ASHRAE Journal - June 2023 - 33

COLUMN IEQ APPLICATIONS
FIGURE 1 Particle Settling Time from NIOSH Aerosol 101.6
Half-life of particles in 8 ft high room
3 µm
1 µm
12 Hours
1.5 Hours
8.2 minutes
Particles settling in turbulent air will
have an exponential decay rate.
5.8 Seconds
to particulate matter (PM) as no other contaminant
comes within a factor of fi ve.5 Fine particles of 2.5 µm
or less in diameter (PM2.5) have increased adverse
health effects because they penetrate more deeply
into the respiratory system. Unlike airborne dust that
quickly settles out, smaller particles stay suspended
in the air for longer time periods. Airborne pathogens
such as SARS-CoV-2 are transmitted by respiratory
aerosols that are predominantly fi ne particles.7-11
People are exposed to particles both outdoors and
indoors.12,13 Although outdoor pollution migrates
indoors,14 particles from indoor sources often dominate
personal exposure.15
Improving Filtration Requirements
Historically, ASHRAE's (non-health-care) indoor
air quality (IAQ) standards required fi ltration for
equipment protection, rather than health benefi ts.
ASHRAE Standards defi ne acceptable indoor air quality
(Standards 62.1, 62.2), specify ventilation and fi ltration
for health-care (Standard 170), and particle fi lter performance
(Standard 52.2).16 More than 90% of people
in the US spend most of their waking hours in buildings
covered by these ASHRAE Standards.17
US Environmental Protection Agency (EPA) requirements
only apply to outdoor air and historically have
been cited by ASHRAE Standard 62 for requirements
for cleaning particles from outdoor air. Table 1 shows
particle information from ASHRAE Standards 62 and
62.1 citing the US EPA. The EPA is currently proposing
to lower the current outdoor air PM2.5 standard.18
Other ASHRAE fi ltration requirements address keeping
mechanical equipment cleaner while current model
codes only regulate smoke generation from fi lters if they
are ignited.19-21
Particles are also generated indoors by occupants
and occupant activities. The WHO guidelines apply
Improved Filtration: Benefi ts and Challenges
Cost-benefi t analyses that consider the impact on
human health of PM2.5 control fi nd air cleaning to be
highly cost-effective.22,23 ASHRAE Research Project 1649
also performed a literature search on effectiveness of
particle fi ltration.24 Figure 2 shows the results of the fi nal
report as benefi t-to-cost ratios.
For more than four decades, ASHRAE ventilation
standards for non-health-care facilities adopted a defi -
nition of acceptable indoor air quality that requires that
there be no known contaminants at harmful concentrations.25
Because we now know more about the health harms of
PM2.5, WHO target concentrations for indoor air should
be addressed in ASHRAE Standards.
There are concerns in the marketplace that increased
fi lter performance indicated by higher MERV ratings
will increase pressure drop and cause various problems
with HVAC systems. ASHRAE Research Project 1649
concluded that " pressure drop is not strongly correlated
with MERV " (Figure 3).
Looking Ahead
The ASHRAE Epidemic Task Force's widely adopted
recommendation to achieve MERV 13 or better levels of
fi ltration for infectious aerosol exposure reduction26
highlighted a signifi cant leadership opportunity
TABLE 1 ASHRAE Standards 62 and 62.1 Particle Information Citing US EPA
YEAR
ANNUAL
PARTICLE
CONCENTRATION
µG/M3
1973*
1981
1989, 1999,
2001, 2004
2007
2010
2013, 2016,
2019, 2022
60
75
50
50
15
-15
-12
*Cites
proposed EPA standard
J U N E 2 0 2 3 ashrae .o rg ASHRAE JOURNAL
33
24-HOUR PARTICLE
CONCENTRATION
µG/M3
150
260
150
150
65
150
35
150
35
10 µm
0.5 µm
100 µm
41 Hours
globally to both outdoor and indoor environments
except for occupational settings. WHO air quality
guideline levels for PM2.5 are 15 µg/m3 24-hour, 5 µg/
m3 annual.
PARTICLE SIZE
Particulate
Particulate
PM10
PM10
PM2.5
PM10
PM2.5
PM10
PM2.5
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