ASHRAE D RP397 198507 Airbone Bacteria in Schools

7/25/2019 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


4/26

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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5

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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7

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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9

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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10

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


14/26

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19/26

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


20/26

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


21/26

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.


22/26

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

ASHRAE D RP397 198507 Airbone Bacteria in Schools

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