Chlorinated water is 10 times as likely to carry the legionella bacteria which causes Legionnaires' disease than water which is treated with the disinfectant mono chloramine, researchers have found.

Introducing the disinfectant into the water supply especially in hospitals and other public places would save thousands of lives, and could reduce outbreaks of Legionnaires' disease by 90 per cent.

The bacteria kills up to 40 per cent of people infected, say researchers from the US Centers for Disease Control and Prevention in Atlanta, Georgia.

They investigated the disinfection methods used by 32 hospitals around the US that had suffered outbreaks of Legionnaires' disease and compared them with those methods used by 48 other hospitals.

They found that water containing chlorine only was 10.2 times more likely to contain legionella, a finding backed up by their laboratory tests which showed that the bacteria were frequently found in chlorinated water, but not in water with monochloramine.

Although monochloramine has been available as a drinking water disinfectant since 1916, researchers found that fewer than 25 per cent of US municipalities add it to their water supply.

It is estimated that between 8000 and 18,000 Americans are infected with Legionnaires' disease every year (Lancet, 1999; 353: 272-7).

People who spend several hours in indoor swimming pools with water sprays run the risk of developing granulomatous pneumonitis, or "lifeguard lung".

The problem is caused by contaminated aerosols released from the sprays. Although chlorine can kill the bacteria, researchers found that people still suffered an immune reaction in the lung.

Symptoms included cough, chest tightness and fever. Most people recover after leaving the pool, but a few who are severely affected can suffer persistent breathing problems (Am J Public Health, 1998; 88: 1795-800).