Legionella found in home water heaters

By MARTIN JOHNSTON health reporter

The bug that can cause the potentially fatal Legionnaires' disease may be living in the hot-water systems of 12 per cent of homes with electric water heating.

In the first study of its kind in this country, researchers at the Institute of Environmental and Scientific Research tested hot water and took tap and shower-head swabs from 100 Wellington homes.

Legionella bacteria were confirmed in six homes and presumed to be present in six more.

Their findings, published in the Medical Journal, are based on 1998 research financed by the Building Industry Authority.

The researchers looked only at homes with electric water tanks, saying they may be more likely than gas ones to harbour the bugs. This is because the elements are suspended in the water, possibly permitting a cooler area of water and sediment at the bottom.

Gas cylinders are heated from below.

But an industry source said the electric elements were generally no more than 200mm from the lowest point. Water entered at the base and left at the top, so any legionella leaving the tank would go through the hottest part.

Legionella can survive temperatures up to about 60 deg, which is why the Building Code requires hot water in new housing to be stored at 60 deg or more.

But the temperature must be cut, usually by cold-water mixers, to 55deg or less at bathroom taps to reduce the risk of scalding.

The study found the water temperature at the tap in many systems, including four that were contaminated, was well over 60deg.

Legionnaires' disease is a form of pneumonia caused by one of the legionella bacteria. Symptoms include aches, diarrhoea, a dry cough, high fever, shaking, chills, thick phlegm and sometimes drowsiness.

It is named after an outbreak that killed 29 members of the American Legion attending a convention in a Philadelphia hotel in 1976.

More than 51 cases were reported in New Zealand last year and our rate is more than double that of Europe.

The disease kills about 12 per cent of previously healthy people who catch it. Outbreaks have been linked with the water or air-conditioning systems of large buildings, although most cases here are caused by soil or potting mix.