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2008 17: 122Indoor and Built EnvironmentL.T. Wong, K.W. Mui, P.S. Hui, W.Y. Chan and A.K.Y. Law

Thermal Environmental Interference with Airborne Bacteria and Fungi Levels in Air-Conditioned Offices  

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Original Paper

Indoor and BuiltuiltEnvironment Indoor Built Environ 2008;17;2:122–127 Accepted: January 17, 2008

Thermal EnvironmentalInterference with AirborneBacteria and Fungi Levels inAir-Conditioned Offices

L.T. Wong K.W. Mui P.S. Hui W.Y. Chan A.K.Y. Law

Department of Building Services Engineering, The Hong Kong Polytechnic University

Hong Kong, China

Key Words

Offices E Air temperature E Bacteria

E Humidity E Microbes

AbstractIndoor airborne bacteria and fungi levels can be

selected as indicators of a healthy indoor environment.

This study investigated the relationships between the

airborne bacteria levels, fungi levels, and thermal

environmental parameters, i.e., air temperature and

relative humidity, in some offices with a Mechanical

Ventilation and Air-Conditioning (MVAC) system

operating. A total of 101 samples were collected from

two typical Hong Kong air-conditioned office premises.

There was evidence that the operation of the MVAC

system would have significant influence on both of the

indoor airborne bacteria and fungi levels. The results

showed that no significant difference in airborne

bacteria and fungi levels was observed between offices

having similar thermal environments (p4 0.05). How-

ever, significantly higher airborne bacteria and fungi

levels were found in the same office during non-office

hours when the air-conditioning system was shut down

(p50.03). It was also reported that the airborne bacteria

and fungi levels would be correlated with the thermal

environmental parameters in some offices (p50.0001).

Introduction

Exposure to indoor airborne bacteria and fungi could

cause sick building syndrome and a wide range of adverse

health effects [1–3]. Indoor airborne bacteria are normally

present due to the presence of humans and fungi are

usually imported from the outdoor environment [4,5].

Although various guidelines on indoor airborne bacteria

level have already been established by different organiza-

tions [6–8], an international threshold for indoor airborne

levels of fungi with regard to assessment of an acceptable

indoor air quality (IAQ) has yet to be unified.

The survival of indoor airborne bacteria and fungi

depends strongly on environmental conditions such as air

temperature and relative humidity [9–11]. Water activity,

a measure of the moistness of building materials, is affected

by relative humidity and its optimal level for indoor airborne

fungi is between 0.83 and 0.96 [10,11]. These thermal

environmental conditions affect the germination, growth,

� SAGE Publications 2008Los Angeles, London, New Delhi and SingaporeDOI: 10.1177/1420326X08089260Accessible online at http://ibe.sagepub.com

Dr KW MuiDepartment of Building Services Engineering, The Hong Kong PolytechnicUniversity, Hong Kong, China. Tel. þ85 227 665 835, Fax þ85 227 657 198.E-Mail behorace@polyu.edu.hk

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and development of indoor micro-organisms [11,12]. Since

Hong Kong is hot, humid, and densely populated with a

dense urban environment, it should provide a favorable

milieu for indoor airborne microbial growth [13,14]. The

optimum air temperature range reported for the germination

and growth of common indoor airborne fungi isolated in

Hong Kong is from 258C to 308C [11].

A poorly designed, improperly maintained, or

microbially contaminated Mechanical Ventilation and

Air-Conditioning (MVAC) system may affect IAQ and

pose health risks by transporting micro-organisms from

the locus of contamination to occupants in the vicinity of

the building [15,16]. Besides, water spray humidifiers

containing stagnant water, filters packed with organic

dust, cooling coils covered with condensation, and any

excessively humid interior can provide moisture and

nutrients for microbial growth [17].

However, questions remain about the level of

confidence in an airborne bacteria and fungi assessment.

Assessment results with limited samples might be deficient

in representing personal exposures. It was shown that an

insufficient number of samples and assessment results with

unknown confidence intervals might even contribute to the

failure of concluding significant associations between

microbial concentrations and personal health [18,19].

Although long-term and comprehensive measurements of

bacteria and fungi levels can be a good approach for

monitoring regional indoor air pollution levels, a rapid

estimate of the office failure rate might be more applicable

to a timely decision on the mitigation actions of the

MVAC system [19–22]. Therefore, instead of recording

a continuous profile the average of repeated sample

values of airborne bacteria and fungi levels could be

taken within an assessment period.

This study investigated the relationship of airborne

bacteria and fungi levels with thermal environmental

parameters in air-conditioned offices. Mathematical expres-

sions were proposed for the estimates of airborne bacteria

and fungi levels correlated with the parameters measured.

The results should be useful in detection of asymptotic

microbiological pollutant problems by a quick estimate of

the probable failure rate of offices over a region against

certain bacteria levels from a few samples, while avoiding

an inappropriate level of reliance on the results.

Review of Field Measurements

One-week measurements of airborne bacteria and fungi

levels were conducted in two in-use air-conditioned office

buildings located near to busy streets of Hong Kong [13].

Two open-plan offices (A and B) without any water

damage or fungus problems were chosen. Air handling

unit (AHU) and fan coil unit (FCU) systems were installed

in both premises. The AHUs were equipped with panel

disposable pre-filters. Each pre-filter that consisted of a

layer of aluminum screen and a layer of fiber filter was

claimed to be cleaned every 3 months and replaced every

half-year. For each of the FCUs, the return air plenum

was equipped with an aluminum screen filter that was also

claimed to be cleaned every 3 months.

A number of 12-hr measurements were performed and

temporal concentration profiles of airborne bacteria and

fungi levels during office hours were reported. In the

absence of a standard measurement protocol, a trial

protocol with four time periods within a day was used [13]:

1. one hour before the air-conditioning systems were

turned on;

2. immediately after the air-conditioning systems were

turned on;

3. immediately after lunch time;

4. immediately after office hours when the air-

conditioning systems were turned off.

Air samples (5min each) were collected at the center of

two offices (1.1m from the floor) by an Anderson 6 Stage

cascade impactor. Tryptocase Soy Agar and Malt Extract

Agar (3% malt extract, 1.5% agar, 0.5% peptone,

supplemented with 0.1mg�mL–1 chloramphenicol)

were used for sampling of airborne bacteria and fungi

respectively. The collected bacteria and fungi samples were

then incubated and quantified as ‘‘Colony-Forming

Units’’ per cubic meter (CFU�m–3) of air.

Results and Discussions

Table 1 summarizes the measured air temperature, the

relative humidity, the airborne bacteria and fungi counts

of the two offices (A and B) during office hours (when the

main MVAC system was operating) and non-office hours

(when the main system was shut down). The air

temperature measured in the office hours was between

198C and 248C and the relative humidity was between 55%

and 80%. The results were not significantly different from

the thermal environment of some other nonresidential air-

conditioned spaces in Hong Kong where the air tempera-

ture was between 19.38C and 25.28C and the relative

humidity between 44% and 90% [20–23].

Temperature Effects on Airborne Microbes in A/C Offices Indoor Built Environ 2008;17:122–127 123

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It was reported that a small air-conditioning unit was

operated during non-office hours to maintain acceptable

relative humidity so that no significant difference in

average relative humidity between office and non-office

hours was reported ( p� 0.77). Nevertheless, the average

air temperature recorded during the non-office hours was

significantly higher ( p� 0.001).

The airborne bacteria levels were 249CFU�m–3 and

280�CFU�m–3 and the fungi levels were 42CFU�m–3 and

52CFU�m–3 for offices A and B in office hours; and for

the non-office hours, the airborne bacteria levels were

429CFU�m–3 and 742CFU�m–3 and the fungi levels were

301CFU�m–3 and 222CFU�m–3 for offices A and B

respectively. The airborne bacteria and fungi counts

could be approximated by a geometric distribution

( p� 0.05) with the percentiles of the measured values as

shown in Figure 1 and the geometric means and standard

deviations as summarized in Table 1.

This study showed that the operation of the MVAC

system in offices could have significant influence on

both indoor airborne bacteria and fungi levels. Table 2

summarizes the t-test p-values for the measurements

from offices A and B. There were parallel observations

in some air-conditioned environments where air

Table 1. Summary of measurement results in two air-conditioned offices

Office A B

period Office hours Non-office hours Office hours Non-office hoursMain HVAC system operations ON OFF ON OFF

Airborne bacteria level (CFU.m–3)Geometric mean GM 249 429 280 742Geometric standard deviation GSD 2.1 2.6 2.0 1.9Sample size N 50 23 16 12

Airborne fungi level (CFU.m–3)Geometric mean GM 42 301 52 222Geometric standard deviation GSD 5.4 6.2 3.1 4.8Sample size N 50 23 16 12

Air temperature (8C)Arithmetic mean AM 22.1 23.2 20.2 22.3Arithmetic standard deviation ASD 1.5 1.3 1.4 1.4

Relative humidity (%)Arithmetic mean AM 66.5 67.9 62.9 66.2Arithmetic standard deviation ASD 4.7 3.6 2.3 6.2

0

20

40

60

80

100

10 100 1000 10,0000

20

40

60

80

100

10 100 1000 10,000

Bacteria level ΦB (CFU.m–3)

Per

cent

ile (

%)

A/C on A/C off

Office A

Office B

Per

cent

ile (

%)

Fungi level ΦF (CFU.m–3)

Fig. 1. Airborne bacteria and fungi levels in two air-conditioned offices.

124 Indoor Built Environ 2008;17:122–127 Wong et al.

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temperature and relative humidity were reported to have

influence on the airborne bacteria levels. From the

results, the bacteria and fungi levels between offices A

and B were comparable ( p4 0.05) when the main

MVAC system was either on (office hours) or off (non-

office hours). For each office by itself (i.e. A or B),

however, both of the airborne bacteria and fungi levels

were significantly higher when the main system was off

( p50.03). It was believed while the main MVAC system

was not in operation, air temperature increased and

water vapors condensed onto the indoor building

materials as well as inside the system; these favored

indoor microbial growth [3,15].

This study observed that both the airborne bacteria

levels �B (CFU�m–3) and fungi levels �F (CFU�m–3) of the

two air-conditioned offices would be correlated with

( p50.0001) the thermal environmental parameters air

temperature T (8C) and relative humidity RH (%) in the

measured ranges 198C�T� 258C and 50%�RH� 80%.

�B ¼ 1:9� 10�7T 4:47RH1:85ð1Þ

�F ¼ 3:3� 10�22T 6:01RH8:71ð2Þ

For the above correlations, the standard errors of airborne

bacteria and fungi levels were "B¼ 438CFU�m–3 and

"F¼ 683CFU�m–3 respectively. The measured levels �B

and �F, plotted together with the predicted levels against

indoor air temperature 198C–268C and relative humidity

45–85%, are shown in Figure 2. They were comparable

with the measurements found earlier in some other

nondomestic A/C spaces as illustrated in the figure

[20–23]. As expected, an office under higher air tempera-

ture and relative humidity within the measured ranges

would give higher bacteria and fungi levels.

Figure 2 also presents the numbers recommended for an

‘‘excellent’’ office environment: thermal environmental con-

ditions in which 208C�T� 25.58C and 40%�RH� 70%

[6]; maximum airborne bacteria level¼ 500CFU�m–3 and

maximum fungi level¼ 200CFU�m–3 [8].

To attain the excellent airborne bacteria and fungi levels

at 60% RH, for instance, the air temperatures should be

23.58C and 23.98C respectively. The results showed that

some thermally excellent air-conditioned spaces would have

expected bacteria and fungi levels beyond the advised limits.

For both offices A and B, correlations with their airborne

bacteria and fungi levels were observed and the results are

shown in Figure 3 ( p50.0005).

�F ¼ 0:18�1:09B ; 70 � �B � 2900 ð3Þ

10

100

1000

10,000

19 20 21 22 23 24 25 261

10

100

1000

10,000

19 20 21 22 23 24 25 26

Air temperature T(°C) Air temperature T(°C)

Bac

teria

leve

l ΦB (C

FU

. m–3

)

Fun

gi le

vel Φ

F (C

FU

. m–3

)

Office AOffice B

× 422 offices in Hong Kong; RH = 55 (20)

85%75%65%55%

RH = 45%

75%

65%

55%

45%

RH = 85%

Recommended ‘excellent’ thermalenvironmental conditions (23)

‘Excellent’

ACGIH guideline

Measurements withRH = 44–90% (13)

Fig. 2. Correlations for the airborne bacteria and fungi levels in two air-conditioned offices.

Table 2. Statistical analysis of airborne bacteria and fungilevels in two air-conditioned offices with various MVAC systemoperation modes

Office designation, operation of MVAC system t-test p-value

Airborne bacteria levelOffice A, ON Office B, ON 40.05Office A, OFF Office B, OFF 40.05Office A, ON Office A, OFF 50.001Office B, ON Office B, OFF 50.03

Airborne fungi levelOffice A, ON Office B, ON 40.5Office A, OFF Office B, OFF 40.6Office A, ON Office A, OFF 50.001Office B, ON Office B, OFF 50.02

Temperature Effects on Airborne Microbes in A/C Offices Indoor Built Environ 2008;17:122–127 125

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Conclusions

In this study, the relationship of airborne bacteria

and fungi levels to thermal environmental parameters

(i.e. air temperature and relative humidity) was investi-

gated in two air-conditioned offices in a sub-tropical

climate. The office thermal environments were found

to be similar and there was no significant difference in

airborne bacteria and fungi levels between the offices at

any time ( p4 0.05). Yet, significantly higher levels of

airborne bacteria and fungi were observed in the

same office when the main MVAC system was shut

down ( p50.03). The measurements showed that the

airborne bacteria and fungi levels would be correlated

with the thermal environmental parameters ( p50.0001).

Using the measurement data, mathematical expressions

were proposed for a rapid estimation of the expected

airborne bacteria and fungi levels under certain thermal

environmental conditions. These expressions were

proven practical tools for epistemic IAQ assessment of

some air-conditioned offices in the same region [24].

Since indoor thermal parameters can be selected as a

reference to identify the performance of the indoor

environment and estimate the failure rates due to

excessive airborne bacteria and fungi levels, this study

should provide a useful source of information for

policymakers to evaluate the IAQ of some air-con-

ditioned offices in a sub-tropical climate.

Acknowledgment

The testing facility described in this work was partially fundedby research grants from The Hong Kong Polytechnic University

(GYE80, GYF21) and from the Research Grants Council of theHKSAR (PolyU 5305/06E). The work described in this paperwas substantially supported by a grant from the Research GrantsCouncil of the Hong Kong Special Administrative Region, China

(PolyU 5248/06E).

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1

10

100

1000

10,000

10 100 1000 10,000

Bacteria level ΦB (CFU.m−3)

Fun

gi le

vel Φ

F (

CF

U. m

−3)

Office AOffice B

Fig. 3. Correlations between the airborne bacteria and fungi levels( p50.0005).

126 Indoor Built Environ 2008;17:122–127 Wong et al.

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