ASHRAE Journal - September 2020 - 31

TECHNICAL FEATURE

FIGURE 1 Relative humidity of 40% to 60% is optimal for human health. Courtesy of Dr. Stephanie Taylor.

High Infectivity of Pathogens

Virus (Lipid Membrane)
Inﬂuenza, Coronavirus, RSV,
Parainﬂuenza., Measles, Rubella,
Herpes
Virus (Non-Lipid Membrane)

Low Infectivity

High Infectivity of Pathogens

Healthiest Humans

Human Illness from Infections

Bacteria (Gram Neg.)
Legionella (Aerosolized)
Bacteria (Gram Pos.)
Mycoplasma (No Cell Wall)

Human Illness from Infections
Adult In-Patients (Acute Care)
Elderly Patients in Long-Term Care
Preschool Children
Poor Human Functioning

Poor Human Functioning

Employee Productivity
Employee Sleep Quality
Student Learning
10

Indoor RH, especially in temperate climate winters,
can stabilize at dry levels of 20% or lower when cold
outdoor air is brought into a building and heated to
temperatures comfortable to lightly clothed people.
New research ﬁndings are revealing the magnitude of
occupant health problems associated with low indoor
RH. While many of these health issues have been noted
in the past,2 they are now receiving additional attention
because new data are reinforcing the signiﬁcance of this
relationship.
Much of the new information about dry air and health
arises from a greater understanding of the coexistence
of microbes with humans. New medical diagnostic tests
implement tools used to sequence the human genome
in 2003. These tests can probe deeply into the causes
of acute infections, chronic inﬂammation and autoimmune disorders. These genetic analysis tests have given
rise to revolutionary information about the coexistence
of vast numbers of microbes, meaning bacteria, viruses

50
60
Relative Humidity (%)

and fungi, on and within our bodies.6
This information directly challenges our long-held
hygiene theory that microbes are disease-causing germs
that need to be eradicated as quickly as possible. We now
know that most microbes in our personal ecosystems,
called our microbiome, are not only beneﬁcial to our
health, but are essential to human survival.
When humans occupy a building, they shed their
microbes from direct contact, skin ﬂakes and expired
droplets into the built environment. Accumulated
occupant microbes combine with those from outdoor
sources, giving rise to dynamic microbial communities known as the building microbiome.7 This building
microbiome is further shaped by ongoing occupant
shedding, ventilation strategies and surface materials
(Figure 1). In addition:
¥ When indoor air is less than 40% RH, the effective,
natural immunological defenses of human airways, eyes
and skin are impaired. The loss of moisture from these

Stephanie H. Taylor M.D., is a Harvard Medical School Incite Health Fellow in Boston. Mike Scoﬁeld, P.E., is president, Conservation Mechanical Systems, Sebastopol, Calif., and is a
consultant to California Hydronics Corp., San Francisco. Patricia T. Graef, P.E., is retired and lives in Fort Myers, Fla.
SEPTEM BER 2020

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ASHRAE Journal - September 2020

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