Friday March 6, 2015
Legionellae are rod-shaped, gram negative bacteria. Over 40
species of Legionella have been identified; L. pneumophila appears
to be the most virulent and is associated with approximately 90% of
cases of Legionellosis. The risk of acquiring Legionnaires' disease
is greater for older persons and for those who smoke tobacco or
have chronic lung disease. Persons whose immune system is
suppressed by certain drugs or by underlying medical conditions
appear to be at particularly high risk.
Legionellae bacteria are commonly present in natural and
man-made aquatic environments. The organism is occasionally found
in other sources, such as mud from streams and potting soils. In
natural water sources and municipal water systems, Legionellae are
generally present in very low or undetectable concentrations.
However, under certain circumstances within manmade water systems,
the concentration of organisms may increase, a process termed
"amplification." Conditions that are favorable for the
amplification of legionellae growth include water temperatures of
25-42 degrees Celsius (°C) (77-108 degrees Fahrenheit), stagnation,
scale and sediment, biofilms, and the presence of amoebae.
Legionellae infect and multiply within several species of
free-living amoebae, as well as ciliated protozoa. The initial site
of infection in humans with Legionnaires' disease is the pulmonary
macrophage. These cells engulf Legionellae, provide an
intracellular environment that is remarkably similar to that within
host protozoa, and allow for multiplication of the bacterium.
Growth in nature in the absence of protozoa and/or in the absence
of complex microbial biofilms has not been demonstrated.
There is an indication that growth of Legionella is influenced
by certain materials. Natural rubbers, wood, and some plastics have
been shown to support the amplification of Legionella, while other
materials such as copper inhibit their growth. Generally,
Legionella thrive in diverse, complex microbial communities because
they require nutrients and protection from the environment.
Controlling the populations of protozoa, and other microorganisms
may be the best means of minimizing Legionella.
Transmission of Legionnaires' Disease
Most data on the transmission of Legionnaires' disease are
derived from investigations of disease outbreaks. These data
suggest that, in most instances, transmission to humans occurs when
water containing the organism is aerosolized in respirable droplets
(1-5 micrometers in diameter) and inhaled by a susceptible host. A
variety of aerosol-producing devices have been associated with
outbreaks of Legionnaires' disease, including cooling towers,
evaporative condensers, showers, whirlpool spas, humidifiers,
decorative fountains, and a grocery store produce mister.
Aspiration of colonized drinking water into the lungs has been
suggested as the mode of transmission in some cases of
hospital-acquired Legionnaires' disease. Numerous investigations
have demonstrated that cooling towers and evaporative condensers
have served as the sources of Legionella-contaminated aerosols
causing outbreaks of community- and hospital- acquired infection. A
number of outbreaks of Legionellosis associated with cooling towers
and evaporative condensers have occurred after these devices have
been restarted following a period of inactivity. Shower heads and
tap faucets can also produce aerosols containing legionellae in
droplets of respirable size.
Common amplifiers (growth factors) associated with building
water systems, including the treatment recommended to minimize the
risk of Legionellosis, are discussed below.
Potable Water Systems
Factors associated with the plumbing system that may influence
the growth of legionellae are as follows:
- Chlorine concentration;
- Temperature; and
- Plumbing system design and materials
Municipal potable water supplies are generally chlorinated to
control the presence of microorganisms associated with sewage.
Legionellae are more tolerant of chlorine than many other bacteria,
and may be present in small numbers in municipal water supplies.
Potable water can also support legionellae growth if the water
temperature is in the range of 77-108°F. Plumbing design and
materials also influence the growth of legionellae.
Growth of legionellae may occur in portions of the system with
infrequent use, in stagnant water, and in portions of the system
with tepid temperatures. Growth may also occur in dead-end lines,
attached hoses, shower nozzles, tap faucets, hot water tanks, and
reservoirs. Rubber washers and fittings, including water hammer
arrestors and rubber hoses with spray attachments, have been shown
to provide sites for growth of legionellae. Organic compounds
leached from plumbing materials may contribute to growth of
heterotrophic bacteria, including legionellae.
Contaminated potable water sources present the greatest risk
when dispersed into the air in a very small droplet size (less than
5 micrometers) that can be inhaled deeply into the lungs. Actions
that may generate small droplets are those that break up the water
stream, i.e., shower nozzles, aerators, spray nozzles, water
impacting on hard surfaces, and bubbles breaking up. Both dead and
living microorganisms, biofilms, and debris may provide nutrient
sources for legionellae growth. When legionellae are found in
plumbing systems, it is common to detect the microbes in the
sediment in hot water tanks, and in peripheral plumbing fixtures
that accumulate sediment.
Where practical in high-risk situations, cold water should be
stored and distributed at temperatures below 20°C (68°F), while hot
water should be stored above 60°C (140°F) and circulated with a
minimum return temperature of 124°F. However, great care should be
taken to avoid scalding problems. One method is to install preset
thermostatic mixing valves. Where buildings cannot be retrofitted,
periodically increasing the temperature to at least 66°C (150°F) or
chlorination followed by flushing should be considered. Systems
should be inspected annually to ensure that thermostats are
functioning properly. Where practical in other situations, hot
water should be stored at temperatures of 120°F or above. Those hot
or cold water systems that incorporate an elevated holding tank
should be inspected and cleaned annually. Lids should fit closely
to exclude foreign materials.
Where decontamination of hot water systems is necessary
(typically due to implication of an outbreak of Legionellosis) the
hot water temperature should be raised to 160~170°F and maintained
at that level while progressively flushing each outlet around the
system. A minimum flush time of five minutes has been recommended
by the Center for Disease Control. However, the optimal flush time
is not known and longer flush times may be necessary.
Emergency Water Systems-safety Showers, Eye Wash Stations, And
Fire Sprinkler Systems
These systems are generally plumbed to the potable water system,
have little or no flow with resulting stagnant conditions, and may
reach temperatures warmer than ambient. The presence of
legionellae, heterotrophic bacteria, and amoebae in these systems
has been documented. When the devices are used, aerosolization is
Safety shower and eye wash stations should be flushed at least
monthly. In the case of fire sprinkler systems, it is recommended
that fire-fighting personnel wear protective respiratory gear and
that non- firefighting personnel exit the burning area. Appropriate
precautions should be taken when checking the operation of fire
Architectural Fountains And Waterfall Systems
In these systems, water is either sprayed in the air or cascades
over a steep media such as rocks, and then it returns to a man-made
pool. These systems are sometimes operated intermittently with
on-time often scheduled only during certain time periods. Systems
that are operated intermittently may encourage greater
Because of the high temperature ranges needed for proliferation
of legionellae bacteria, outdoor fountains and pools in hotter
climates, and indoor fountains and pools subject to sources of heat
may be susceptible to legionellae growth. Temperature increases may
be facilitated by heat from pump/filter systems themselves.
Intermittent operation may also create situations where temperature
increases occur in isolated areas of the system. Fountains are
subject to contamination from a wide variety of potential nutrient
sources, including materials scrubbed from the air and returned to
the pool with the falling water droplets as well as organic and
inorganic materials dropped, thrown, or blown into the pool.
The recommended treatment for fountains includes:
- Regular cleaning is recommended; and
- Use of filters should be considered; however, systems with a
small water volume may be drained, and refilled with fresh water
every few weeks in lieu of filtering.
Microbial fouling control is important, especially where the
conditions are such that there are significant periods of time when
the temperature of the fountain water is in the range that is
favorable for the amplification of legionellae growth. When
biocidal treatment is employed for microbial fouling control, the
biocide must be registered with the United States Environmental
Protection Agency (USEPA) for use in decorative fountains.
Cooling Towers Including Fluid Coolers (closed-circuit Cooling
Towers) And Evaporative Condensers
Evaporative heat rejection equipment such as cooling towers and
evaporative condensers have been implicated in numerous outbreaks
of Legionnaires' disease, and studies have shown that detectable
levels of legionellae are present in many of these devices.
A cooling tower is an evaporative heat transfer device in which
atmospheric air cools warm water, with direct contact between the
water and the air, by evaporating part of the water. Air movement
through such a tower is typically achieved by fans, although some
large cooling towers rely on natural draft circulation of air.
Cooling towers typically use some media, referred to as "fill," to
achieve improved contact between the water and the cooling air. The
typical temperature of the water in cooling towers ranges from 85°F
to 95 °F although temperatures can be above 120 °F and below 70°F
depending on system heat load, ambient temperature, and system
Closed-circuit cooling towers and evaporative condensers are
also evaporative heat transfer devices. Both are similar to
conventional cooling towers, but there is one very significant
difference. The process fluid (either a liquid such as water, an
ethylene glycol/water mixture, oil, etc., or a condensing
refrigerant) does not directly contact the cooling air. Rather, the
process fluid is contained inside a coil assembly. Water is drawn
from the basin and pumped to a spray distribution system over the
coil assembly while the cooling air is blown or drawn over the coil
by fans. Removal of heat is achieved by evaporating part of the
water. Water temperature in closed-circuit cooling towers and
evaporative condensers is similar to that in cooling towers.
Cooling towers and evaporative condensers incorporate inertial
stripping devices called drift eliminators to remove water droplets
generated within the unit. While the effectiveness of these
eliminators can vary significantly with the design (new
state-of-the-art eliminators are significantly more efficient than
older designs) and the condition of the eliminators, it should be
assumed that some water droplets in the size range of less than 5
micrometers leave the unit. In addition, some larger droplets
leaving the unit may be reduced to 5 micrometers or less by
Because cooling towers and evaporative condensers are highly
effective air scrubbers and because they move large volumes of air,
organic material and other debris can be accumulated. This material
may serve as a nutrient source for legionellae growth. Diverse
biofilms, which can support the growth of legionellae, may be
present on heat exchanger surfaces, structural surfaces, sump
surfaces, and other miscellaneous surfaces.
The key recommendations are that the system be maintained clean
and that a biocidal treatment program be developed and implemented.
It is also recommended that the services of a qualified water
treatment specialist be used to define and oversee the treatment.
Keeping the system clean reduces the nutrients available for
Legionella growth. Regular visual inspections should be made for
general cleanliness. The cold water basin of the unit should be
cleaned when any buildup of dirt, organic matter, or other debris
is visible or found through sampling. Mechanical filtration may be
used to help reduce these solids. Strainers, cartridge filters,
sand filters, centrifugal-gravity-type separators, and bag-type
filters can be used to assist in removal of debris. The drift
eliminators should also be inspected regularly and cleaned if
required or replaced if deteriorated or damaged.
An effective water treatment program allows more efficient
operation due to lower fouling, a longer system life due to
decreased corrosion, and safer operation of the system due to
reduced chances of microbial exposure to the public.
Control of scaling and corrosion is necessary in many water
treatment programs. Scale such as calcium carbonate and/or other
minerals containing silica, magnesium, and phosphate may
precipitate onto heat exchanger and piping surfaces. Scaling can be
minimized by use of inhibitors containing phosphonates, phosphates,
and polymers to keep calcium and carbonate and other minerals in
solution. Corrosion can be minimized by the use of inhibitors such
as phosphate, azoles, molybdenum, and zinc. Scale and corrosion
inhibitors are effective if microbial fouling and biofilm
development are properly controlled. Microbial fouling can
influence scaling and corrosion processes and can affect the
performance of inhibitors. Microbial biofilms on surfaces can
consume certain inhibitors (such as phosphates, phosphonates, and
azoles), prevent access of inhibitors to surfaces, create localized
oxygen-depleted zones, change the pH near surfaces, and accumulate
or trap deposits onto surfaces.
Microbial fouling is controlled by the use of biocides, which
are compounds selected for their ability to kill microbes while
having relatively low toxicity for plants and animals. In the USA,
the Environmental Protection Agency has regulatory authority for
biocides and requires registration of all biocides. In addition,
registration is required in each state where the biocide will be
distributed. Non-oxidizing biocides include many organic compounds
registered with the USEPA for cooling water applications. These
biocides function in a number of ways, including reacting with
intracellular enzymes, solubilizing cell membranes, and
precipitating essential proteins in microbial cell walls. Properly
used, non-oxidizing biocides are effective for control of the
microbial fouling process in cooling water systems. It is generally
good practice to regularly alternate the biocides used for a
cooling water system to avoid the selection and growth of resistant
strains of microbes. The alternating biocide approach has been
emphasized with the rationale that the population that survives the
biocide treatment one week is susceptible to the alternate biocide
a week or two later. Alternating the dose and frequency of the same
biocide is also used to achieve this goal.
Equally important to controlling scale and corrosion is keeping
the system clean and free of sediment. Common sources of sediment
include materials scrubbed from the air (dirt, leaves, paper,
kitchen or other organic exhaust), precipitated solids (calcium,
magnesium, carbonate silica), and corrosion products (rust).
Microbes including bacteria, protozoa, algae, and (infrequently)
fungi can grow in cooling systems and use the above materials as
nutrients. Consequently, it is desirable to either prevent the
entry of the material into the system or to remove it from the
When the system is to be shut down for a period of more than
three days, it is recommended that the entire system (cooling
tower, system piping, heat exchangers, etc.) be drained to waste.
When draining the system is not practical during shutdowns of short
duration, the stagnant cooling water must be pretreated with an
appropriate biocide regimen before tower start-up.