During the UK's national Swimfit week in June I took my 14-month-old son to our local pool. It was so dirty I refused to take him in. Sunlight streaming in through the window illuminated a cloud of swirling matter of what looked like particles of feces and coagulated mucous. After blaming the problem on exceptional crowding that week, plus the absence of the staff member responsible for filtration, the manager gave me a refund.

But esthetically displeasing dirty water in commercial swimming pools and spas is not as big a health hazard as it looks unless the disinfection process isn't working properly, that is.In most public swimming pools this is automatically monitored; nevertheless, in 1984 the US health department found that 90 per cent of 30 commercial spas investigated in Portland, Oregon, were not adequately chemically maintained and over half were contaminated. In a 1985 study of 50 public spas in San Diego, 24 per cent were a source of parasitic infection and more than half were under-chlorinated and contained unhealthy bacteria. In 1987, 48 cases of severe gastroenteritis in Wilmington, North Carolina, were traced back to a local pool/spa. In the same year, two people died from Legionnaire's disease contracted from a contaminated whirlpool spa in Vermont.

These incidents are probably only the tip of the iceberg, as most cases go unreported and are unlikely to be traced back to a swimming pool, anyway.

Water contamination is caused by chlorine, the usual disinfectant, reacting with organic matter such as sweat, urine, blood, feces, mucous and skin cells to form chloramines. These can cause eye and throat irritation, and it is the chloramines which produce that strong odour present in many swimming pools, rather than the chlorine itself. The more people in the pool, and the more chlorine added, the greater the number of chloramines.

The only way to prevent the build-up of chloramines is to constantly dilute the contaminated water with clean water. In Germany, where they have a strict litres-of-clean-water-per-bather dilution policy, public pool and spa-water quality either meets or exceeds U S Environmental Protection Agency (EPA) drinking water standards (Pool and Spa News, October 7, 1985). Germany also uses ozone in its pool disinfection processes, which reduces the amount of chlorine needed.

Such high standards are unlikely to be achieved in the UK due to compulsory competitive tendering, which means pool maintenance contracts are awarded to those who can provide the most "cost-effective" service. Topping up chlorine levels is far cheaper than replacing and reheating water.

Besides producing chloramines, chlorine and organic matter also react together to produce chloroform, a known carcinogen in animals. Chloroform has been classified by the International Agency for Research on Cancer (IARC) and the EPA as a probable carcinogen in humans. As with chloramines, the higher the levels of chlorine and organic matter, the higher the level of chloroform in the water and surrounding atmosphere. Swimming for one hour in a chlorinated pool is said to far exceed chloroform exposure from all other environmental sources (Toxicology Letters, 1994; 72: 375-80).

Very little research has been done into the effects of chloroform on humans. Nevertheless, it appears that chloroform can cause vomiting, fainting, dizziness and irritation of the eyes, suppression of the central nervous system, liver cancer and damage to the reproductive system, according to Dr Paul Johnston, Principal Scientist at the Greenpeace Research Laboratory at Exeter University. Johnston found in one of his own studies that in younger competition swimmers, an enzyme related to kidney function was higher than normal, indicating possible kidney damage, which could get worse in later life.

Another study actually found evidence of possible kidney damage in younger swimmers. It concluded that "swimming in chlorinated pools at an early stage of development may have undesirable consequences later in life" (Toxicology Letters, 1994; 72: 375-80.)

Blood and exhaled air samples from swimmers in training have shown substantially elevated levels of chloroform, which enters the body through the respiratory system, skin and gastrointestinal tract. Swimmers who breathe under stress for a long time are most at risk, as chloroform vapour is concentrated immediately above the water surface. (Archives of Environmental Health, May/June 1990, and July/August 1993).

It stands to reason that exposure to chloroform in the air is going to be greater in confined spaces. A Korean study did indeed find that air chloroform levels in indoor swimming pools were between 70 and 240 times higher than air chloroform levels in outdoor pools (Journal of Exposure Analysis and Environmental Epidemiology, 1994; 4 (4): 491-502).

If you're reluctant to give up the health benefits of swimming, as I am, the best advice is to find a pool which uses the ozone method of disinfection.

Ozone is more commonly used in pools built in the last 10 years, many aimed at families. Otherwise, Dr Philip Penny, a member of the UK's Pool Water Treatment Advisory Group and medical advisor to the Amateur Swimming Association, advises taking note of a pool smell first. "You should only be able to smell a chlorinous odour for a minute or two before your nose gets used to it," he says. "If you can smell it for any longer than that, the pool is probably too heavily contaminated."