Handbook of air conditioning and refrigeration / Shan K

24.4 CHAPTER TWENTY-FOUR
24.3 ELIMINATING MICROBIAL GROWTH
Basics
Microorganisms or microbes include fungi, bacteria, and viruses. Fungi and some bacteria form
spores. Mold is minute fungi growing on moist organic substance. According to Kowalski and Bahnfleth
(1998), disease-causing microorganisms transmit respiratory irritation and other serious health
pathogens via the airborne route from two kinds of sources: Most contagious pathogens come from
human hosts, and most noncommunicable pathogens come from the environment—the HVAC&R
and building-related environment.
At the ASHRAE Journal Health Building/IAQ 1997 Conference, Dr. Bascom reported that “In
the United States and Australia, asthma rates have nearly doubled in the past 15 years.”
Contagious Respiratory Microorganisms
Microbial Growth
Kowalski and Bahnfleth (1998) noted that most contagious respiratory microorganisms induce their
human host to aerosolize large quantities of infectious bioaerosols by irritation, which causes cough
and sneezing. A single sneeze can generate 100,000 floating bioaerosols, and many carry viable microorganisms.
A single cough typically produces only 1 percent of these bioaerosols but are ten
times more frequent than sneezes. Keeping a distance from the infected person is a simple and useful
method to reduce the risk of being infected.
Spores are characteristically larger and more resistant to impacts that will destroy viruses and bacteria.
According to Kowalski and Bahnfleth (1998), noncommunicable diseases are almost entirely
due to fungal or actinomycete spores and environmental and agricultural bacteria. Spores form the
most important group in noncommunicable diseases. Microbes are commonly present in outdoor
air. Normally, indoor air spore levels tend to be from 10 to 100 percent of outdoor spore levels
when the species mix indoors reflects outdoors and are mostly less than 200 colony forming units
(CFU) per m 3 because of transport from outdoors to indoors due to ventilation, infiltration, and people,
and because of microbial growth indoors. In the California Healthy Building Study from
Godish (1995), a naturally ventilated building had an indoor spore level of about 80 percent that of
outdoors, mechanical ventilated building about 65 percent, and an air conditioned building about 15
percent.
Spores germinate and grow in the presence of water and nutrients—dirt and debris. The key
factor is water. Water may exist inside a building or an HVAC&R system because of the following:
● Water leaks.
● Entrained water droplets.
● Wet coil face velocity exceeding 550 fpm (2.75 m/s).
● Wetted interior surface.
● Negative space pressure, due to improper airflow balance, extracts hot and humid outdoor air
which contacts with the indoor cold surfaces and causes surface condensation.
● Clogged condensate pan and drain system.
● Oversized humidifier or improperly designed direct evaporative cooler.
The result is a “damp building” which is characterized by dampened materials, damp surfaces,
mold, and microbial growth. Legionnella is a kind of pathogen transmitted via airborne route because
the cooling tower water system is not properly treated and maintained.