ASHRAE Journal - September 2019 - 16

the literature, particularly as they
FIGURE 2 Outdoor air concentration of PM2.5 in Santa Barbara, Calif., during 2017.
pertain to air cleaners.
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Portable air cleaners can improve
Sunday
health outcomes. Numerous air
Monday
Tuesday
cleaner intervention studies have
Wednesday
found statistically significant assoThursday
ciations between the use of porFriday
Saturday
table air cleaners in homes and (1)
reductions in indoor particulate
Good (<=12.0 mg/m3) 285 Days
Moderate (12.1 to 35.4 mg/m3) 67 Days
matter exposure and (2) at least one
Unhealthy for Sensitive Groups (35.5 to 55.4 mg/m3) 3 Days
measure or marker of improved
Unhealthy (55.5 to 150.4 mg/m3) 9 Days
health outcomes. For example,
Very Unhealthy (>=150.5 mg/m3) 1 Days
Allen et al. deployed portable HEPA
air filters and placebo filtration
FIGURE 3 Wildfire in Southern California-2017.
in a randomized crossover intervention study of 45
healthy adults in a woodsmoke-impacted community
during consecutive seven-day periods of filtered and
non-filtered air.8 Portable HEPA filters reduced indoor
PM2.5 concentrations by 60% on average. That level of
PM2.5 filtration was associated with improved endothelial function (9.4% increase in reactive hyperemia
index) and decreased concentrations of inflammatory
biomarkers (32.6% decrease in C-reactive protein).
Other studies of interventions with portable units have
shown similar effects. Much less is known about central air filtration, primarily because it has been studied
far less than portable air cleaning units that have high
rated as having a high CADR for smoke. Then operCADRs.
Short term exposure can also affect health. Most studies have ate them when or if smoke builds up in the outdoor
documented the negative effects of long-term exposure. air. More comprehensive measures beyond air filtration are also discussed in separate EPA guidance for
However, there is also direct epidemiological evidence
response to wildfires.10 Readers interested in further
that short term exposure affects health. Schwartz, et
details of health effects of PM2.5 are encouraged to conal.,9 performed an analysis of U.S. mortality (7.5 million
deaths in 135 U.S. cities) and compared death rates with
sult the studies referenced in the technical summary of
locally-reported outdoor air concentration of PM2.5 at
the EPA guidance.
the time of death. Over the period examined, each 10 µg/
Indoor PM2.5 Concentrations Frequently Exceed Outdoor
m3 increase in daily PM2.5 concentrations was associated with a 0.6 to 1.5% increase in daily deaths (approxi- Concentrations
Indoors, PM2.5 concentrations vary widely dependmately 112,500 individuals).
ing on many factors. In the absence of indoor sources,
To put this finding into perspective, consider the
indoor concentrations are sometimes lower than outmeasurements of outdoor airborne particulate shown
doors, especially in modern tight houses which greatly
in Figure 2. In Santa Barbara Calif., during December
reduce particle infiltration from outdoors. But studies
2017, airborne PM2.5 increased from less than 12 µg/m3
3
to over 150 µg/m because of the local wildfire. The
have also measured indoor concentration at higher
health-relevance of such short-term exposure suggests levels than outdoor concentrations. One review of 28
that in areas prone to wildfires, it would be wise to
large-scale field studies measuring indoor and outprepare by acquiring one or more portable air cleaners door concentration in non-smoking homes in North
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ASHRAE Journal - September 2019

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