ASHRAE D RP397 198507 Airbone Bacteria in Schools
Transcript of ASHRAE D RP397 198507 Airbone Bacteria in Schools
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FINAL RETORT - RP-397
THE.
EFFECT OP VENTILATION AND RELATIVE HUMIDITY
UPON AIRBORNE BACTERIA UJ SCHOOLS
G.E. Green, Dept. of Mechanical Engineering
University of Saskatchewan
Saskatoon, Saskatchewan S7N OWO
K. Sareen, KS Engineering Inc.
Saskatoon, Saskatchewan
A.D.
Osborne, Dept. of Veterinary Microbiology
University of Saskatchewan
Saskatoon, Saskatchewan S7N OWO
ABSTRACT
It is hypothesized that the increase in respiratory diseases in
winter is caused by the reduced indoor relative humidity increasing
the survival time of airborne bacteria *and viruses. This paper
reports the effect of ventilation and relatives, humidity upon the
number of airborne colony forming units per m (cfu/m ) in six
schools.
The airborne bacteria found were mainly non-pathogenic with
a few pathogenic bac^ria appearing in December, January and February.
The number of cfu/ra were mainly a function of occupancy. In
non-occupied periods the number of cfu/m were in the range of 40 to
70,this increased to 300 to 700 as soon as the students entered the
classroom. The average level of the cfu/m was largely a function of
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FINAL REPORT - RP-397
THE EFFECT OF INDOOR RELATIVE HUMIDITY ON SURVIVAL
OF AIRBORNE MICROORGANISMS AND RELATED ABSENTEEISM
IN SCHOOLS AND HOSPITALS
G.H. GREEN
DEPT.
OF MECHANICAL ENGINEERING
UNIVERSITY OF SASKATCHEWAN
SASKATOON, SASKATCHEWAN, CANADA S7N 0W0
INTRODUCTION
Airborne infections are caused by bacteria and virus that survive
their transport through the air, find a favorable landing site, and
cause an infection. Survival depends upon a number of factors, and
this reports the investigation of two of them, indoor relative
humidity and ventilation. Relative humidity affects the survival of
airborne microorganisms, and the general rule seems to be that the
midrange of relative humidity is the most lethal. A number of
investigators attribute the increase in respiratory infections in
winter to the lower indoor relative humidity of that season permitting
an increased survival of airborne infectious microorganisms. Studies
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2
MEASUHEMENTS AM) METHODS - SCHOOLS
A preliminary investigation began in March 1983 with measurement
in 12 schools taken randomly with respect to time of day and class
occupancy. Twelve schools were selected because it would have
provided a large enough sample of students for statistical significant
test.
However, it was noted that the airborne bacteria counts were a
function of the time of day. Airborne bacteria counts in the schools
increased rapidly up to the recess period, decreased, then increased
until noon, and decreased rapidly during the noon hour. The airborne
counts were obviously a function of the number of students, so daily
hour readings had to be taken, which was not possible for 12 schools.
Therefore, the study was reduced to the six schools whose heating and
ventilation characteristics are shown in Table 1. A more extensive
measurement study was undertaken of school (HC) for more detailed
analysis of airborne counts.
The increased number of measurements six to ten for each school
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3
taken before the class started08:30,then repeated at09:30, 10:00,
11:00, 11:30, 12:15, 12:40,and12:35.
Relative humidity was measured with an aspirating psychometer
with readings taken during the microbial sampling period.
The airborne bacteria counts were made with an Andersen sampler
that collects air at 28.3 L/min through six stage with decreasing
diamter holes in each succeeding plate. As the jet velocity increases
with the decreasing diameters of the holes, smaller particles are
recovered at each successive stage. Below each plate, a surface
containing the nutrient agar receives the airborne particles separated
according to their aerodynamic dimensions. Samples were taken for
5-15 minutes, then the petri plates were removed, incubated, and the
colonies counted by the 'positive hole' method.
The reproducibility of the Andersen sampler was confirmed by
using two additional samplers side by side. The results were within
3
10%
over the entire range of up to 2200 colony-forming units/m
(cfu/m
3
).
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4
air-change rate was determined by using N
2
0 as a tracer gas. A
predetermined quantity of N
2
0 was released into the air and additional
mixing was created in empty classrooms with two 24-inch propeller
fans. After a mixing period, a sample was taken every five minutes by
pumping the room air into a sample bag for later analysis. Five
samples were taken in each test. Analysis of the gas was made with an
infrared analysis instrument. Results were consistent and checked
with measured airflow quantities from the diffusers. The combined
room air-change rate resulting from the infiltration and recirculation
will be referred to as the recirculation rate, as the air is mostly
recirculated in the study of the school, HC.
Since the natural death of airborne bacteria is a logorithmic
function (described in the next
section),
the decay rate of viable
bacteria in an empty classroom was determined by taking samples at
3
selected time intervals. These cfu/m were plotted on semilog paper
to determine the equivalent air change rate of the decay of the
viables.
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each of the three hospitals under investigation. To limit the
readings, the levels in the intensive care unit in each of three
hospitals were taken once per month with readings every hour from 8:00
am to 12:30 pm.
The hospitals were: Hospital A was cooled in the summer with
central air conditioning and the same system ventilated and humidified
the rooms in winter. Heating in the winter season was done with
radiators.
The windows were of the double pane non-opening type.
Hospital B and C were not air conditioned, not humidified, and the
windows and storm windows were openable. Hospital A had 185 staff,
and hospitals B and C, 650 and 1560 respectively.
It was not possible to measure ventilation rates in the hospitals
as their duct systems were too extensive and the hospitals much too
large to conduct a tracer gas technique for air change rates for this
investigation.
More extensive readings wwould have to be taken in hospitals to
justify a paper on the results.
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6
bacteria relationship between airborne counts and absenteeism, and (2)
the airborne pathogens and their relationship to relative humidity.
The trends of absenteeismas a measure of illness^-and airborne
bacteria concentration shown in Figures 1 and 2 of this report may be
of great significance but is not considered reliable because it was
obtained from six classes of 20 students each which is too small for
statistically significant results.
3
Figure 1 shows an increase in absenteeism with increase in cfu/m
3
(colony forming units per meter ) . Figure 2 shows the same
absenteeism data plotted versus airchange or recirculation rate and
the one point Q.E. stands out. There is reason to believe that the
discharge rate determined with the room unoccupied is not the correct
value for the occupied state of this classroom. (Air change rates
were done on unoccupied rooms because N
2
0 was used as the tracer gas.)
The trend of the data would suggest there is a relationship between
the number of monthly non-pathogenic cfu/m measured in this
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other pathogens and since they appear in very low numbers, it may be
difficult or impossible with the known methods.
RESULTS HOPITALS
The concentration of non-pathogenic airborne bacteria in the
intensive care units of the hospitals for the period August to
September, 1983, are shown in Figures 3,4 and 5. The counts are low
compared to schools, occurring in the range of 30 to 200 cfu/m . High
readings were found wwhen activities took place in the ICU. The data
of Hospital C shows what happened when workmen began repairs to the
mechanical systems in the ceiling space. People entering the unit
also caused excursions from the basic levels. The reduction on the
3
cfu/m with reduction in relative humidity was similar to the schools
and no pathogens were found in the hospitals.
CONCLUSIONS
Schools
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8
3. Ordinary filters with high air recirculation rates are highly
effective in removing airborne bacteria from rooms. They may
constitute a means for protecting building occupants from
bacterial pathogens produced by sprays, humidifiers and other
conditioning equipment that could produce illness causing airborne
bacteria.
4.
A decrease of indoor relative humidity reduced the number of
3
cfu/fri in the air. Other studies that show the opposite did not
take into account the variation in number of occupants and the
effect of ventilation air recirculation.
5. This study did not substantiate the hypothesis that the rediction
in absenteeism and respiratory illnesses in winter with increased
indoor relative humidity is caused by the reduction in lowered
survival time of airborne micro-organisms with increased humidity.
6. Pathogenic bacteria were only found in the mid-winter season.
While this investigation revealed many aspects of airborne
bacterial contamination in schools that may be significant to all
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microorganisms generated in the classroom of R.P. 397 can be used to
study the airborne microorganism removal of filters in the field,
examining individual filters sealing and other practical problems.
It was not possible in R.P. 397 to determine if there was a
relationship between he number of non-pathogenic airborne bacteria
and illness, but the trend of reduced illness with reduced
non-pathogenic airborne microorganisms is of sufficient importance to
warrant further studies.
The apparent higher level of airborne microorganisms that
occurred in the one school with a high nap carpet warrants further
studies.
Schools because of their high occupancy generates high levels of
non-pathogenic airborne bacteria and assuming that this is a
reflection of pathogens, it indicates that schools may be a good type
of building to investigate the transmission of airborne diseases.
Hospitals
The large size and diverse characteristics of the various
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UTILIZATION OF HP-397
The Effect of Ventilation and Relative Humidity upon Airborne
Bacteria in Schools
1. This research is related to:
a) Fundamentals Volume/ Chapter 8, Physiological Principles,
Comfort and Health, Section indoor Climate and Health; Chapter
11,Air Contaminants Section, Airborne Microorganism.
b) Equipment Volume, Chapter 10 , Air Cleaners, Section Filters.
c) Application Volume, Chapter 6, Education Facilities Ventilation
Requirement; Chapter 7, Health Facilities.
2. This research extends our present knowledge of the levels of
non-pathogentic bacteria in schools. It was demonstrated that the
filter was quite effective in removing bacteria from the air and
that the classroom recirculation air change rate through the
filter was the greatest factor in lowering airborne bacteria
levels.
Our knowledge of the effect of relative humidity upon the
life of airborne bacteria was increased.
3. This work bears out the recirculation concept in hospital
operating room prevented by, Luciano, J.R., New Concept in French
Hospital Operating Room HVAC Systems, ASHRAE Journal, Feb. 1984,
p.
3 0, and suggests a new approach in the chapter Health
Facilities in the application volume.
4. Further investigations suggested by RP 397 are:
a) The place of filters in removing airborne legionella bacteria
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.
JU-USLE
J.
School Heating and Ventilating Systans
p -
School
C
HC
KG
NP
QE
W
Heating System
Natural Gas
Warm - Air
Natural Gas
Warm - Air
Steam Radiators
Hot Water
Radiators
Unit Ventilators
Unit Ventilators
Humidifying
System
Air Washer
Air Washer
Air Washer
None
None
None
Outdoor Air
Per Person
1.5 L/sec
3 cfm
1.5 L/sec.
3 cfm
5.0 L/sec
100 cfm
1.5 L/sec
3 cfm
*
*
Recirculation
Rate Ac/Mr
2.6
3.0
All Fresh Air
1.22
3.00
0.63
0.63
Room Vol .
Per Student
15
8.2
15
13
13.6
15
525
287
525
455-
476
527
* Uni t ventilator controls we re set with minimum fresh air whe n heating was re quired
opening to admit outdoor air as cooling was required.
+ 20 students was average in all schools
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No.2887(R P-397)
The Effect of Venti lation
and Relative H um idity
upon Airborne Bacter ia in Schools
G.H.
Green K. S areen A .D. Osborne
ASHRAE Fellow
ABSTRACT
It is hypothesized that the increase in respiratory diseases in winter is caused by the
reduced indoor relative humidity increasing the survival tine o f airborne bacteria and
viruses. This paper report s the effect o f ven tilation and relative hu midity upon the number
of airborne colony-forming unitsper m (cfu/m ) in sixschools. Th e airborne bacte ria found
were mainly nonpathogenic with.a fe w pathoge nic bacteria appearing in December, Januar y, and
February. The number o f cfu/m were mainly a function o f occupancy: in noncccupied per iods,
the numberso f cfu/m were in the range of 40 to 70, which increased to 300 to 700 as scon a s
the studen ts enteredthe cla ssroom. Th e average level of the cfu/m w as largely a function o f
theair recirculation rat e, becauset h e filter remove d 90% of the airborne bacte ria. Ordinary
filters
a re
very effective
in
removing bacteria from
air and
could
b e
used
a s a
protection
from sources disseminating pathogenic bacteria in ventilating system s. Th e number o f
classroom airborne bacteria w a s reduc ed slightly a s the relative humidity de crea sed, which
doesn ot support the hypoth esis proposed for the increase o f respiratory illnessesin winter.
They we re, however, mainly nonpathogenic,and the pathogens may have a different survival
pattern with relative humidity. In general, t he level o f airborne bac ter ia in the classroom
wa s mainlya function of occupancy and air recirculation rate through a filte r.
INTHTPCTIOW
Airborne infectionsa re caused by bacteria and virus that survive their transpor t thr ough the
air , finda favorable landing site, an d cause a n infection. Survival dep end s upo n a number o f
factors,a nd this paper investigates t w o of them, indoor relative humidity and ventilation.
Relative humidity affectsthe survival of airborne microorgan isms,and the general rule seems
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the results must be considered tentat ive.
The objective of this study is to investigate the effect of relative humidity and
vent ilation rates on the concentrat ion of airborne bacte ria in eleme ntary schoo ls.
MEASUREMENTS AND METHODS
With the objective of getting random samples of airborne bacteria and relative humidity, the
study began with measurements of airborne bacteria, relative humidity, dust, and surface
bacteria in 12 schools. A single classroom in each school of about 20 Grade 3 students of
ages 7, 8 , or 9 was select ed.
It was noted that the airborne bacteria counts were a function of the time of day (Figure
6 Airborne bacteria counts in the schools increased rapidly up to the recess period,
dec reased, then increased until noon, and decreased rapidly during the noon hou r. The
airborne counts were obviously a function of the number of student s, so daily hourly readings
had t o be taken, which was not possible for 12 schools. Therefore, th e study wa s reduced to
the six schools whose heating and ventilation characteristics are shown in Table 2. A more
extensive measurement study was undertaken of school (EC) for more detailed analysis of
airborne co unts.
Readings of relative humidity, dust, and airborne counts were taken before the class
started 08:30, then repeated at 09:30, 10:00, 11:00, 11 :30, 12:1 5, 12: 40, and 12:35. A
typical plot of the results is shown in (Figure 6 ).
Relative humidity was measured with an aspirating psychometer w ith re adings taken during
the micro bial sampling period.
The airborne bacteria coun ts were mad e with an Andersen sampler that col lec ts air at 28.3
I/minthrough six stages with decreasing diameter ho les in each succeeding p lat e. As the jet
velo city increases with the decreasing diamet ers of the ho les, smaller par ticles a re recovered
at ea ch successive stage. Below each plate , a surface containing the nutrient agar r eceives
the airborne particles separated according to their aerodynamic dime nsions. Samples were
taken for 5-15 minutes, then the petri plates were remo ved, incubated, and the colonies
counte d by the 'positive hol e' method (Andersen1968).
The reproducibility of the Anderson sampler was confirmed by using two additional
samplers side by side^ The ^results were within 1 0% over the entire r ange o f up to 2200
colo ny-forming units/m (cfu/ra ) .
Airborne du st samples were taken simultaneously, with the airborne bacteria collection
with th e sampler. A respirable aerosol mass monitor was used to determ ine the respirable mass
cont ent of the air. Mass content of the air was registered in Mg/m by a piezoelectric
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same school.
The mechanism of the death of viable particles, which have been studied in an aerosol
chamber, has shown two slopes: the initial dec ay, ^ , and a later decay constant, K_. In
this study, a mean de cay wa s determined because of t he length of time required for airborne
bacte ria sampling.
The loss of viable a irborne bacteria in the air is considered as the sum of the dilut ion
loss due to the recirculated air rate and the decay l oss due to the natural occurring deat h in
the air, which has been shown to be a function of a number o f factor s, one of which is
rel ative hum idity, which h as a significant effect (Akers and Dimnick1 969).
The study of the separate effect of air changes per hour and relative humidity was
carried out by determining the slope, K, of the decay curve s. In Figure 8, a study of the
effect of air-change rate in a single school at a constant relative humidity is shown. The
test began at 11:55, so the cfu/m are at a maximum; then the students leave, so the occupancy
is zero. Curves A was are obtained by turning off the ventilation fans so that the dec ay
repre sents the lowest air-change rate, caused by infiltration onl y, wh ich was found with
tracer-gas decay equ al t o, 0.30 AC/hr. -At the same time, samples of air we re taken with the
sampler to find the decay ra te of cfu/m with no occu pancy . This slope is given by Curve A'
equ al to 1.80 AC/hr. The loss of viability due to biological deat h is represented by the
difference in slopes between A and A'. The deca y rate was equivalent t o 1.5 = (1.8-0.3)
AC/hr. Repeating the test with th e ventilation fans on , Curve B is the decay r ate repr esented
by the dilution of the ventilation air and Curves B' are the combined effect o f ven tilation
and biological death.- The biological decay rate of curve B is 5.62 - 3.0 = 2.6 2 AC/hr .
The increase in cfu/m resulting from a reduc tion in recircu lation ra te is shown in
Figur e 9.
Figure 10 shows the reduction in the cfu/m that occurred in one school with reduct ion in
indoor humidity. All the data showed the same trend , that is, a reduction in indoor relat ive
humidity resulted in a reduc tion of viable airborne bacter ia.
Several species of the non-pathogenic airborne viables from the schools were grown in
cultu re so they could be atomized into the air of a chamber that could be kept at a selected
relative humidity. Samples taken at time intervals established th e viability of the bacter ia
at various humidities. The tests showed that the survival time of bacteria decreased with
lowered relative humidity.
Respirable dust level s of 10 micron or l ess showed a relationship to the cfu/m increased
as the duct content increased as shown in Figure U . Whether
the bacteria and their
associated skin cells contributed to the dust cou nt or vice versa is not kno wn.
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3. Ordinary filters with high air recirculation rates are highly effective in removing
airborne bacteria from rooms. They may constitute a means for protecting building occupants
from bacterial pathogens produced by sprays, humidifiers, and other conditioning equipment
that could produce illness by causing airborne bacteria.
4.
A decrease in indoor relative humidity reduced the number of cfu/m in the air.
Other studies that showed the opposite did not take into account the variation in number of
occupants and the effect of ventilation and air recirculation.
5. This study did not substantiate the hypothesis that the reduction in absenteeism and
respiratory illnesses in winter with increased indoor relative humidity is caused by the
reduction in airborne microorganisms with increased humidity.
6. Pathogenic bacteria were only found in the midwinter season.
Vfliile this investigation revealed many aspects of airborne bacterial contamination in
schools that may be significant to all buildings, further investigations will have to be
undertaken to substantiate the findings.
Andersen, A.A., New Sampler for the Collection, Sizing, and Enumeration of Viable Airborne
Particles, Journal of Bacterilogy, V. 76 , 1968, p.
471-84.
Akers, A.B.; and Dimmick, R.L., An Introduction to Experimental Areobiology, Wiley, 1969, p.
280.
Dang, V.B.; Guberan, E. ; and Sweetman P.M., L'humidification de L'air des Locant Previent
elle les Maladies Respiratoires Pendant L'hiver, Schweizerische Medizinische
Wbchenschrift, 108, No . 22, 1978.
Druett,
H.A., The Inhalation and Retention of Particles in the Human Respiratory System,
Airborne Microbes, in P.H. Gregory and J.L. Montieth, eds., 17th Symposium of Society
for General Microbiology, London, Cambridge Univ. Press, 1967.
Green,
G.H., The Positive and Negative Effects of Building Humidification, ASHRAE Trans.,
1982,Vol. 88., Pti 1.
Green, G.H., Winter Humidities and the Related Absenteeism in Canadian Hospitals, Digest of
the 3rd CMBES Canadian Clinical Engineering Conference, Sept. 1981, Saskatoon,
Saskatchewan.
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TABLE
2
School
Heating and Ventilating Systems
School
C
HC
KG
NP
QE
.
Heating System
Natural Gas
Warm - Air
Natural Gas
Warm - Air
Steam Radiators
Hot Water
Radiators
Unit Ventilators
Unit Ventilators
Humidifying
System
Air Washer
Air Washer
Air Washer
None
None
None
Outdoor Air
Per Person
l.S L/sec
3 cfm
l.S L/sec
3 cfm
5.0 L/sec
100 cfm
l.S L/sec
3 cfm
*
*
Recirculation
Rate Ac/Mr
2.6
3.0
All Fresh Air
1.22
3.00
0.63
0.63
Room Vol.
Per Student
_ 3
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S2 7
V )
m