ASHRAE Journal - February 2014 - 64

COLUMN IAQ APPLICATIONS

FIGURE 2 Association between air change rate in winter and cold infection in sixpeople room.

30%

6%
Percent

P=0.002
4%

Percent1

15%

15
10

10%

3 People Per Room
4 People Per Room
6 People Per Room

20%

Occupancy Level

25%

10%

0%
2%

1st Quartile
(0.046 - 0.38)

2nd Quartile
(0.38 - 0.69)

3rd Quartile
(0.69 - 1.28)

4th Quartile
(1.28 - 6.93)

AOR2 (95% CI)

FIGURE 1 Comparison of common cold incidence for different occupancy levels
over a 12 month time frame.

Air Change Rate in Winter (h-1)
0%
6 to 10 Times

>10 Times

study. However, one building was not included in
the analysis because measurements were performed
in only nine rooms. On average, there were 1,140
occupants in each dorm building. A mean ventilation rate of 5 L/s (11 cfm) per person was associated
with ≥ six common colds per year in 5% of occupants,
compared to 35% of occupants for 1 L/s (2 cfm) per
person.

Discussion and Limitations
Although it is widely held that people in crowded
spaces have more respiratory infections,4 there are few
studies on this topic. Our study is among the first published suggesting a relationship between occupancy
levels, ventilation rates, and respiratory infections.
With six occupants instead of three in a 20 m2 (215 ft2)
dorm room, the proportion of occupants reporting
more than six common colds in the previous 12 months
doubled.
When the level of crowding is adjusted for, a lower
ventilation rate is associated with an increased risk
of the common cold. The suggested relationship
between dorm ventilation rate and common cold
infections among occupants could perhaps be
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1Proportion of occupants with ≥ 6 common colds in the previous 12 months.
2Odds ratios were adjusted for gender, age, family member allergy history, exposure to
environmental tobacco smoke and building age.
AOR: Adjusted odds ratio; CI: Confidence interval.

extrapolated to other crowded public premises with
lower ventilation rates, making this an important
public health topic (e.g., for schools, daycare
centers). A crucial question is whether the increased
frequency of common cold in crowded places is due
to direct contact (or via surfaces), rather than via
droplets or droplet nuclei. The strong association
with ventilation in this study indicates that airborne
transmission is an important and perhaps the
dominant route.
There were imperfections and limitations in our
data collection. In some rooms, occupants may
have had the window open during the night measurements in winter. Or perhaps the incidence
of common cold was influenced by a pandemic
of influenza. The common cold infections were
self-reported without clinical confirmation. These
sources of error, however, would shift our findings
towards the null hypothesis, i.e., that there was no
association between common cold infections and dorm
crowdedness or ventilation rate. Thus, it is likely that
more measurements and more accurate data on types of
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ASHRAE Journal - February 2014

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