Beware of Legionella bacteria

Legionnaires' disease killed 34 people in 1976 in Pennsylvania. It occurred at a Legionnaires' convention, which, sadly, is how the name arose. After massive studies, the cause was narrowed to a hotel cooling tower. Since then, Legionnaires' disease has been out of the public consciousness, but evidence has now shown that it is not limited to cooling towers.

Legionella bacteria are naturally occurring in almost all freshwater, and every year the U.S. Centers for Disease Control (CDC) reports that 8,000-10,000 Americans are hospitalized after being exposed to these airborne noncontagious bacteria; 5 percent to 30 percent of those people die. Deadly serious as it is, the symptoms resemble flu or pneumonia and, as such, have been historically underreported and underdiagnosed.

CDC says exposure to legionella is highest in the summer and fall, and the most vulnerable people are the elderly, smokers and those with weakened immune systems.

The most common way that humans are exposed to the bacteria is through aspiration (choking), and the most common sources of the bacteria in homes are whirlpool spas and in water used for drinking and bathing.

Where does bacteria thrive?

Legionella thrives in warm water and is not present in air conditioning systems or condensation on car windows, or other hidden dirty places we would expect bacteria. So where does it thrive? In your hot-water tank and in the hot-water system.

The ideal temperature range for the bacteria to grow is 95-115 degrees, and growth will occur at up to 122 degrees. Below 68 degrees, it can survive, but it goes dormant. From 122 to 131 degrees, it will survive, but will not multiply. At 140 degrees, it will die within 32 minutes, and at 151 degrees, it dies within two minutes. The "disinfection range" is 158-176 degrees.

Ironically, law requires that we set water-heater thermostats at 120 degrees. This helps prevent scalding and saves energy. Both admirable goals for sure, but 120 degrees is right in the growth range, and evidence is showing that we could be sacrificing the health of many for the protection of a few. Anti-scald devices can help with the latter, and may be a better solution in the long run.

Radiant heat systems

In our area, many recently built town homes have a newer style heating system that is sometimes referred to as "hydronic" or "radiant" and may be making our bacteria exposure risk greater. A traditional radiant heating system uses a boiler — water in the boiler, pipes and radiators never mixes with the potable water.

Conversely, most of the newer hydronic systems in town houses are what is called an "open system," with a tank or on-demand heater doing double duty as boiler and domestic hot-water supplier.

In an open system, a pump drives water to the buried pipes or wall-mounted convectors in the living spaces. Water flows from the tank through the heating system and eventually is drawn off for domestic use, being replenished continually.

During summer and fall (coincidentally, the same time as our higher rate of legionella infection, and higher incoming temperature of cold water), when no heating is necessary and the pump is not running, water in the heating system stagnates at slightly higher than room temperature in the pipes.

Some of that water may be drawn out, but certainly not all of it.

With open systems, the only protection against legionella we have is a timer that is required to flush the stagnant water from the heating side, where it can then be drawn out on the domestic side. Without it, the water would sit untouched for several months.

I can assure you that a large percentage of these timers and pumps are miswired and do not function from day one. Even when working correctly, a large percentage of homeowners do not know how to operate the timer correctly, or understand the need for it, and just shut the timer off.

Adding to the issue is that most of the open systems are plumbed with a flexible plastic pipe material called cross linked polyethylene (PEX).

As an aside, the newest research is showing that some PEX "B" piping may be having structural failures in local public-housing projects and elsewhere (stay tuned — this does not affect type A or C which is more common).

More directly related to legionella, PEX piping that is lacking an oxygen barrier may be actually encouraging more bacterial growth inside the pipes.

Until the jury returns, so to speak, temperature control is key, as is ensuring the timer on an open system is activating the pump and flushing the system. And silly as it sounds, avoid choking in the shower or anywhere around misting warmer water.

Darrell Hay is a local home inspector. Send e-mail to dhay@seattletimes.com. Sorry, no personal replies.