ASHRAE Journal - September 2020 - 32

TECHNICAL FEATURE

tissues leaves occupants significantly
FIGURE 2 In ambient RH of 50%, the respiratory system is optimally protected from Influenza A disease.8
more vulnerable to infectious, inflammatory and allergic diseases8 (Figure 2).
* A dry air-environment with low RH
Improved Mucociliary
Clearance
carries more infectious bioaerosols than
hydrated air. Airborne pathogens stay
Enhanced Protection
buoyant and viable for longer periods of
Induced by Interferon
time in dry atmospheres, and they can
be spread more readily within the indoor
Decreased Inflammation
and Tissue Damage
Enhanced
human breathing zone.9
Tissue
* For reasons still not completely
Repair
understood, when ambient RH is low, the
viruses and bacteria in aerosolized drop* Legionnaires' disease outbreaks have focused the
10
lets are more virulent to a secondary host.
public's attention on hazards associated with building
* Humans have a vast surface area that loses signifiwater and HVAC systems.
cant water to the environment through breathing and
Nevertheless, we now know more clearly than ever that
skin exposure.11 In dry air, this insensible water losses
dry interior environments are harmful to our health,
result in mild dehydration that stresses brain funcproductivity and learning.
tioning,12 impairs natural respiratory tract immunity
Adding water vapor to hydrate indoor air to 40% to 60%
(discussed above), decreases skin health and wound
RH benefits occupants in several ways. First, balanced
healing13 and overstimulates blood clotting, which is
air-hydration directly supports healthy and functional
associated with poorer outcomes of heart attacks and
skin and mucosal barriers8 necessary to resist invading
pathogens and facilitate appropriate immune system
strokes.14
In summary, dry indoor air is harmful to occupant
responses to microbes. In addition, properly hydrated
health by causing skin and mucus membrane water
air reduces skin production of unhealthy stress horlosses and by creating building conditions that foster the mones,16 improves cognitive abilities, which increase
survival and transmission of pathogenic microbes. Thus, employee productivity and student learning17 and supventilation choices clearly link human health to the built ports healthy eye physiology. Additionally, RH of 40% to
60% fosters a building microbiome with communities of
environment. Despite these findings of harm, understanding and remediating dry indoor environments has diverse microorganisms, which are beneficial to human
health.
been largely overlooked by both building and clinical
Despite the health benefits from balanced indoor air
professionals. The reasons for this are several-fold:
* While the temperature of surrounding air is imme- hydration, there are building-related concerns about
providing indoor humidification. Water in buildings,
diately evident to nerve endings in our skin, the degree
even in the form of vapor, generally has a bad reputaof dryness is not felt and, therefore, is not experienced
15
tion because poorly insulated building envelopes or cold
as discomfort.
* Acute infections from pathogens that we now know
surfaces can allow unintended condensation to occur.
are transmitted via airborne infectious aerosols in dry
Building owners and managers worry about this conair have historically been incorrectly attributed to condensation and associated mold growth.
tact transmission routes alone.
While liquid water will support mold growth, water in
* Chronic illnesses associated with dry air often arise
the vapor form will not because fungal organisms are not
slowly and, therefore, are not immediately associated
hygroscopic when the RH is between 40% and 60%.18 The
effective and rational solution to inhibit mold growth,
with low humidity indoor environments.
therefore, is to adequately insulate cold building surfaces
* Providing humidification requires dedicated HVAC
and use vapor barriers to prevent high water activity in
equipment (humidifiers) that must be run (using water
interstitial spaces. The result should not be an overdry
and energy) and maintained (by human attention) for
indoor environment that compromises occupant health.
safety.
32

ASHRAE JOURNAL

ashrae.org

SEPTEM BER 2020
http://www.ashrae.org
ASHRAE Journal - September 2020

Table of Contents for the Digital Edition of ASHRAE Journal - September 2020
