The risk of contracting Legionnaires' Disease from residential plumbing is rare, but some inexpensive measures can be taken.

Although large piping networks are more conducive to the growth of Legionella bacteria (legionellae) than smaller systems, plumbing in houses and apartments still can harbor the bacteria, and have indeed been implicated in cases of Legionnaires' disease.

Biofilm is a primary factor associated with legionellae in plumbing systems. Biofilm is a slimy coating that is formed as microbes attach to underwater surfaces (e.g., the inside of a pipe). Biofilm and scale that form in valves and fittings and on pipe walls not only feed legionellae, but also protect it from hot water and chlorine. Fragments of the outer layer of biofilm can flake off and flow into the water, releasing potentially high levels of legionellae and other microbes into the system.

Although environmental studies have shown that a significant percentage of home plumbing systems may be contaminated with legionellae, epidemiologic studies have shown that the risk of contracting Legionnaires' disease in the home is probably minimal, provided that the legionellae levels are low and the persons exposed are nonsmokers in generally good health.

Preventive measures involve designing, operating, and maintaining plumbing systems to minimize legionellae growth. Most preventive measures seek to: 1) avoid water temperatures in which legionellae grow best; 2) eliminate scale, sediment or biofilm; 3) avoid materials that provide food sources for legionellae; or 4) minimize water stagnation.

Some measures cost nothing and should be implemented out of good sense. Expensive measures could be a waste of money for healthy, nonsmoking adults who are at low risk of contracting Legionnaires' disease. On the other hand, high-risk individuals should consider taking every reasonable precaution.

The following risk-reduction options apply only to small plumbing systems (e.g., those used for houses and other small buildings) rather than apartment buildings or other large buildings that have central water heating systems.

Hot Water Temperatures

In large buildings, keeping water at 140 degrees F in the plumbing system will not always control legionellae, but high-temperature water has been effective in single-family residences.

At these temperatures, however, scalding is a big risk. Do not set the water heater at 140 degrees F if the house is occupied by children or others who may open a hot water faucet unaware of the risk of scalding.

Check the water temperature at faucets farthest from the water heater to ensure that temperature remains sufficiently high as it travels through the system. Also check faucets nearest to the water heater to make sure that the water is not too hot (temperatures should not exceed 144 degrees F).

Replacing standard faucet and shower fixtures with thermostatic mixing valves may allow a homeowner to maintain hot water at 140 degree F from the water heater to the mixing valve, but deliver water at a lower temperature to reduce the risk of scalding.

Water Heaters

Studies indicate that homes with gas water heaters are less likely to have legionellae than are homes with electric water heaters. Of 211 homes surveyed in the Quebec City area, legionellae were found in none of the 33 houses with gas water heaters but were found in 69 (33 percent) of the 178 houses with electric water heaters.

Gas heaters benefit from the location of their heat source below the water tank. Thus, the bottom of the tank, where sediment accumulates, is sufficiently hot to prevent legionellae growth. In contrast, most electric units have heating elements on the side of the tank, so the sediment at the bottom is cool enough to allow legionellae growth. Electric water heaters also tend to have lower temperatures than do gas heaters.

Water heaters should be drained and cleaned annually to remove sediment and scale. Excessive scale buildup cannot be removed by flushing, so it is important to do the full cleaning at least once a year.

Water Softeners

Water softeners may indirectly help minimize legionellae by reducing scale, biofilm and iron.

Softeners cannot be relied upon to lower legionella risk by reducing calcium and magnesium in the water. One study indicated an association between legionella and calcium and magnesium in hospitals, but in two separate studies of Pittsburgh homes, no association was found between legionella and levels of calcium and magnesium.

More research is needed to determine if water softening tanks provide a habitat for legionellae growth.

Water Filters

Don't rely on water filters to control legionellae. A 1-micron filter will remove some types of bacteria, but more research is needed. The typical home water filter will not block legionellae. What's more, legionellae and other bacteria can grow well on carbon filters and on the sediment that builds up in other types of filters, so it's important to clean or replace filters at proper intervals.

Plumbing Repairs

A study of 146 adults showed a higher risk of contracting Legionnaires' disease shortly after home plumbing alterations or repairs were made. Repair work can loosen biofilm from piping and plumbing fixtures, releasing high levels of legionellae into the water.

Consider conducting a heat-flush procedure after making plumbing repairs or changes. The procedure can minimize the risk by killing and flushing the legionellae that were released during the repairs. New piping installed as part of the repairs should be heat flushed prior to use to remove any contaminants that accumulated in the piping between the time it was manufactured and installed.

A heat-flush procedure is generally performed by flushing every tap for at least 30 minutes with 158-degree F water. If the water heater does not have the capacity to flush all taps simultaneously (most home water heaters don't), flush one or two taps at a time for 15 minutes, beginning with those closest to the water heater and ending with the farthest taps. If the piping is old or in poor condition, heat the water to 140 degrees F instead of 158 degrees F. Be sure that every person in the house is aware of the scalding risk and that children or other potential scald victims are not present.

Ultraviolet Treatment

UV may be appropriate in homes occupied by "immunocompromised" persons. The unit should be installed on the incoming water line to treat all water used in the house (i.e., rather than treating only the kitchen faucet).

An ultraviolet unit effectively kills any legionellae in the water that flows through it, but is not effective for large building plumbing systems already contaminated with legionellae. By the time the legionellae-free water leaving the unit reaches distant points in a large piping system, it will be re-contaminated with legionellae growing in scale and biofilm. Thus, UV units are best suited for small systems that are free of scale and biofilm (e.g., a new house).

Hot Water Recirculation

Installing a hot water recirculation line in new home systems may lower legionella risk. A European study found that homes with hot water recirculation systems were less susceptible to legionellae growth than were homes without them. Be sure to extend the recirculation line to the point farthest from the water heater.

Water Sampling

Testing water for legionella is expensive, generally $100-$150 per sample. Routine sampling of home water is unnecessary, particularly for healthy nonsmoking adults. However, if a member of a household contracts Legionnaires' disease, a test may be beneficial to rule out the home water supplies as the source of contamination. If legionellae are found, disinfecting the plumbing system should be considered.