Preventing Mold Growth in Hospitals
by Mark Skaer ,
November 3, 2008
Consultant Encourages Scrutiny of HVAC Design in Health Care Facilities
Mark Nunnelly will be the first to tell you that there are no guarantees to
yield a zero mold growth in a health care facility. To prove his point, all
the president of Nunnelly & Associates, based in Birmingham, Ala., has to do
is turn to American Society of Heating, Refrigerating and Air-Conditioning
Engineers’ (ASHRAE’s) HVAC Design Manual for Hospitals and Clinics,
which states, “health care facilities are environments of controlled
In other words, the hazards — molds, bacteria, viruses, etc. — are there.
The challenges, maintains Nunnelly, is controlling these hazards to a level,
as he put it, “deemed safe for the facility’s occupants, patients, and
“Oftentimes, the root causes of mold-related problems in health care
facilities can be traced back to shortcomings with the initial facility
design,” said Nunnelly, who specializes in humidity control consulting.
“These shortcomings often pertain to the building structure itself — the
building envelope, the building materials used, etc. Other design
shortcomings, however, are often related to the HVAC system and its ability,
or lack thereof, to control the patient care environment to conditions that
are not conducive to mold growth.”
Even if all of the bases were covered properly during the design phase, and
the facility was built in accordance with the design standards and
documents, mold can often still be found in a building because of the way in
which the building and its HVAC system are being operated and maintained, he
In Nunnelly’s estimation, many design issues in health care facilities can
lead to a lack of moisture control within a hospital that can potentially
lead to mold problems. These issues could be building envelope- and
construction-related, such as poor or improper insulation leading to wall
surface condensation. The problems might be improper roof drainage, leaky
window and door systems, improper installation of vapor retarders, or any
number of envelope-related issues, he said. It might very well be related to
having too much water and/or humidity entering the building during the
Zeroing in strictly on the HVAC system’s design and operation, Nunnelly
believes there are a number of issues that must be resolved before the
proper HVAC system can be designed, whether it be intended for the surgical
suite, the patient rooms, or the administration offices.
“Initially, the proper ambient design conditions must be selected,” he said.
“Too often, only the peak cooling design conditions are considered for
sizing the capacity requirements of the system. These ambient conditions are
listed in the ASHRAE Handbook — Fundamentals as the dry bulb
temperatures with mean coincident wet bulb temperatures, representing
conditions on hot, mostly sunny days. These conditions are used in sizing
cooling equipment, such as chillers or air conditioning equipment for
“In some climates, this might be satisfactory. However, in geographic areas
known for higher humidity levels, considering only this cooling condition
might not be sufficient.”
According to Nunnelly, extreme dew point temperature conditions may occur on
days with moderate dry bulb temperatures, resulting in high relative
humidities and peak absolute moisture loads from the weather. He said these
values from tables found in the Fundamentals handbook are useful for
humidity control applications, such as desiccant cooling and
dehumidification, cooling-based dehumidification, and fresh air ventilation
“These values can also be used as a checkpoint when analyzing the behavior
of cooling systems at part-load conditions, particularly when such systems
are used for humidity control as a byproduct of temperature control,” he
said, noting that the psychrometric chart for Memphis, Tenn. (see Figure 1),
highlights what a significant impact the selected ambient extremes can make.
As seen in Figure 1, if the space were to be maintained at a condition of
75˚F dry bulb (db) and 50 percent relative humidity (rh), the ventilation
air being introduced during the extreme dew point condition would require
much more dehumidification capacity than the ventilation air at the extreme
cooling condition, he explained. This means almost 52 percent more
dehumidification capacity is required just to deliver the ventilation air at
a space “neutral” humidity level.
“Not considering the more difficult peak dew point conditions of the ambient
air is one of the major reasons that HVAC systems in the hospitals cannot
adequately control the humidity during much of the year,” said Nunnelly.
DEALING WITH COOLER CONDITIONS
Figure 1: Psychrometric chart for Memphis, Tenn.
Nunnelly maintains that another reason humidity and condensate problems are
being seen in hospitals, particularly within surgical suites, is that the
rooms are being kept at cooler conditions than for that which the engineer
designed the HVAC system. Most of the time only the general requirements
from the American Institute of Architects (AIA) guidelines are considered
for the surgical environment’s temperature and humidity range, for example
68°db and 50 percent rh.
“However, nothing in these guidelines shall be construed as precluding the
use of temperatures lower than those noted when the patients’ comfort and
medical conditions make lower temperatures desirable. Some surgeons may
require room temperatures that are outside of the indicated range,” he said.
“All operating room design conditions shall be developed in consultation
with surgeons, anesthesiologists, and nursing staff. The designer must ask
the end users of the facilities what the anticipated space conditions will
be.” If the HVAC system had only been designed to maintain the space at the
higher room temperatures, Nunnelly said then once the room’s temperature is
actually lowered (e.g., 60˚ to 62˚db), then the HVAC system’s capacity would
very likely not be able to reach the necessary lower absolute moisture
levels for the space.
“If this occurs, the moisture in the air will begin to condense on the
cooler surfaces of the ceiling supply air diffusers, the suspended fixtures,
and even on the surgical equipment and tables,” he said.
“Condensate raining from the ceiling, after being in contact with the dirt,
dust, and mold spores is certain to cause concerns regarding the sterility
of the surgical environment.”
He noted that this same phenomenon can happen in other parts of the
hospital, including kitchen and dining, labs, and even patient rooms.
However, the surgical environment is generally the more common area of
concern, he said.
“This is the primary reason the desiccant-based dehumidification systems are
becoming much more of a standard in the surgical suites,” he added.
Nunnelly’s bottom line is this: Proper selection, sizing, and application of
the HVAC systems are essential to ensuring the humidity is controlled to a
level to where there is minimal chance the moisture in the air will condense
and provide the mold spores with the moisture it needs to flourish.
Mechanical HVAC systems — including chilled-water systems and/or direct
expansion systems — must be selected with the cooling coils capable of
supplying air at dew points low enough to actually compensate for the
moisture gain within the spaces (i.e., latent gain), he noted.
Nunnelly used the following as an example. If the operating space is to be
maintained with a condition of 60˚ db and 50 percent rh, then a
chilled-water coil only capable of delivering air off the cooling coils at
50˚ to 52˚ (saturated) will not satisfy the need. He said the supply air
would have to be delivered at an absolute humidity level of less than 41.3°
“This would require either a low-temperature chiller or a desiccant-based
dehumidification/cooling system,” said the consultant.
For a facility only trying to maintain the space at 75˚ db and 50 percent rh
(i.e., 55˚ dew point), Nunnelly said the coils must still be capable of
delivering air with a dew point less than 55˚ in order to absorb the space
“If the moisture is not controlled sufficiently by the HVAC system, this
could spell disaster with regard to trying to keep significant mold growth
out of the facility,” he said.
Once the air-handling equipment has been properly sized and selected, care
must be taken to ensure that the supply air ducts, the chilled water lines
(supply and return), and the refrigerant lines are well insulated, he said.
With the cooler air inside the ducts or water inside the chilled-water
piping, Nunnelly warned that condensation can easily form on the outside of
the ducts and/or pipes.
“The dew point temperature of the air surrounding the cooler ducts and pipes
could easily be higher than the surface temperature of the ducts and pipes,”
he said. “Condensation will occur when this happens.”
If the ducts and piping happen to be in the ceiling space, the condensate
can drip onto a surface that is loaded with mold — ceiling tiles, dry wall
boards, insulation, plywood, etc. — “and all of the necessary elements are
there for mold growth,” he said.
HUMIDITY CONTROL MATTERS
In his estimation, once the HVAC system has been selected and properly
sized, serious consideration must then be given to the controllability of
the systems. It is his belief that most HVAC systems are installed with the
primary function of either cooling or heating, with the humidity control
simply as a secondary function of the system.
“This is evidenced by the fact that most HVAC systems are installed only
with a thermostat installed to operate the system,” said Nunnelly.
“Including humidity control devices in the controls package for the HVAC
system is almost nonexistent. How can you control the humidity if it is not
even being measured?”
Also, far too often, the supply and return air ducts aren’t sealed or
insulated properly during construction, he said. On the return side of the
equipment, leaky ducts will draw in far more moisture than the cooling coils
were designed to remove. The result, he said, is a higher than designed and
desired humidity level in the space.
“In an effort to save installation dollars, the return duct is often deleted
from the plans and the interstitial space between the suspended ceiling and
the roof assembly, or the floor assembly above, is used as a return plenum,”
“Little thought is given to the fact that this interstitial space is now
under a substantial negative pressure and this could cause moisture-laden
ambient air to be drawn into the space from an outside wall through cracks
and holes if this space is not sealed adequately. This infiltration air is
completely unconditioned at this point.”
Nunnelly is quick to point out there is more to consider, but all should
start with the basics first.
“The HVAC system, the heart and lungs of the facility, must be selected and
sized properly for its very important task,” he summarized. “And that task
is to control the environment to promote the healthiest conditions possible
for the patients and the health care providers.”
For more information, contact Nunnelly at email@example.com.
Sidebar: Mold Dangers
According to the Environmental Protection Agency (EPA), many mold types are
very opportunistic and will often infect those people with weakened immune
systems (i.e., immuno-compromised individuals). Aspergillus fumigatus,
for example, has been known to infect the lungs of immuno-compromised
individuals. These individuals inhale the mold spores, which start growing
in their lungs.
Trichoderma has also been known to infect immuno-compromised
children. Other health effects can range from simple headaches, sinus, or
breathing problems to more severe problems like skin rashes, bleeding of the
lungs, cancer, and even death.
There is evidence that a direct association exists between exposure to mold
and adverse health effects. Mayo Clinic investigators have recently found
that inhaled fungus and mold trapped in the nasal mucus are a major cause of
sinusitis. There is also other suggestive evidence that there is an
association between exposure to mold and other adverse health effects.
Sidebar: Pay Attention
In addition to design issues, consultant Mark Nunnelly said maintaining a
clean environment, including a clean HVAC system, will pay great dividends
toward minimizing the potential for mold growth in hospitals.
“Food for mold spores is generally in abundance within the facility, but
there is no reason to spoon-feed the mold spores,” said the president of
Nunnelly & Associates. “Keep the air handlers’ filters clean, and keep the
dust and dirt out of the space as much as possible.”
If remodeling projects are underway within the hospital, even if it involves
simply running new cables above a ceiling system, Nunnelly encouraged the
practice of sealing off the construction areas to make certain the airborne
debris is unable to get into the areas that should be clean.
“As a preventive measure, consider engaging the services of an industrial
hygienist to assist,” he suggested.
In an effort to conserve energy, Nunnelly said oftentimes the chiller is
allowed to operate with a slightly elevated leaving water temperature (e.g.,
elevating from 44˚ to 46˚, possibly). While this will indeed save on the
energy bill, and it might be sufficient for keeping the space temperature
under control, “it could fail miserably at controlling the moisture within
the space, especially within the operating rooms,” he said.
He added, “If the humidity within the space should exceed the desired
maximum acceptable level — for instance, 55 percent to 60 percent rh — then
there should be a humidity sensor and transmitter within the space that
could override the temperature controls and then lower the chilled-water
temperature in order to keep the humidity under control.”
In the overall pictures, he said building pressurization is another critical
factor to monitor in a hospital, as it can greatly affect the
controllability of the environment. If the building pressure is allowed to
become negative due to supply filters being loaded, supply fans running too
slow, or return fans running too fast, humid and dirty air can be drawn into
the building through cracks and openings, he explained.
“This air is completely unconditioned and can provide several of the
necessary ingredients to promote mold growth,” he said, meaning moisture,
more spores, and nutrients. “Dynamic pressure differential monitoring must
take place in order to ensure the building is under the necessary positive
He advised also to regularly have a testing, adjusting, and balancing (TAB)
evaluation performed on the facility’s HVAC systems. At the same time,
examine those drain pans.
“It is far too common to see water standing in drain pans due to either
clogged condensate lines, pans that are not sloped, or drain line
connections placed too high above the drain pan’s floor,” he said. “If there
is water standing in the pans, then this water can become a breeding ground
for hazardous bacteria and molds. Much of this standing water will also be
evaporated back into the supply airstream, saturating the supply duct,
making it a prime breeding ground for undesirable hazards.”
Even though some of these issues might be obvious, Nunnelly said these are
some of the more common issues found in the design and operation of the
health care facilities.
“Excessive mold growth has been found in too many instances within the
facilities that should be
the healthiest of them all,” he said. “Greater attention must be paid, in
the design and in the operational phases, to the HVAC system’s capability to
actively control the humidity within health care facilities.”
Publication date: 11/03/2008