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47. An Assessment of Airborne Infectious Isolation Rooms in Minnesota Hospitals

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A performance assessment of airborne
infection isolation rooms
Stefan A. Saravia, MPH,
a
Peter C. Raynor, PhD, MSEE,
a
and Andrew J. Streifel, MPH
b
Minneapolis, Minnesota
Background: Airborne infection isolation rooms (AIIRs) help prevent the spread of infectious agents in hospitals. The performance
of 678 AIIRs was evaluated and compared with construction design guidelines.
Methods: The pressure differentials (DP) between the isolation rooms and adjacent areas were measured, and ventilation and con-
struction details were recorded for each room. Ultrafine particle concentrations were evaluated in the rooms, surrounding areas,
and ventilation systems serving the rooms. Measurements were analyzed as a function of room parameters.
Results: Only 32% of the isolation rooms achieved the recommended DPof22.5 Pascals (Pa) relative to surrounding areas. AIIRs
with solid ceilings had an average DPof24.4 Pa, which was significantly higher than the average DPof22.0 Pa for rooms with
dropped ceilings (P5.0002). Isolation room ultrafine particle concentrations were more highly correlated with particle levels in
surrounding areas (R
2
50.817) than in the ventilation systems serving the rooms (R
2
50.441). Almost all ventilation filters serving
AIIRs collected fewer particles than anticipated.
Conclusion: The results indicate that hospitals are not all maintaining AIIRs to correspond with current guidelines. The findings
also support the contention that having tightly sealed rooms helps maintain appropriate pressure differentials. (Am J Infect Control
2007;35:324-31.)
To limit airborne transmission of infectious agents in
health care facilities, heating, ventilating, and air-condi-
tioning (HVAC) systems are used to establish airborne
infection isolation rooms (AIIRs). Properly functioning
AIIRs require consistent negative-pressure differentials
relative to the surrounding areas and sufficient air
changes per hour.
HVAC systems provide fresh, conditioned, and fil-
tered air to a building through supply ducts. Air-han-
dling units move air through the system. Filters in an
air-handling unit are a main line of defense against
the spread of infectious disease in hospitals. Filter
manufacturers rate their products according to the
type of test conducted to determine the efficiency of
the filters. The American Society of Heating, Refrigerat-
ing, and Air-Conditioning Engineers (ASHRAE) dust
spot efficiency rating measures a filter’s ability to cap-
ture atmospheric dust particles.
1
The rating is given as
the percentage of particles collected. Generally, stan-
dard prefilters are rated between 20% and 40%,
whereas final filters may be rated above 80%. High-
efficiency particulate air (HEPA) filters capture at least
99.97% of 0.3-mm diameter particles.
In addition to the supply ductwork, a system of re-
turn or exhaust ductwork is also found within a build-
ing. At many facilities, air exhausted from AIIRs is
expelled directly outdoors. In others, exhausted air is
passed through HEPA filters before it is returned to
the supply system to limit the spread of infectious
agents within the facility. AIIRs operate by having the
return ducts remove air from the rooms at higher rates
than the supply ducts add air. To balance the flows, air
enters from outside an AIIR through cracks under doors
or other points of entry. This creates a slight negative
pressure within the AIIR relative to the surrounding
areas.
Streifel and Marshall
2
indicated that the most impor-
tant parameters for an AIIR are room pressure, room
ventilation rate, filtration, and directed airflow. Streifel
et al
3
concluded that self-closing doors and perma-
nently sealed windows are critical for maintaining
adequate pressure differential. The American Institute
of Architects (AIA),
4
which publishes design guidelines
considered to be the standard of care for new isolation
rooms, stressed these parameters and others consid-
ered essential for construction or renovation of AIIRs.
The Centers for Disease Control and Prevention
(CDC)
5
also emphasized ventilation controls and rec-
ommended infection control procedures that should
be implemented when patients require these rooms.
A few researchers have investigated the perfor-
mance of AIIRs. Using smoke sticks, Fraser et al
6
found
that 45% of 115 negative-pressure isolation rooms
tested were actually positively pressured relative to
surrounding areas. When evaluating tuberculosis
From the Division of Environmental Health Sciences, School of Public
Health,
a
and the Department of Environmental Health and Safety,
b
University of Minnesota, Minneapolis, MN.
Address correspondence to Peter C. Raynor, PhD, MSEE, Division of
Environmental Health Sciences, University of Minnesota, Mayo
MC 807, 420 Delaware St. SE, Minneapolis, MN 55455. E-mail:
praynor@umn.edu.
0196-6553/$32.00
Copyright ª2007 by the Association for Professionals in Infection
Control & Epidemiology, Inc.
doi:10.1016/j.ajic.2006.10.012
324
isolation rooms, Sutton et al
7
determined that 28% of
25 rooms evaluated were positively pressured. Similar
qualitative analyses by Pavelchak et al
8
indicated that
38% of the 140 AIIRs they evaluated exhibited outward
flow from under the door. One deficiency that contrib-
uted to problems in these rooms was that continuous
airflow monitoring equipment did not function prop-
erly. Using a micromanometer, Rice et al
9
measured
pressure differentials for 4 AIIRs for 2- to 3-month
periods in both summer and winter. Although the
mean pressure difference was 20.3 Pa, measurements
ranged from 21.3 Pa to 10.7 Pa. All of these studies
indicated that improperly functioning AIIRs are
common.
With increasing concerns about bioterrorism and
emerging infectious diseases, more attention has
been given to the proper functioning of AIIRs in prep-
aration for disease outbreaks. The purpose of this study
was to evaluate the operation of AIIRs in hospitals in a
post-9/11 environment and in comparison with the
most recent recommendations issued by the AIA and
the CDC. To fulfill this purpose, the study had 4 specific
aims: (1) establish benchmarks to evaluate AIIR perfor-
mance; (2) develop a questionnaire to provide prelimi-
nary information about participating hospitals, their
HVAC systems, and their AIIRs; (3) visit each participat-
ing hospital to make measurements and record obser-
vations; and (4) analyze data from the questionnaires
and site visits.
METHODS
The first 3 aims were data collection steps. Methods
for accomplishing these aims will be discussed first.
Approaches for analyzing the data, the fourth aim,
will be discussed afterward.
Data collection
Benchmarks for AIIR performance were based on
what the authors viewed as the most critical para-
meters in AIA and CDC recommendations.
4,5
These 6
‘‘essential’’ parameters are shown in Table 1.
The next task was to develop an on-line question-
naire for hospitals to complete before site visits. The
purpose of this questionnaire was to acquire important
information regarding a hospital and its AIIRs before a
site visit to help plan the visit and decrease the time
needed for the visit. Because the questionnaire was
used for broader purposes than the study discussed
here, it contained sections not only about AIIRs in
emergency departments and other patient care areas
but also sections on portable HEPA filter machines, por-
table anterooms, mobile isolation transport systems,
surge capacity areas, and emergency department triage
and holding areas.
Performance relative to parameters 2 and 4 in Table
1was self-reported by the hospitals on the survey. Air
changes per hour (ACH) in the AIIRs were self-reported
by the hospitals rather than being measured during the
site visit because of time considerations. In some cases,
the ACH were measured by facility staff. In others, the
reported values were design specifications. However,
many facilities had no information about the air-
change rates in their AIIRs.
Upon completion of the surveys, site visits were con-
ducted. The AIIRs and air-handling units serving them
were inspected visually and measurements were taken
by the researchers to assess parameters 1, 3, 5, and 6
in Ta b l e 1 and to confirm responses for parameter 4.
Other nonessential parameters, discussed below, were
measured as well.
The pressure differential was measured using a
DG-700 Pressure and Flow Gauge (The Energy Conser-
vatory, Minneapolis, MN) sensitive to 0.1 Pa. A 6-inch
metal probe was attached to the unit by a 12-inch
length of tubing. The probe was inserted through the
undercut of the closed door leading to the area that
was to be measured. After the instrument came to equi-
librium, a 5-second average of the pressure differential
was recorded. The pressure differentials for all of the
doors leading to the isolation room were measured, in-
cluding the door from the corridor to the anteroom,
when present, and the door from the anteroom to the
isolation room. If the AIIR had doors to both an ante-
room and a hallway, the pressure differential to the
hallway was recorded. When access to an AIIR was
strictly through an anteroom, the pressure differential
for the AIIR was recorded as the greater of the differen-
tial from the hallway to the anteroom or the differential
from the anteroom to the isolation room.
In addition to measuring the pressure differential of
the isolation room, particle number concentrations
were measured inside the isolation rooms using a
P-Trak Ultrafine Particle Counter (TSI Inc., Shoreview,
MN), which counts particles with diameters ranging
Table 1. Critical parameters for benchmarking AIIR
performance.
All AIIRs should .. .
1. Have a negative pressure differential between the isolation room and the
surrounding areas of at least 2.5 Pa
4,5
2. Have at least 12 air changes per hour
4,5
3. Have self-closing doors leading into the isolation rooms
4,5
4. Have a permanently installed pressure monitor
4
5. Not have a system installed allowing the room to switch from
negative to positive pressure or function as both an isolation
room and a protective environment room
4,5
6. Have ASHRAE dust spot tested filters of at least 90% efficiency
installed in the supply air unit that serves the AIIR
4,5
Saravia, Raynor, and Streifel June 2007 325
from approximately 0.02 to 1 mm. This size range in-
cludes individual viruses and smaller bacteria. Particles
were measured in occupied isolation rooms by in-
serting a telescoping wand that adjusts from 18 inches
to 3 feet through the undercut of the door. The real-
time particle concentrations were observed until the
displayed concentration was steady, after which a 10-
second average measurement was taken. For AIIRs
that were not occupied, the room was entered, the par-
ticle counter was allowed to come to equilibrium, and
a measurement was collected. Particles were also mea-
sured in the corridor outside of the isolation room.
Room parameters relevant to the AIIRs were
recorded. The ceiling type of the isolation room, ante-
room, and bathroom were observed and recorded as ei-
ther dropped (acoustic laid in) or solid (hard or plaster).
The type of ventilation serving the isolation room,
anteroom, and bathroom was also recorded, eg, supply
only, supply and exhaust, exhaust only, or none.
Whether the door leading to the isolation room was
self-closing or not was marked. Finally, whether the
room was used as a protective environment for
immunocompromised patients as well as an AIIR was
recorded.
Air-handling units that served 1 or some of the
patient care areas in the hospitals were inspected. The
facilities engineer was asked for the percentage of air
being recirculated in the system versus fresh air, the
ASHRAE-rated filtration efficiency of the installed fil-
ters, and the areas served by the air-handling unit. The
efficiency of the filters in the air-handling units was
measured using the P-Trak. For air-handling units with
predrilled service ports, the probe of the particle
counter was inserted into the duct and allowed to
come to equilibrium. A 10-second average concentra-
tion was then measured. For units that did not have pre-
drilled ports, measurements were collected from the
downstream side of the filter bank with the access
door slightly ajar to allow the probe into the unit. Air
downstream from the filter flowed out of the door
with sufficient velocity to prevent any contaminant air
from the surrounding area to enter and skew the results.
Measurements were not collected from the mixing
chambers upstream from the filter bank if access ports
were not available because of the drawing of air from
outside of the system. For sufficiently large ventilation
systems, the unit was physically entered and the door
closed. Ten-second average measurements were then
measured after the P-Trak had come to equilibrium.
Data analysis
The data set included measurements and observa-
tions for 678 AIIRs. Only partial data were available
for many of the rooms.
Summary statistics were calculated for the factors
evaluated. For pressure differential and ACH, the
percentages of rooms meeting the recommendations,
average values, and standard deviations were deter-
mined. For self-closing doors, permanently installed
pressure monitors, rooms that switched from negative
to positive pressure, and air-handling units having ASH-
RAE-tested filters of at least 90% dust spot efficiency
installed, the percentages of rooms meeting the recom-
mendations were determined.
The efficiencies of the filters installed in the air-
handling systems measured using the P-Trak were
calculated using the equation
hfilter 51003cpre2cpost
cpre ð1Þ
in which h
filter
is the filter efficiency, c
pre
is the parti-
cle concentration upstream from the filters, and c
post
is the particle concentration downstream from the
filters.
An initial data set including the first 55 AIIRs visited
was evaluated statistically to determine whether any
correlations existed between pressure differential or
isolation room particle concentrations and various
room parameters. Based on these results, the null hy-
potheses shown in Table 2 were developed and tested
with data from as many AIIRs as possible from the
full data set.
Hypothesis 1 was tested using both a 2-sample ttest
assuming unequal variances and the Mann-Whitney U
nonparametric test. For hypotheses 2 to 5, relationships
between variables were compared using linear regres-
sion. Pvalues for slope and intercept were calculated,
and correlation coefficients (R
2
) were determined. Parti-
cle concentration data used to test hypotheses 3 and 4
were converted to the logarithm of particle concentra-
tion because the readings appeared to be distributed
lognormally. For the single data point with zero parti-
cles, which would be undefined when its logarithm
was taken, a particle concentration of 0.5 particle/cm
3
was utilized for conversion to a logarithmic value.
Most air-handling units served more than 1 isolation
room. Therefore, to test hypothesis 3, the logarithm of
particle concentration downstream from an air-han-
dling unit was compared with the logarithm of the aver-
age of the particle concentrations in the rooms served
by that air handler. For hypotheses 6 to 9, x
2
analyses
were used to determine whether meeting the guidelines
for various room variables had a significant impact on
meeting the recommended pressure differential of
22.5 Pa.
The numbers of rooms for which data were available
for each test are listed in Table 2. These numbers are
lower than the 678 rooms in the database because of
326 Vol. 35 No. 5 Saravia, Raynor, and Streifel
incomplete self-reporting of data by the hospitals and,
in just a few cases, errors in measurements. The num-
ber of comparisons for hypothesis 3 is small because of
the aforementioned averaging of particle concentra-
tions for AIIRs served by a single air-handling unit.
RESULTS
The data collected from all hospitals were compared
against the current design guidelines
4,5
to determine
the percentages of isolation rooms operating according
to current standards. These results are presented in
Table 3. Only 32% of the rooms assessed were found
to have the recommended pressure differential of
22.5 Pa. In addition, 58 rooms, approximately 9% of
those evaluated, were positively pressurized.
Isolation rooms with solid ceilings had an average
pressure differential of 24.4 Pa, which was signifi-
cantly higher than the differential of 22.0 Pa measured
for rooms with drop ceilings according to the 2-sample
ttest with unequal variances (P5.0002). The differ-
ence in the distribution of pressure differentials was
also significant by the Mann-Whitney Utest (P5
.0002). The pressure differentials for isolation rooms
with solid ceilings were found to have a significantly
higher variance, 56.3 Pa
2
, than those for rooms with
drop ceilings, 12.2 Pa
2
(P,.0001).
A regression was performed to determine whether
ACH was a significant predictor of pressure differential
(DP). When this relationship was evaluated for 366
rooms that had data for both DP and ACH, a significant
relationship was found (P5.021). However, the rela-
tionship with air-change rate explained only a small
portion of the variance in DP for the complete data set
(R
2
50.015). Figure 1 shows the data for pressure differ-
ential plotted against air-change rate. The regression
line for the data is shown together with a dark shaded
region representing the 95% confidence band for the
prediction of the line. The lighter shaded region repre-
sents the 95% prediction band for estimating DP for a
single AIIR if information about its ACH already exists.
We conducted x
2
tests to determine whether room
and ventilation parameters could affect the likelihood
that AIIRs achieve the recommended pressure differen-
tial of 22.5 Pa. The associations of 2 parameters with
pressure differential were significant statistically. Rooms
with at least the recommended 12 ACH achieved the rec-
ommended pressure differential 37.6% of the time,
whereas rooms with less than 12 ACH achieved the rec-
ommended pressure only 22.8% of the time (x
2
59.56,
P5.0020). In addition, 43.3% of AIIRs with solid ceilings
achieved the recommended pressure differential com-
pared with only 28.5% of rooms with drop ceilings
(x
2
511.86, P5.0006). Significant associations with
meeting the recommended pressure differential were
not found for having permanently sealed windows
(x
2
52.15, P5.14) or for the presence of an anteroom
(x
2
50.05, P5.83).
Airflow relationships were studied further by inves-
tigating ultrafine particle concentrations. The relation-
ship between logarithms of particle concentrations in
the isolation rooms and logarithms of particle concen-
trations directly after the final filters in air-handling
units supplying the rooms was analyzed. The relation-
ship, shown in Fig 2 for a total of 107 pairs of data, was
significant (P,.0001) with an R
2
of 0.441. The Figure
shows the data, a 1:1 line, and a regression line for the
data with 95% confidence bands. The AIIR particle
concentrations were generally higher than the postfil-
ter concentrations, especially when the postfilter
counts were low. The regression line was significantly
different from the 1:1 line.
The relationship between logarithms of particle
concentrations in hallways or anterooms adjacent to
the door of the AIIRs was also compared with the log-
arithms of particle concentrations in the AIIRs. Results
are presented in Fig 3 for the 569 pairs of data. The re-
lationship was significant (P,.0001) with R
2
50.817.
The 1:1 line falls within the narrow 95% confidence
bands for the regression line.
Although 93% of the air-handling systems assessed
had the proper ASHRAE-rated final filters installed, few
of these filters performed at the rated filtration effi-
ciency. The filter efficiency calculated according to
Table 2. Null hypotheses tested statistically and number
of observations used in each analysis
1 The type of ceiling does not influence the negative
pressure differential of the isolation room.
n5634
2 Air changes per hour do not influence the negative-
pressure differential of the isolation room.
n5366
3 Isolation room particle concentrations are not
influenced by particle concentrations in the
air that is supplied to the room.
n5107
4 Isolation room particle concentrations are not
influenced by particle concentrations in air from
surrounding areas, eg, anterooms or corridors.
n5569
5 Filtration efficiency measured with the P-Trak
corresponds to the ASHRAE-rated
filter efficiency.
n5112
6 Having the recommended ACH does not have
a significant influence on achieving the
recommended pressure differential.
n5366
7 Having an anteroom does not have a significant
influence on achieving the recommended
pressure differential.
n5666
8 Having permanently sealed windows does not have
a significant influence on achieving the
recommended pressure differential.
n5582
9 Having solid ceilings does not have a significant
influence on achieving the recommended
pressure differential.
n5634
Saravia, Raynor, and Streifel June 2007 327
equation 1was less than the rated efficiency for 107 of
112 filter banks tested. A geometric average of the
change in particle penetration indicated that filters
allowed, on average, 437% more particles to penetrate
than ratings would have indicated. This value was sig-
nificant statistically (P,.0001). Figure 4 shows the
relationship between measured and rated efficiencies
of the filters.
The trend indicating that installed filters are not
operating as specified was particularly noticeable for
the HEPA filters that were tested. Of 9 filter banks
with HEPA filtration, none reached the 99.97%
efficiency required for HEPA status, although 1 was
close. In fact, only 4 of the 9 filter banks achieved
99% efficiency.
DISCUSSION
Results in Ta b l e 3 indicate that not all hospitals are
operating AIIRs that meet today’s standards. If it is ac-
cepted that the current standards reflect essential
Fig 1. Pressure differential versus air changes per
hour. The thick line represents the regression
between the 2 variables. The dark shaded region is
the 95% confidence band for the mean value of
pressure differential at each air-change rate. The
light shaded region is the 95% prediction band for an
individual room’s pressure differential if the
air-change rate for the room is known.
Fig 2. Open triangles represent particle
concentrations in the AIIRs versus particle
concentrations immediately downstream from the
filters in the air-handling units serving the AIIRs. The
thick line represents the regression between the 2
variables. The shaded region is the 95% confidence
band for the regression line. The 1:1 line is also
displayed.
Table 3. Performance of AIIRs agent functional criteria
Functional criteria
Percentage of rooms meeting the functional criteria
(n 5number of rooms evaluated for a criterion)
Pressure differential between isolation room and surrounding areas greater
(more negative) than 2.5 Pa
32% (n 5672)
At least 12 air changes per hour 51% (n 5370)
Permanently installed pressure monitor 76% (n 5566)
Ventilation system does not allow room to be used for infectious isolation and
protective isolation
90% (n 5560)
Self-closing doors are installed 36% (n 5621)
Final filters are rated at $90% efficient 93% (n 5403)
328 Vol. 35 No. 5 Saravia, Raynor, and Streifel
criteria, facility managers should assure the functional-
ity of their AIIRs. Hospitals should strive to achieve the
ventilation criteria recommended by the AIA and the
CDC, an effort that is sometimes difficult because of
the expense of renovations versus the expected risk re-
lated to the current condition of their AIIRs. However, a
response is too late once an infectious patient enters a
health care facility. Best practice includes preparation
and response development appropriate to the risk. Al-
though risks are uncertain for many infectious agents,
risk management principles are already used to pre-
pare protocols for more common agents, such as Myco-
bacterium tuberculosis, to assure that infectious disease
management components are ready at any time.
Although the finding that 9% of rooms were posi-
tively pressurized is disturbing, this value is an im-
provement over the percentages of rooms measured
as positively pressured in earlier investigations.
6-8
One possible explanation for this reduction in the per-
centage of positively pressured rooms is that hospitals
are learning over time how to make their AIIRs func-
tion more effectively. Other potential explanations
include the better pressure measurement techniques
used in this study and the capabilities and resources
of the population of hospitals evaluated in this study.
Rooms with laid-in ceilings are more likely to have
air leakage because of wire chases and pipe runs.
This may be the reason that AIIRs with solid ceilings
have a higher pressure differential on average. Tighter
construction in AIIRs may help hospitals attain the
desired pressure differential.
The poor correlation between DP and ACH suggests
that, despite the statistically significant relationship,
air-change rate was not a good predictor for pressure
differential. In Fig 1, the narrowness of the confidence
band indicates that the mean value of DP at each ACH is
well understood for the AIIRs tested. However, like the
poor correlation coefficient, the width of the prediction
band indicates that air-change rate is almost useless for
predicting the pressure differential for an individual
Fig 3. Open triangles represent particle
concentrations in the AIIRs versus particle
concentrations in the areas adjacent to the AIIRs.
The thick line represents the regression between
the 2 variables. The shaded region is the 95%
confidence band for the regression line. The 1:1 line
is also displayed. Fig 4. Triangles represent measured filter bank
efficiency in air-handling units serving AIIRs versus
dust spot efficiency reported by manufacturers. The
dashed line is a 1:1 relationship between measured
efficiency and manufacturer-reported efficiency.
Saravia, Raynor, and Streifel June 2007 329
room. These results indicate that simply increasing
exhaust flow from an AIIR, which increases ACH, in
an attempt to increase pressure differential may be
ineffective.
Although having a high air-change rate in an AIIR
does not by itself mean that the room will adequately
protect health facility staff, patients, and visitors, high
ventilation flows may have other important benefits.
Facilities that exceed the recommended 12 ACH should
be able to achieve the recommended negative-pressure
differential of 22.5 Pa by checking for leakage and seal-
ing areas in the room while adjusting the ventilation bal-
ance, if necessary. Having more airflow to work with
should allow a ventilation system balancer to achieve
the recommended negative DP more easily. In addition,
adequate airflows are important for minimizing air
migration out of AIIRs when their doors are opened.
10
Although results from the x
2
tests show that ante-
rooms may not significantly influence the steady-state
pressure differential of AIIRs, their importance in limit-
ing the migration of air from the AIIR to the corridor
when doors are opened has been documented.
3
Figures 2 and 3 indicate that AIIR particle concentra-
tions are more closely related to particle concentra-
tions in surrounding areas than to concentrations in
the air supplied to the rooms by the HVAC system.
Experimental factors may have influenced this result.
Some unoccupied isolation rooms were entered, and
the measured particle counts may reflect the air that
was drawn into the room when the door was opened.
For those rooms that were not entered, the probe was
inserted through the undercut of the door. The particles
sampled in this way may reflect concentrations influ-
enced more directly by air penetrating under the door
than concentrations in the rest of the room.
The apparent influence of surrounding area particle
levels on the isolation room concentrations is impor-
tant from an infection control standpoint. The closer
correlation between AIIR and surrounding area particle
concentrations than between AIIR and postfilter con-
centrations suggests that more of the air entering the
AIIR is coming from the surrounding areas than from
the supply air. This, in turn, suggests that typical AIIRs
have many leaks where air can enter the room. Such
leaks make the attainment of a satisfactory pressure
differential difficult, limit the maximum pressure dif-
ferential that can be attained, and require the use of a
greater exhaust flow than would otherwise be neces-
sary. The result also suggests that the areas surround-
ing AIIRs should be supplied with air that has passed
through 90% efficient filters.
The filter efficiencies measured in the air-handling
units are typically lower than the efficiencies indicated
by manufacturers. Some of the lower measured effi-
ciency may be linked to the utilization of the ultrafine
particle counter to evaluate efficiency in this study ver-
sus methodology for the ASHRAE dust spot efficiency
rating. Although the diameters of particles measured
during the dust spot procedure and using the
P-Trak overlap, the particle sizes assessed using the
P-Trak are smaller on average. Whether this particle
size difference has a positive, a negative, or no bias
on the relationship of the 2 efficiency measures is
uncertain. However, the anticipated extent of any mea-
surement bias cannot account fully for the differences
observed in Fig 4.
Much of the reduction in filtration efficiency of the
tested filters is likely due to predictable changes in
the efficiency of HVAC filters. Raynor and Chae
11
found
that, over a 19-week period, filters made from synthetic
fibers that carry electrostatic charges exhibited sub-
stantial efficiency losses as they became loaded with
particles in an operating HVAC system. The findings
could also mean that filters are not being installed
properly, leaving gaps between filter housing and filter
racks through which particles can penetrate. As hospi-
tals install higher efficiency filters, this becomes even
more important because of the greater pressure drop
across the higher efficiency filters. Increased pressure
drop will cause air to bypass the filters via gaps be-
tween filters and degraded filter seals more easily
than with less efficient filters with lower pressure drop.
For several points in Fig 4, the measured efficiency
was found to be less than zero, indicating the possibil-
ity of particle resuspension or generation downstream
from filter banks. This could be related to unclean duct-
work or overly dusty filters.
The finding that even HEPA filters are not reducing
particle levels as much as anticipated may have impor-
tant consequences for those hospitals that use HEPA fil-
tration before recirculating potentially contaminated
air exhausted from AIIRs to hospital environments. If
this air is not being filtered properly, opportunities
for airborne transmission are more likely. In addition,
HEPA filters are counted on in other parts of hospitals
to reduce levels of infectious agents. For example,
HEPA filters are used in operating suites to protect
patients from airborne agents. A significant health con-
cern may be posed if users assume these filters collect
all particles with at least 99.97% efficiency. The instal-
lation of HEPA filters in operating suites must be man-
aged to ensure an aseptic environment for the patient.
CONCLUSION
Not all hospitals are prepared to meet current air-
borne infectious disease standards. This is not neces-
sarily surprising given the fact that there are no rules
or incentives in place for hospitals to make these
expensive renovations. The organizations responsible
330 Vol. 35 No. 5 Saravia, Raynor, and Streifel
for accrediting health care facilities should consider
inspecting some of the important ventilation parame-
ters involved with AIIRs.
The majority of AIIRs tested in this study did not
achieve the negative-pressure differential of 2.5 Pa rec-
ommended by the AIA. In addition, results indicated
that AIIRs with drop ceilings are less likely to achieve
the recommended pressure differential than those
with solid ceilings. The ‘‘tightness’’ of the room should
be considered when constructing or renovating AIIRs.
It is a concern that hospital ventilation systems are
often not filtering particles at the expected efficiency
for the installed filters. Means and methods for facilities
and infection control staff should be developed to
ensure that the air filters are properly installed in their
HVAC systems and that they are changed at regular
intervals.
The information obtained from this study is valuable
in developing a risk mitigation plan in case of emerging
or pandemic infectious diseases. The federal Bioterror-
ism Hospital Preparedness Program provides incen-
tives for AIIR preparedness because problems are
difficult to fix in the middle of an incident.
The authors thank Jeanne Anderson, Kathleen Harriman, Franci Livingston, Fernando
Nacionales, and James Loveland of the Minnesota Department of Health for their
assistance with this study.
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The efficiency and pressure drop of filters made from polyolefin fibers carrying electrical charges were compared with efficiency and pressure drop for filters made from uncharged glass fibers to determine if the efficiency of the charged filters changed with use. Thirty glass fiber filters and 30 polyolefin fiber filters were placed in different, but nearly identical, air-handling units that supplied outside air to a large building. Using two kinds of real-time aerosol counting and sizing instruments, the efficiency of both sets of filters was measured repeatedly for more than 19 weeks while the air-handling units operated almost continuously. Pressure drop was recorded by the ventilation system's computer control. Measurements showed that the efficiency of the glass fiber filters remained almost constant with time. However, the charged polyolefin fiber filters exhibited large efficiency reductions with time before the efficiency began to increase again toward the end of the test. For particles 0.6 microm in diameter, the efficiency of the polyolefin fiber filters declined from 85% to 45% after 11 weeks before recovering to 65% at the end of the test. The pressure drops of the glass fiber filters increased by about 0.40 in. H2O, whereas the pressure drop of the polyolefin fiber filters increased by only 0.28 in. H2O. The results indicate that dust loading reduces the effectiveness of electrical charges on filter fibers.
1: gravimetric and dust-spot procedures for testing air-cleaning devices used in general ventilation for removing particulate matter. Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers
  • Ashrae Standard
ASHRAE. ASHRAE Standard 52.1: gravimetric and dust-spot procedures for testing air-cleaning devices used in general ventilation for removing particulate matter. Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers; 1992. p. 32.
Design, construction and operation of healthy buildings, solutions to global and regional concerns
  • A J Streifel
  • J W Marshall
Streifel AJ, Marshall JW. Parameters for ventilation controlled environments in hospitals. In: Moschandreas DJ, editor. Design, construction and operation of healthy buildings, solutions to global and regional concerns. Atlanta: American Society of Heating, Refrigeration, and Air-Conditioning Engineers Press; 1998. p. 305-9.
Guidelines for design and construction of health care facilities. Washington: The American Institute of Architects
  • Facility Guidelines Institute
Facility Guidelines Institute. Guidelines for design and construction of health care facilities. Washington: The American Institute of Architects; 2006. p. 294.