HEPA
3
5.7
~~
EP 1 9.9 Electrostatic precipitator unit housing a woven polypropylene prefilter, electronic cell with ionizing
section and collector plates, followed by
56
g impregnated carbon filter
Electrostatic precipitator unit housing a prefilter, electronic cell with ionizing section and collector
plates, followed by 142 g granular carbon filter
Electrostatic precipitator unit housing a prefilter, electronic cell with ionizing section and collector
plates, followed by
56
g
granular activated carbon filter
EP2 9.1
EP3 9.3
oz
1‘ 6.4 Ozone generator with prefilter, primary glass media filter, a 0.45 kg granular activated carbon
filter, ion generating pulsating section, and ozone generating section
Ozone generator with prefilter, ion generating pulsating section, and ozone generating section
Four-stage filter bed (prefilter, electret filter, post filter and a
6
oz. Impregnated activated carbon
filter), followed by switchable ionizer section
oz
2‘ 2.1
EFI/IONlb 7.5
ION 2
0
Ion generator with carbon treated “receptor” strip designed to collect particles
EF2 8.5/6.4d Four-stage filter bed (prefilter with 170 g of activated carbon, nonwoven polyester fiber filter,
borosilicate fiber filter, and nonwoven charged polypropylene); optional adsorbent filters may be
inserted (8.2 kg of activated carbon were used in all the gas contaminant tests, except for potassium
permanganate used in formaldehyde tests)
6000
volt static electric charge is established on a screen within filter media
EF3 9.6
a
The HEPA 1 airflow rate decreased to 6.1 m3/min after
800
hours
of
continuous operation
The EFl/ION 1 unit was tested as a combined electretiion (EFl/IONl) system when the unit was fully engaged, as an electret
filter only (EF1) by disengaging the ionizer mechanism, and as an ionizer only (ION 1) by removing the electret filter
Ozone generators were always tested at maximum settings on the device (maximum ozone output)
The EF2 unit airflow rate decreased to 6.4 m3/min when the 8.2 kg of sorbent was added
ing fan was installed to provide mixing in the
chamber to avoid air stratification.
The chamber was flushed with outside air prior
to each experiment. Inlet and exhaust ports were
then sealed for static mode operation during the ex-
periments (heat pump fan remained on low during
all tests). The air exchange rate within the chamber
in a static mode was measured by means
of
carbon
dioxide as a tracer gas according to ASTM Stan-
dard E741 (ASTM, 1983). The maximum
chamber air exchange rate suggested by
AHAM
is
0.05
air changesk (ach). The environmental
chamber used for this project had an exchange rate
of
0.02
ach. The air cleaner was positioned in the
center of the chamber on a table at a base height of
0.91 m fkom the floor for each test conducted.
Air,
for measurement purposes, was drawn through Te-
flon lines located near the center
of
the chamber
and routed to a common mixing manifold for analy-
sis. Between test periods, the chamber was cleaned
with a mild detergent and anti-static spray used on
all wall surfaces to minimize sink effects and emis-
sions from chamber walls.
Each portable indoor air cleaner was tested as to
its effectiveness in removing specific particulate and
gaseous contaminants. Levels of ozone emitted by
the ozone generators were monitored and reported
in a previous paper by Shaughnessy and Oatman
(1991). Analytes and test conditions are summar-
ized in Table
2.
Sensory testing incorporated the use
of
human
response to specific chamber test conditions. Two
sensory panels consisting
of
five persons each were
used. One panel performed their tests during morn-
ings; the other panel tested in the afternoons for
seven test conditions.
182
Shaughnessy et al.: Effectiveness of Portable Indoor Air Cleaners: Sensory
Testing
Results
Table
2
Summary
of
chamber tests
Contaminant Type
test Initial chamber Instrumentation
concentration Sampling period Chamber T("C)/
%
R.H.
Dust Arizona 200 to 400 Particle 20 minutes 25?2"C/
road dust particleslcc measurement
@
1 min 3024%
(0.5-3.0 pm) systems laser intervals
Aerosol
Spectrometer
Model
#Las-X
CRT
Pollen Paper 5 to 15 Allergenco 30 minutes 24? 1"CI
Mulberry particledcc samplair
(1
min on, 3025%
Broussonetia
1 min
off)
Papyrifera
(10-12 pm)
Total
Calvatia
5 to 15 Allergenco 30 minutes 242 l"C1
spores
Cyathiformis
particledcc samplair
(1
min
on,
3025%
mores (4-5 wm)
1
min
off3
Viable
Penicillium
2000
to
3000 Andersen
spores
Chrysogenum
CFU/m3 sampler
mores (2-3 um)
20
minutes 252 l"C1
(1 min on, 2521%
1 min
off)
~ ~ ~ ~~~ ~ ~ ~
25
2
3"Cl
ETS IR4F research 3300-5
700
Particle 20 minutes
(particulate) cigarettes particleslcc measurement
@
1 min 2924%
systems laser intervals
Aerosol
Spectrometer
model
#Las-X
CRT
ETS IR1 research 100 to 150
pg/
Collected on
I00
minutes 2523"Cl
(nicotine cigarettes m3
XAD-4
sample
(8
min sample 3559%
&
vinyl tubes
@
1.25 LPM periods with
pyridine*) for 8 rnin 2 min delay
analyzed by GC between periods)
determination
(Ogden, 1989)
120 minutes 2553"C/
Formaldehyde Formaldehyde 4 PPm CEA instruments
vapor model TGM 555; 2857%
range of 0-5 pprn
HCHO, 20.1 ppm
Nitrogen Nitrogen 4 PPm CEA instruments
120
minutes 25f-1°C/
dioxide dioxide gas model TGM
555;
2652%
range of
0-5
ppm
NOz,
20.1 ppm
Carbon Carbon 9.5 ppm Thermo 180 minutes 2551"Ci
monoxide monoxide environmental 2725%
gas model 48; range
of
1-10 ppm CO,
20.1 ppm
*
Vinyl pyridine analysis completed for only one run of test on selected units
The seven chamber test conditions consisted
of:
6,
Smoke; with
OZl
air cleaner
7.
Smoke; with
EP1
air cleaner
An
equal number of female and male non-smokers
served as sensory panel members. Prior to the pan-
elists entering the testing facility, smoke from four
machine-smoked cigarettes was pumped into the
chamber. Air from the chamber was then circulated
1.
No
smoke; no air cleaner operation
2.
Smoke; no air cleaner
3.
Smoke; with
EFl/IONl
air cleaner
4.
Smoke; with
EF2
air cleaner (loaded with
8.2
kg
5.
Smoke; with
HEPAl
air cleaner
of
activated carbon)
Shaughnessy et
01,:
Effectiveness of Portable Indoor
Alr
Cleaners: Sensory Testing Results
183
through three sniff boxes. The amount of smoke
was constant throughout the tests. Panelists could
not see inside the chamber, as the opening was
covered throughout the sensory testing. Further,
panelists were not aware of the operation of air
cleaners, nor were they given any information about
any of the test conditions. The procedures used for
sensory testing were similar to those used by other
researchers using the sniff box methodology (Cain,
et al., 1981;
Kurtz
and Savoca, 1988).
The panelists opened the
sniff
boxes, sampled the
air in the smoke-filled chamber (Trial 1), then filled
out a questionnaire. Panelists waited for approxi-
mately
20
minutes in another building while the air
cleaners were engaged. Each panelist sampled the
air a second time (Trial
2),
filled out the ques-
tionnaire, and waited another
20
minutes as the air
cleaner continued to run. Panelists then sampled
the air a third time (Trial
3)
and filled out a third
questionnaire for each test condition. Particulate
concentrations were monitored during each test
run. Clean Asr Delivery Rates (CADR) with respect
to the individual air cleaners were found to be com-
parable to CADRs determined
in
initial chamber
tests for ETS particulate removal. Data were also
gathered for
two
control conditions: (1) smoke with
no air cleaner in operation, and
(2)
no smoke with
no air cleaner in operation. A code (number) for
each condition was posted during each testing ses-
sion.
Data
Analysis
The
AHAM
standard develops an effectiveness fac-
tor termed the Clean Air Delivery Rate (CADR)
and is basically a quantitative measure of the air
cleaner performance by the
AHAM
test procedure
(AHAM,
1989).
It
is a measure of the number of
m3/min of air the unit cleans
of
a specific material.
For example, if an air cleaner has a CADR of 10
for dust particles, this corresponds to a reduction in
the dust particle concentration equivalent to adding
10 m3/min
of
clean (ventilation) air each minute.
The CADR is based primarily on effectiveness in
removing smoke and dust; however the same deri-
vation has been applied to gaseous contaminant re-
moval (Daisey and Hodgson, 1988). The rate equa-
tion for decay of a particular contaminant of con-
centration, C, within an enclosed environmental
chamber can be given by: C,=Ci e-kt, where C,=
concentration at time,
t;
Ci=initial concentration at
t=O;
k=decay constant (min-'); t=time (minutes).
By
plotting
In
C,
vs
t
for a specific test and calculat-
ing the slope of the line obtained through a linear
regression analysis, the decay constant,
k,
is deter-
mined. Tests are initially performed to find the
natural decay constant
(h)
for the pollutant of in-
terest. The air cleaner is then introduced into the
chamber and the tests repeated to find an experi-
mental decay constant
(k).
The CADR is then
found by: CADR=V
(k-k)
or ECR=V(k-h)
where V=volume of chamber.
Due to the variance in airflow rates among the air
cleaners, it was important (for purposes of compari-
son) to calculate and report CADR's
and
system
efficiencies of the air cleaners. The system efficiency
is the CADR divided by the actual airflow through
the air cleaner.
All chamber tests (unless otherwise noted) were
run
in triplicate. Standard deviations of the CADRs
were calculated and subsequently used to determine
95% confidence intervals for each reported CADR
and efficiency.
The two groups of panelists
in
the sensory study
were asked to evaluate the air from the sniff boxes
along four dimensions (1) odor strength,
(2)
nasal
irritation,
(3)
eye irritation and
(4)
overall ac-
ceptability of the air. Odor strength, nasal irritation,
and eye irritation were scaled fi-om O=none to 10=
extremely unacceptable. Overall air acceptability
was scaled from 1 =extremely acceptable to lO=ex-
tremely unacceptable. The evaluation forms were
modeled after those used by Cain et al. (1981) and
Kurtz
and Savoca (1988). For each of the seven
conditions, panelists filled out three questionnaires.
The primary statistic used in this analysis was the
t-test, which measures the differences between
mean sensory rating scores. The t-tests indicated
the probability of the mean score differences occur-
ring by chance. Analysis of variance (ANOVA) was
used to test for sensory differences in each of the
three trials, using each of the five air cleaners. Un-
like the t-test, ANOVA allowed simultaneous testing
of multiple sensory dimensions (odor, nasal, eye,
overall).
Results
and
Discussion
Air
Chamber
Testing
Table
3
presents chamber test results for each of the
air cleaners tested. The Clean
Air
Delivery Rates
and efficiencies are reported for chamber tests with
respect to contaminant removal. One air change per
hour (ach) in a residential room of
60
m3 corre-
Table
3
Air cleaner test resultsa
Code name Flow rate Dust Pollen Total spores Viable spores ETS (Particulate)
CADR
%
CADR
%
CADR
%
CADR
%
CADR
%
(m3/min) eff. (m3/min) eff. (m3/min) eff. (m3/min) eff. (m3/min) eff.
(m3/min)
HEPA
1 8.2 6.5720.11 8021 7.0821.42 86217 6.8821.30 84216 6.1 150.99 742 12 6.7920.08 8321
HEPA
2 5.1 3.9920.14 7853 3.8220.79 75216 4.7051.30 92226 3.5450.06 9651
3.9620.34 7827
(w/59
kg
carbon)
HEPA
3 5.7 4.3020.28 7625 4.0820.28 72248 4.0520.45 7228 4.7021.44 83226 4.6120.11 8222
EP
1 9.9 9.14'0.45 9224 9.4021.27 95213 9.3721.30 95213 9.4524.47 95245 8.3220.17 8422
EP
2 9.1 7.7320.23 8523 7.5022.60 83229 6.71
2
1.1 74212 7.3921.56 82217 7.4420.17 8222
EP
3 9.3 5.6320.08 6021 3.6521.75 39219 5.35'0.59 5726 6.0020.37 6424 5.5220.02 59.120.3
oz
1 6.4 1.8720.06 2921 3.1420.74 49212 2.6620.88 42514 2.4321.84 38229 1.3320.06 20.920.9
oz
2 2.1
No
effect
-
No effect
-
No
effect
-
No effect
-
No
effect
-
ION
1 7.5 0.3720.1 1 522 1.8151.25 25217 1.3620.20 1823 1.6120.90 2225 0.3720.01 5.020.2
ION
2
0
No effect
EFIlION
1 6.5 4.0820.28 6324 4.7321.84 73224 5.2921.08 81217 4.7520.59 7329 3.8820.14 6022
EF
1 6.5 3.5450.1 1 5452 5.1251.78 79527 4.8422.89 74244 5.8921.98 90230 1.1020.01 16.920.2
EF
2 8.5 5.3520.06 6321 7.7522.32 91227 5.7220.85 67210 6.2520.93 71
2
11 4.5620.11 5542 1
72 1
EF
3 9.6' 2.5820.17 5223 4.7022.18 49223 5.4621.73 57218 4.7520.51 4925
-
0.2520.06
-
No
effect
-
No effect
-
No
effect
-
0.7150.08
Table
3
(cont.)
Code name Flow rate ETS (nicotine) ETS (vinyl pyridine) Formaldehyde Carbon monoxide Nitrogen dioxide
CADR
%
CADR
%
CADR
%
CADR
%
CADR
%
(m3/min) eff. (m3/min) eff.' (m3/min) eff. (m3/min) eff. (m3/min) eff.
(m3/min)
HEPA
1
HEPA
2
(~159
kg carbon)
HEPA
3
EP
1
El?
2
EP
3
oz
1
oz
2
ION
1
ION
2
EFYION
1
EF
1
EF
2
EF
3
8.2
5.1
5.7
9.9
9.1
9.3
6.4
2.1
7.5
0
6.5
6.5
8. 5/6.4b
9.6
0.1720.08
0.4220.34
No effect
0.2820.17
0.2820.08
No
effect
0.6220.23
0.18d
No
data
No
effect
0.1720.14
No data
0.2520.17
No
effect
22 1
827
-
322
32
1
1024
824
-
-
-
322
423
-
-
0.95
1.45
No
data
0.16
No
data
No
data
0.95
No data
No
data
No data
1.35
No
data
1.57
No
data
11.6
28.4
-
1.6
-
-
14.9
-
-
-
20.7
24.7
No
data
-
No effect
1.5320.40
No effect
No effect
0.03120.003
0.04820.017
No
effect
No effect
No
effect
No effect
No
effect
No effect
1.5620.17
No
effect
-
3028
-
-
0.320.03
0.520.2
-
-
-
-
-
-
2423
-
No effect
No
effect
No effect
No effect
No effect
No
effect
No effect
No effect
No effect
No effect
No
effect
No
effect
No effect
No effect
0.7920.17
2.4921.36
0.51 20.17
No
effect
0.1720.06
0.05720.028
0.4220.14
0.1720.08
No effect
No effect
0.8520.06
0.31 k0.14
2.7220.48
No effect
1022
49527
923
1.920.6
0.620.3
722
854
-
-
-
1321
522
4328
-
~~
a
All CADR reported values represent the average of
3
test runs. The degree
of
uncertainty
(+/-
value) represents the
95%
confidence interval for CAD& and
%
efficiencies
The EF
2
Unit flow rate decreased to
6.4
m3/min when
8.2
kg
of activated carbon were added for the gaseous contaminant tests.
KMn04
sorbent was used in the HCHO tests
The airflow rate on the EF3 unit was
4.9
m3/min for the dust tests only
Average of
two
test runs only
CADR reported values for the vinyl pyridine represent only one test run
A
W
e
%
0
73
4
0'
E
-
3
a
0
0,
v,
(D
3
(n
2
i
Shaughnessy et
01.:
Effectiveness of Portable Indoor Air Cleaners: Sensory Testing Results
185
sponds to an infiltration rate of 1.0 m3/min. Since
1 ach is about the minimum ventilation rate that
may have an effect in assisting to reduce a moderate
contaminant problem, this represents the minimum
CADR
(1.0
m3/min for a 60 m3 room) that an air
purifier should have in order to be effective in re-
moving air contaminants (Whitby et al., 1983).
In
general, air cleaners were most effective at re-
moving particulate phase contaminants and least ef-
fective in the gaseous phase contaminant removal.
The HEPA and electrostatic precipitator units ex-
hibited the highest CADRs and efficiencies on par-
ticulate removal (smoke, dust, pollen and spores),
while ozone and ion systems were least effective.
The effectiveness of the OZ1 unit
in
removing par-
ticulates may possibly be attributed to the filter me-
dium incorporated in the unit as opposed to any
effect of the ozone dissemination. Electret filters
were also somewhat effective at particulate capture
(consistently
>50%
efficiency observed). The pol-
len, spores, and viable spore tests showed the least
repeatability in chamber tests due to the irregular
decay of the contaminants as a hction of the larger
particle diameters.
As
discussed above, filtration of VOC’s and other
vapor constituents pose a difficult challenge as com-
pared to particulates. The air cleaning systems load-
ed with additional carbon sorbent (HEPA 2, EF2
units) proved most effective in removal of the vinyl
pyridine, formaldehyde and nitrogen dioxide. (The
EF2 unit was loaded with
KMn04
for the form-
aldehyde tests.) The ozone generators were margin-
ally effective (ranging from
7
to 15% efficient) with
respect to nicotine, vinyl pyridine, and nitrogen
dioxide contaminants, yet displayed no effect with
respect to formaldehyde removal. Weschler and
Hodgson (1992) have recently raised important
considerations with respect to ozone reactions
within an enclosed environment. Essentially they
have shown that ozone chemistry is complex and
while
O3
may react to reduce some VOC’s, it will
also react with various organics to produce other
VOC’s (aldehydes). The end effect may be an over-
all increase in TVOC within a space. For example,
the chamber tests have indicated some degree of re-
moval of nitrogen dioxide due to the ozone gener-
ators. Ozone/N02 chemistry would suggest that the
NO2 is being converted to NO3 which could ulti-
mately produce nitric acid
(HNO,)
vapor in the air.
These secondary reactions were not studied or
monitored in these limited experiments. None of
the air cleaning systems were effective at carbon
monoxide removal.
This
result was expected based
on the sorbents used. Nicotine’s lack of aflinity for
carbon can be observed in the very low efficiencies
displayed by the air cleaning systems with
this
sor-
bent.
It is important to note that air cleaners with the
highest
CADR
values did not always represent
those with the highest efficiencies. Due to differ-
ences in air cleaner flow rates, high CADR values
may relate to air cleaners with the high efficiencies
and/or high airflow rates (with moderate efficiency,
as shown by the equation: (System efficiency)
X
(Air
cleaner flow rate)=CADR.
To
truly compare air
cleaner technologies, CADRs
and
efficiencies must
be examined.
A specific concern regarding electrostatic precipi-
tators has been the potential emission of ozone due
to the occurrence of electrical arcing. The electro-
static precipitators examined in
this
project emitted
no appreciable levels of ozone.
Less
than 30 ppb
was detected after 1 hour of continuous chamber
operation. Further experiments should be con-
ducted to monitor emissions since the ionizing wires
become fouled with contaminant build-up after ex-
tended operation.
Retesting Experiments
Two air cleaning systems (HEPA 1 and EPl) that
proved most effective in tobacco smoke particulate
removal were placed in operation in typical settings
(residential bedroom for the HEPA and smoking of-
fice room for the EP). The air cleaners were used
intermittently on a daily basis when people were
present, totaling
800
hours of actual operation
spanning over a
six
month period of time. The units
were brought back into the chamber environment
and retested for smoke particulate removal. The air
cleaner flow rate remained constant for the EP sys-
tem; however, the flow rate for the HEPA system
decreased from
8.3
m3/min to 6.1 m3/min as the
HEPA filter had steadily loaded with the extended
operation of the unit.
Noticeable drops in CADR’s were observed in
smoke particulate removal comparing the initial to
“used operation” states. A 25% CADR reduction
(fiom 6.8 to 5.1 m3/min) was measured for the
HEPA 1 system, while a 38% reduction (from 294
to 182) was measured for the EP1 system. The EP
system efficiency suffered a 32% drop between the
initial and retest results. The HEPA system ef-
ficiency did not change due to the lower airflow rate
186
Shaughnessy et al.: Effectiveness
of
Portable Indoor Air Cleaners: Sensory Testing Results
Table
4
Comparisons
of
air cleaners
by
trial
F
value Significance
Trial 1
:
Smoke-filled chamber
Odor strength
Nasal irritation
Eye irritation
Overall acceptability
Odor strength
Nasal irritation
Eye irritation
Overall acceptability
Odor strength
Trial 2: Initial air cleaner period
Trial 3: Second air cleaner period
0.190
0.344
0.457
1.272
3.422
1.288
1.653
3.163
4.471
0.942
0.846
0.767
0.295
0.015*
0.289
0.177
0.022*
0.004*
Nasal irritation 2.608 0.048*
Eye irritation 3.821 0.009*
*
Signifies that the differences between mean scores for the air
cleaners are statistically significant at the conventional 0.05 or
0.01 levels. In Trials 2 and 3, panelists detected significant
differences in the four sensory dimensions they rated. Ex-
ample: the differences in “overall acceptability” ratings in Trial
3 would have happened by chance only 2 times in 1,000 trials,
or; there are 2 chances in 1,000 that these differences are not
genuine
Overall acceptability 5.096 0.002*
processed by the air cleaner as the filter became
coated; however the
CADR
was considerably re-
duced which would relate to a net loss in clean air
output of the unit and a significant decrease in con-
taminant removal with extended use of the unit.
The electrostatic precipitator unit was cleaned ac-
cording to manufacturer’s instructions following the
retest experiment, and then retested again within
the chamber. The
EP1
air cleaner demonstrated a
net system efficiency improvement of
17%
(from
52%
to
69%)
due to the cleaning of the unit. This
reinforces the need for routine maintenance of the
air cleaning systems with continued operation.
Sensory
Testing
The data were first examined to ascertain possible
rating differences between the
two
sensory panels.
No
statistically significant differences were found,
and the data were pooled in order to provide a
larger number
of
cases for analysis. The analysis of
variance results in Table
4
showed that no statisti-
cally significant differences
(.05
or less) were found
for odor strength, nasal or eye irritation, or overall
air acceptability for the smoke-filled chamber trials.
This finding added validity to the sensory data, in
that panelists seemed to be experiencing similar
sensations. The data for Trial
2
and Trial
3
indi-
Table
5
Odor strength/intensity mean scores: trials
1,
2,
and
3.
(Scale range=O (non)
to
10
(extreme),
20
minute trial intervals)
Condition Mean odor scores
Trial 1
1.
No
smoke; no air cleaner operation 0.70
2. Smoke; no air cleaner 5.60
3. Smoke; with EFl/IONl air cleaner 5.30
4. Smoke; with EF2 air cleaner 6.00
(loaded with 8.2 kg of activated
carbon)
5.
Smoke; with HEPAl air cleaner
6. Smoke; with
OZ1
air cleaner
7. Smoke; with EPl air cleaner
5.80
5.60
5.50
Trial 2 Trial 3
0.40 0.40
4.90
5.50
4.80 4.00
3.60b 2.90‘
5.00
4.80
5.40 4.60
6.30
6.50
Table6
Nasal irritation/intensity mean scores: triols
1,
2,
and
3.
(Scale range=O (none) to
10
(extreme],
20
minute trial inter-
vols)
Condition Mean nasal irritation
scores
Trial 1 Trial 2 Trial 3
1.
No smoke; no air cleaner operation
0.10
0.00
0.00
2.
Smoke; no air cleaner 3.30 2.90 3.30
3. Smoke; with EFl/IONl air cleaner 2.60 2.70 1.90
4. Smoke; with EF2 air cleaner 3.60 1.90 1.40”
(loaded with 8.2 kg
of
activated
carbon)
5.
Smoke; with HEPAl air cleaner 3.40 3.00 2.40
6. Smoke; with
OZ
1 air cleaner 3.50 3.20 3.00
7. Smoke; with EP1 air cleaner 3.50 3.80 4.10
a=Difference from Trial 1 significant at
0.05
level
b=Difference from Trial 1 significant at 0.01 level
‘=Difference from Trial
1
significant at 0.001 level
cated that the air cleaners were producing different
sensory ratings among the panelists.
In order to determine which cleaner or cleaners
produced these differences, t-tests were used to test
for sensory differences between each of the three
snif€ing trials for each condition. The data for each
of the four sensory dimensions for the seven con-
ditions are shown in Tables
5
through
8.
The data in Table
5
show that significantly differ-
ent odor strength responses across the three trials
were found for only the
EF2
air cleaner. The
EF2
unit produced significantly lower nasal irritation and
eye irritation evaluations
in
the second cleaning
trial, but not in the first (Tables
6
and
7).
On overall
air acceptability perceptions (Table
8)
,
the
EF2
unit
again produced statistically significant decreases be-
tween Trial
1
and
3.
Another assessment of the relative performance
of the air cleaners involved sensory rating compari-
sons between the control conditions and the five
cleaner conditions across three trials. The control
Shaughnessy et al.: Effectiveness of Portable Indoor Air Cleaners: Sensory Testing Results
187
Table
7
Eye ivritation/intensity mean scores: trials
1,
2,
and
3.
[Scale ronge=O [non)
to
10
[extreme),
20
minute trial intervals)
Table
9
Comparison of smoke, no cleaner control
and
EF2
cleaner an trial
3
Condition Mean eye irritation
scores Sensory dimension Control Dustfree t
mean mean value
Trial 1 Trial 2 Trial
3
1. No smoke; no air cleaner operation
0.00
0.00
0.00
2. Smoke; no air cleaner 2.20 2.10
2.50
3.
Smoke; with EFliIONl air cleaner 2.30 2.00
1.50
4. Smoke; with EF2 air cleaner 2.80 1.70
0.80b
(loaded with 8.2 kg
of
activated
carbon)
5.
Smoke; with HEPAl air cleaner 2.90 2.50 2.10
6. Smoke; with OZ1 air cleaner
3.50
3.40 2.70
7. Smoke; with EP1 air cleaner 2.70 3.40 4.00
Table
8
Overall air acceptability mean scores: :rials
1,
2,
and
3.
(Scole range=
1
(Extremely acceptable) to
10
[extremely un-
acceptable],
20
minute trial intervals)
Condition Mean eye irritation
scores
Trial 1 Trial 2 Trial
3
1. No smoke;
no
air cleaner operation 1.80 1.40 1.30
2. Smoke; no air cleaner 6.10
5.30
6.10
3. Smoke; with EFMONl air cleaner
5.50
5.60 4.40
4. Smoke; with EF2 air cleaner 6.10 4.60 3.70a
(loaded with 8.2 kg
of
activated
carbon)
5.
Smoke; with HEPAl air cleaner 7.20 6.00
5.80a
6. Smoke; with
OZ
1 air cleaner 6.70 6.40 5.60
7. Smoke; with EP1 air cleaner 6.80 6.90 7.10
“=Difference from Trial 1 significant at
0.5
level
b=Difference from Trial 1 significant at 0.1 level
conditions were
(1)
smoke, no cleaner and
(2)
no
smoke, no cleaner.
No
significant differences in sensory perceptions
were found between the first control condition
(smoke, no cleaner) and any of the cleaner con-
ditions for trial one (the smoke-filled chamber) or
trial
two
(initial air cleaner operation period). How-
ever, on the third trial (second air cleaner operation
period), significant differences were found between
the control and
EF2
air cleaner conditions for all
sensory dimensions (Table
9).
The second set of comparisons involved the “no
smoke,
no
cleaner” control condition and the five
air cleaners. The question was whether any of the
air cleaners could produce sensory ratings similar to
those found in the “no smoke, no cleaner” con-
dition. The first trial (smoke-filled chamber) and
trial
2
(initial air cleaner operation period) resulted
in significant differences across all four sensory di-
mensions for each air cleaner.
In
the third trial (sec-
ond air cleaner period), the
EF2
did not differ sig-
Odor strength
5.00
2.90 3.66**
Nasal irritation 3.30 1.40 2.32*
Eye irritation 2.50
0.80
2.23*
Overall air acceDtabilitv 6.10 3.70 3.13**
~~~ ~
*
significant at
0.05
level
**
significant at 0.01 level
nificantly from the control on the eye irritation di-
mension. Thus, the
EF2
electret produced eye
irritation sensory ratings which did not differ signifi-
cantly from those produced by the “no smoke”
chamber.
It is important to note that the
EF2
unit was the
only unit that incorporated a significant amount of
sorbent
(8.2
kg of activated carbon; filter face area=
.80
m2; carbon bed thickness=.025 m; air velocity
@
filter face=.
125
dsec; dwell time=
.20
sec). The
amount of carbon may potentially represent the de-
termining factor in the improved sensory perception
response. It is interesting that the
EF2
unit did not
have the highest
CADR
with respect to smoke par-
ticulate; however, due to the carbon sorbent, many
of the gaseous contaminants would be considerably
reduced. These data would suggest that the gaseous
fraction of environmental tobacco smoke plays a
critical role in sensory perception response.
Conclusions
The following general conclusions regarding the use
of portable air cleaners for removal of specific gas-
eous and particulate contaminants can be drawn
based on chamber tests reported here.
Portable indoor air cleaning systems are not
equally effective in removing indoor pollutants.
The clean air delivery rates and efficiencies
varied by a considerable margin depending on
the system tested.
In general, the
HEPA
and electrostatic precipi-
tator systems exhibited the highest removal ef-
ficiencies with dust, smoke particulate, spores,
pollen and viable spores. The electret filters
closely followed the
HEPA
and
EP
types in re-
moval efficiencies. Ionizers (ion only) and ozon-
ators were least effective.
Air
cleaners with sorbent addition were margin-
188
Shaughnessy et al
-
Effectiveness of Portable Indoor
Air
Cleaners: Sensory Testing Results
ally effective with vapor constituents (excepting
carbon monoxide and nicotine).
Air
cleaner sys-
tem tests represent initial use results. Release of
previously adsorbed vapors as the sorbent loads
with extended operation must be considered in
maintaining the cleaners.
Significant reduction of CADRs (smoke par-
ticulate) were noted for a HEPA and
EP
(smoke
particulate) after extended operation. Routine
maintenance is paramount for continued ef-
ficient operation of the air cleaners.
The results of the sensory data analysis indicated
that the EF2 electret received the best ratings
across the dimensions of odor strength, nasal
ir-
ritation, eye irritation, and overall air ac-
ceptability. These differences were generally
found between Trial
1
(smoke-filled chamber)
and Trial
3
(after second cleaning period). Due
to the EF2 electret unit being the only air
cleaner loaded with activated carbon, these data
would suggest that the gaseous fraction of en-
vironmental tobacco smoke plays a critical role
in sensory perception response.
Acknowledgments
This
work was performed at the University of Tulsa
Center for Environmental Research and Tech-
nology (CERT). Funding for completion of this
project was made possible through the Center for
Indoor
Air
Research, Linthicum, Maryland,
U.S.
Their support was deeply appreciated. We also ac-
knowledge and appreciate the cooperation of the
Association of Home Appliance Manufacturers
(AHAM)
and its member companies in providing
units for testing at no charge. Conclusions of
this
study are solely those of CERT and do not necess-
arily represent the views of nor are they attributable
to
AHAM
or
its
member companies. The authors
also extend their sincere thanks to
Mr.
Kenneth
Rouk for
his
steadfast efforts in chamber test experi-
ments over the course of
two
years.
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... For health-care facilities without adequate natural or mechanical ventilation, the following approaches can be considered [32]. ...
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Although green innovation and technology is not new, so far very limited information is available regarding the diversified approaches for green technologies and engineering. This book highlights the challenges and opportunities, offering a roadmap for using various approaches in the most cost effective way. The book discusses the interrelationship between a circular economy and green technologies. It presents the dimensions of green innovations and illustrates the challenges of industrialization, especially in terms of material synthesis and utilized processes. It covers the current environmental and health challenges of societies and describes the role of stakeholders in developing sustainable societies and industries. This book provides a line of approach to core and interdisciplinary students, academicians, research scientists, and various industry personnel to present their ideas of green innovations with a common vision of sustainable development of community and industries in mind. Features Discusses the interrelationship between a circular economy and green technologies Presents the dimensions of green innovations Illustrates the challenges of industrialization, especially in terms of material synthesis and utilized processes Covers the current environmental and health challenges of societies Offers the identification and role of stakeholders in the sustainable development of societies and industries
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Air pollution due to suspended particles in the built environment threatens people’s health. The collection of submicron particles using air cleaners is especially difficult due to their tiny sizes. Surface patterned structures can enhance submicron particle deposition, thus improving the performance of an air cleaner. In this study, a semi-circular micro-structured surface was designed and optimized by considering particle deposition enhancement and reduction of pressure differential. The maximum efficiency ratio at the pitch-to-height ratio ([Formula: see text]) of 3 reached 1137 for the 1 µm particles but only 3.4 for 0.01 µm particles. The main mechanism of enhancing submicron particle deposition depends on the capture of the recirculation wake induced by semi-circular structures and entrainments of large turbulence kinetic energy to the near-wall regions. Submicron particles was observed to deposit between cavities on the semi-circular surface at a small [Formula: see text]. This study shows that surface patterned structures in the semi-circular shape at small [Formula: see text] can enhance the submicron deposition efficiency, thus providing a possible approach to improve the overall efficiency of air cleaners.
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The coronavirus disease 2019 (COVID-19) pandemic shook the world in ways not seen since the pandemic influenza of 1918–1919. As of late August 2020, over 25 million persons had been infected, and we will see the global death toll exceed one million by the end of 2020. Both are minimum estimates. All segments of society have been drastically affected. Schools worldwide have been forced to close due to illness and absenteeism, transmission and risk to vulnerable members of the school community, and community concerns. The decision to reopen school during a pandemic will have a tremendous impact on children’s safety, growth, and well-being. Not opening invites social isolation and suboptimal educational experiences, especially for youth whose computing assets and online access are limited and those with special needs. The opening has hazards as well, and the mitigation of these risks is the topic of this chapter. Opening schools requires careful considerations of benefits, risks, and precautions. Guiding principles for safety and strategic application of the principles in each educational niche are critical issues to consider during school reopening. The fundamental principles of disease control involve school-directed initiatives (physical distancing and mask use, hand/face and surface cleansing, administrative controls, engineering controls) and individual-level risk reduction approaches to maximize adherence to new guidelines. The school-initiated “top-down” approaches and the individual-level “bottom-up” approaches must be synergized, as no single method will ensure safety. We discuss how to effectively layer strategies in each educational space to increase safety. Since the vulnerability of children has been heightened during this pandemic crisis, we highlight the special considerations for mental health support that should be considered by schools. The safety principles, disease control strategies, and other critical issues discussed here will serve as a starting point for developing a safe, comprehensive, and feasible reopening plan.
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Experiments on occupancy odor addressed the question of why required ventilation rate per occupant increased progressively with increases in the number of persons in a space. In order to investigate ventilation requirements under approximately ideal conditions, we constructed an aluminum-lined environmental chamber with excellent control over environmental conditions and a ventilation system that provided rapid and uniform mixing of air. Psychophysical experiments on occupancy odor explored 47 different combinations of occupancy density, temperature and humidity, and ventilation rate. The experiments collected judgements both from visitors, who smelled air from the chamber only once every few minutes, and from occupants, who remained in the chamber for an hour at a time. The judgements of visitors revealed that occupancy odor increased only gradually over time and rarely reached very high or objectionable levels. Judgements of occupants also revealed rather minor dissatisfaction. Only during combinations of high temperature and humidity did objectionability become more than a minor issue to either group. Experiments on cigarette smoking explored rates of 4, 8, and 16 cigarettes per hour under various environmental conditions and with ventilation rates as high as 68 cfm (34 L.s/sup -1/) per occupant. As soon as occupants lit cigarettes in the chamber, the odor level increased dramatically. At ventilation rates far greater than necessary to control occupancy odor, the odor from cigarette smoking remained quite intense. In general, the odor proved impossible to control adequately even with a ventilation rate of 68 cfm (34 L.s/sup -1/) per occupant (4 occupants) and even when only one occupant smoked at a time. As in the case of occupancy odor, a combination of high temperature and humidity exacerbated the odor problem.
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This paper describes methodologies for the particle-size-dependent performance measurement of air cleaners. It discusses the use of laser-optical particle counters and condensation-nucleus counters with preseparators in evaluating air cleaners. The selection of a challenge aerosol is particularly important; materials include nebulized dissolved solids in water or low-vapor-pressure organics. Another area of increased attention is the generation of ozone by electronic air cleaners. Ozone production rates from a wire-plate-electrode configuration are summarized. Activated carbon adsorbents have been used in commercial air cleaners to remove organic materials. The absorption of low concentrations (below 200 ppb) of materials such as benzene and acetaldehyde obtained in laboratory isotherm experiments is reviewed.
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A gas chromatographic method for determination of vapor phase nicotine in environmental tobacco smoke (ETS) was collaboratively studied by 6 laboratories. Nicotine is desorbed from XAD-4 sample tubes with ethyl acetate containing triethylamine and determined by gas chromatography with nitrogen-selective detection. Each collaborator received blind duplicate samples at each of 6 nicotine concentrations. Three concentrations were generated by spiking XAD-4 tubes with known amounts of nicotine; the remaining 3 concentrations were ETS samples obtained in a carefully controlled environmental chamber containing sidestream and exhaled mainstream smoke from 1R4F Kentucky reference cigarettes. Repeatability and reproducibility relative standard deviations ranged from 4.4 to 11.1% and from 7.0 to 11.1%, respectively, for nicotine concentrations evaluated (up to 6 micrograms/cu m). The method has been adopted official first action.
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A rapid gas chromatographic procedure with an analysis time of 5 min was developed for the determination of environmental nicotine collected on sorbent tubes containing XAD-4 resin. In validating this procedure, severe temporal losses of nicotine were observed for solutions in glass sample vials waiting in a queue in an autosampler tray for analysis. These losses were traced to adsorptive interactions of nicotine with the glass surface of the vials. The use of N-ethylnornicotine as the internal standard or the addition of triethylamine to all solutions were both successful in producing constant response ratios of nicotine to internal standard. Owing to the limited availability and expense of N-ethylnornicotine, our current procedure calls for the addition of triethylamine to all nicotine solutions at the 0.01% V/V level and the use of quinoline as internal standard.
Test Method of Determining Air Leakage Rate by Tracer Dilution, ASTM Standard E741, American Society for Testing Indoor air as a source of annoyance
  • Developed
  • Iit
  • Institute
  • Il Chicago
  • Astm And Materials
  • Pa Philadelphia
  • W S Cain
Developed by IIT Research Institute, Chicago, IL. ASTM (1983) Test Method of Determining Air Leakage Rate by Tracer Dilution, ASTM Standard E741, American Society for Testing and Materials, Philadelphia, PA. Cain, W.S. (1987) " Indoor air as a source of annoyance ". In: Environmental Annoyance: Characterization, Measurement, and Control, New York, Elsevier Science Publishers, pp. 189-200.
Air Cleaner Efficiencies for Re-moval of Nitrogen Dioxide and Wata& eganic Compounds. Pre-sented at the Annual APCA Meeting
  • J Daisey
  • A Hodgson
Daisey, J. and Hodgson, A. (1 988) Air Cleaner Efficiencies for Re-moval of Nitrogen Dioxide and Wata& eganic Compounds. Pre-sented at the Annual APCA Meeting, Dallas, TX, June 19-24.