ASHRAE Journal - April 2021 - 58

COLUMN IEQ APPLICATIONS

was 1,150 ppm when doors/windows were closed and 717
ppm when open. They observed that improved ventilation,
as indicated by reduced CO2 concentration, resulted
in improved sleep depth, sleep efficiency and fewer
awakenings. Note that the lower sleep efficiency (SE),
increased number of awakenings (NOA) and lower depth
of sleep indicated in Figure 1 were reported at higher CO2
level of 1,150 ppm.
Xiong, et al.,7 conducted a field study in bedrooms
in non-student accommodations during a summer in
the southern hemisphere. Their results showed that
deep sleep was negatively associated with lower ventilation rates as indicated by raised CO2 concentrations.
Bedroom CO2 concentrations were also compared under
different combinations of bedroom door/window and
occupancy status. The average CO2 concentrations with
the window/door closed were 1,327 ppm for shared
occupancy and 1,004 ppm for single occupancy. For both
shared and single occupancy, the door/window open
condition resulted in similar bedroom CO2 concentrations of around 500 ppm.
Liao, et al.,8 examined the effects of window opening in the bedroom during sleep, and sleep quality was
measured both with a wrist-worn sleep tracker and
polysomnography (PSG), which is a multiparametric test
used in sleep studies and as a diagnostic tool in sleep
medicine. The results indicated that the participants
snored significantly less and awoke less often when
sleeping with the window open. Note that the increased
snoring and NOA indicated in Figure 1 was reported at
the high CO2 level of 1,654 ppm. CO2 levels were around
600 ppm with open windows and 1,654 ppm with closed
windows. The median CO2 levels with the window closed
were 2.6 times higher than with the window open.
Xu, et al.,9 investigated the impact of indoor CO2
levels on sleep quality in climate chambers furnished
and decorated as bedrooms for three CO2 concentration conditions (800 ppm, 1,900 ppm and 3,000 ppm).
Their analysis of the subjective questionnaire and PSG
data showed that a comprehensive score of sleep quality
decreased, sleep onset latency increased and slow wave
sleep decreased as the CO2 concentration increased.
Zhang, et al.,10 conducted a field study in bedrooms
during spring and autumn. The results showed that
wake stage (%) was shorter and sleep efficiency, ease
of awakening (EOA) and sleep satisfaction were higher
with higher ventilation indicated by lower CO2. Note
58

ASHRAE JOURNAL

ashrae.org

APRI L 2021

that the lower SE, increased % of wakefulness, lower
scores of EOA and lower sleep satisfaction indicated in
Figure 1 were reported at high CO2 level of 2,012 ppm.
Two groups were compared, below and above CO2 concentration of 1,000 ppm, for which the average was 672
ppm and 2,012 ppm, respectively. The combined effect
of thermal conditions and ventilation was not examined
although it was observed that a neutral to warm sensation before sleep enhanced sleep quality.
Lan, et al.,11 and Zhou, et al.,12 showed that providing
outdoor air directly to a sleeping person's breathing zone
resulted in better sleep quality, as indicated by subjective evaluation, skin temperature, heart rate and heart
rate variability. However, they did not observe any significant differences in objectively measured sleep quality and did not report the bedroom CO2 concentration.
Hence, their studies were not included in Figure 1.

Summary of Relationship Between
Bedroom Air Quality and Sleep
Figure 1, illustrating the effects on sleep quality,
describes a relationship between sleep quality and
bedroom IAQ and ventilation rate as indicated by the
resulting CO2 concentration. Due to the different ways
in which the CO2 level during the whole sleeping period
was described in these studies, such as average value,
max value, min value etc., Figure 1 shows what the mean
CO2 concentration would have been in the high/low
ventilation conditions. Overlaps between the yellow and
blue circles indicate that the difference between the
high and low conditions in terms of reported CO2 levels
was less than in the other studies.
Figure 1 shows that, in general, poor bedroom ventilation, as indicated by increased levels of CO2, has negative effects on several aspects of sleep and can also result
in poor cognitive performance the next day. Different
effects have been observed at different IAQ levels, but
generally no negative effects have been seen at and
above ventilation rates at which the CO2 concentration
in the bedroom was 835 ppm or lower. It may thus be
recommended that the lowest rate of bedroom ventilation should be maintained at the level ensuring that the
bedroom CO2 concentration is no higher than 835 ppm.
Above this level, negative effects are more likely
to occur, and significant effects have already been
observed when the CO2 concentration is as low as
1,004 ppm. Between about 1,005 ppm and 2,585 ppm,

http://www.ashrae.org

ASHRAE Journal - April 2021

Table of Contents for the Digital Edition of ASHRAE Journal - April 2021