ASHRAE Journal - September 2019 - 18

TECHNICAL FEATURE

America noted that in 17 studies, indoor concentrations
were between 1.5 and 3.5 times higher indoors than
outdoors.11
These results are not surprising, since there are many
particle sources present in most homes. Heating food in
the oven and cooking on the stove are generally the largest contributors, which is why it's important to operate
the range hood exhaust when cooking. Also, resuspension of settled particles can raise indoor concentration
above the outdoor level. For example, walking across
floors can account for a significant increase in PM2.5
concentration in the breathing zones of both adults and
infants. Also, simply sitting down in upholstered furniture generates a cloud of resuspended particles, as does
movement while sleeping.
Interestingly, researchers in Taiwan recently quantified the increase in airborne particle concentration
that comes from children bouncing on their beds.12
In a field study of 60 occupied bedrooms, researchers found increases in airborne PM2.5 concentrations
of 353 mg/m3 using "standardized percussion" as an
approximation of bouncing children. (One can only
regret the lost opportunity for STEM education by using
researchers rather than actual children to whack beds in
60 houses with sticks!)

Operating Hours Limit Filtration Effectiveness
Portable air cleaners and central systems equipped
with high efficiency filters can improve indoor air quality by removing small particles. But not if the system fan
is not operating.
For example, public health interventions using portable air cleaners have shown improvements in health
markers (5% to 10% compared to control groups in some
short-term studies), and sometimes more substantial
improvements (close to 50% improvement in markers
for inflammation and cardiovascular function in other
studies). But without consumer education, the portable
air cleaners may not be turned on, or may not be operated in bedrooms where they usually provide the greatest benefit.
For central HVAC systems, there is very little research
that documents a health benefit from better filtration.
While logic suggests that better filters can make a positive difference, achieving and documenting measurable
improvements is apparently difficult. Perhaps this is
because central systems have been less-studied, and
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S E P T E M B E R 2 0 19

because fans operate for a surprisingly short number of
hours over the course of a year.
For example, in a 2018 study of three years of run-time
records from 7,000 residential forced-air heating and
cooling systems in North America, Touchie and Siegel13
found that typically, system fans operate for less than
20% of the hours in a year. Clearly, if air is not being
filtered and recirculated through occupied spaces for
80% of the year, central systems are challenged to make
a significant reduction in PM2.5 exposure. In light of
this limitation, the updated EPA guidance recommends
using a MERV 13 filter or higher to provide a more useful
degree of fine particle removal during its few operating
hours.
Further, many homes are now heated and cooled with
the increasingly popular mini-split systems. These
homes face a different challenge with respect to fine
particle control. In that class of equipment, fans often
operate continuously, using a strategy of variable but
continuous airflow to provide air mixing. Although this
provides a large number of filtration hours, the washable screens in most of these units are designed for
equipment protection rather than for removal of fine
particles. The revised EPA guidance suggests that adding high-CADR portable units to specific spaces can be a
practical alternative, when heating and cooling equipment does not accommodate air filtration at MERV 13 or
above.

Residential Central System Recommendations
The EPA recommends using a filter rated at MERV 13
or higher in central systems. To be rated at MERV 13, the
filter must have achieved a defined level of fine particle
removal efficiency.
That simple recommendation, more or less understandable by consumers is based on a (very) long examination and evaluation of the technical and economic
issues for both existing and new systems by the EPA
and it's consulting team. Most HVAC professionals will
understand the confounding and critical issues of run
time, filter bypass, airflow velocity, filter cleanliness, air
distribution resistance versus fan static pressure, along
with the variability of building airtightness and domestic cleaning and furnishing preferences. For example,
professionals know that if the filter is not snugly-fitted
into the filter slot, or if the air velocity through the filter
is not ideal, or if the filter is clogged because it has not

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ASHRAE Journal - September 2019

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