Molecule of the Month: Bubbling with health at Blackpool: Germs don't like it, but water companies do. John Emsley examines bleach, the chemical agent with a hundred uses

BLEACH really does kill all known germs, and has been doing so for almost a century. At the beginning of this month, ICI signed a pounds 1m contract with North West Water to supply Blackpool with bleach. The town will add 2,000 gallons of bleach to the 20 million gallons of sewage it pumps out to sea each day through the main outfall pipe which discharges 800m (730 yards) from the famous golden sands.

In future, bathers in the Blackpool briny will be safe from water-borne infections at least.

Bleach made from chlorine was first used to disinfect tap water at Maidstone in 1897 during an outbreak of typhoid, and its success was confirmed when it helped control another epidemic at Lincoln a few years later. Eventually chlorine became the method of purifying drinking water throughout Britain and most of the developed world.

Today it is frowned upon by some environmentalists because it reacts chemically with other matter present in water to form traces of compounds suspected of being carcinogenic. Some UK local authorities forbid the use of bleach in schools and even hospitals, fearing the release of dangerous chlorine fumes.

Bleach is made by bubbling chlorine up a column down which trickles a solution of sodium hydroxide, an alkali.

The two react to form sodium hypochlorite in which the negative hypochlorite half consists of an oxygen and a chlorine atom joined together. This is a strong oxidising agent, which is stable for several months provided it is not exposed to heat, sunlight or metals.

Because it is made from chlorine, ordinary bleach is sometimes misnamed chlorine bleach, to distinguish it from peroxide bleach, which is a solution of hydrogen peroxide. In bleach there is no free chlorine gas as such, although this can be released again from the bleach by a strong acid. The bubbles that rise from bleach when it is being used normally are oxygen.

The largest manufacturer of chlorine gas and bleach in the UK is ICI. It makes a million tons of chlorine each year and turns about 25,000 tons into hypochlorite bleach, according to Matt Kendall, business manager of the company's chloralkali division.

ICI sells most bleach to firms such as Lever, which uses it for household cleaning agents. Bleach is also used industrially to remove ink from recycled paper, to whiten cotton, one of the first uses to which it was put, and to treat water. Water can be chlorinated either with bleach or with chlorine gas itself; in both cases the result is a dilute but highly effective solution of hypochlorite.

Viruses and bacteria are highly sensitive to oxidation, and attack by hypochlorite molecules normally kills them. For this reason hypochlorite will keep water free of germs at very low concentrations and for a long time, which is why it is still preferred over short-lived oxidising agents such as hydrogen peroxide and ozone.

Bleach is ideal for sterilising kitchen surfaces, soiled garments, sinks and lavatories. Thickened bleach is produced by adding a surfactant, which also helps in the cleaning action.

If there is still no substitute for hypochlorite bleach as a disinfectant, why is it being actively discouraged? One reason is its ability to convert organic residues in water into organochlorine compounds, which some environmentalists say are a long-term threat to public health.

Robert Morris, of the Medical College of Wisconsin, Milwaukee, reported last month that there were more cases of bladder and rectal cancer among people drinking heavily chlorinated river water than those drinking lightly chlorinated water from springs and wells.

River water in the US has an average of 50 parts per billion (ppb) of organochlorine compounds; in the UK it is much less than this. Both the US Environmental Protection Agency and the UK authorities set a limit of 100 ppb for chloroform-type chemicals in drinking water.

Some organochlorine compounds have caused cancer among those heavily exposed to them in industry, but at low levels in chlorinated water the risk of them killing anyone is almost negligible. The most common organochlorine residue in water is chloroform, but even at 100 ppb you would consume only a sixth of an ounce of chloroform in a lifetime from this source. (Chlorodyne, a patent cure-all on sale in Britain for more than a century, contained 14 per cent chloroform until a few years ago.)

In 1991, the International Agency for Research on Cancer (IARC), which is part of the World Health Organisation, published an evaluation of the risks posed by organochlorines in drinking water. It concluded that there was insufficient evidence to cause alarm.

Michael Fielding, of the Water Research Centre at Medmenham, Buckinghamshire, who contributed to the IARC report, says that if there are any health risks, they are very low, and have to be offset against much greater health risks associated with drinking unchlorinated water.

Alarmed by the supposed threat of organochlorines, the Peruvian government stopped chlorinating drinking water last year. As a result there was an outbreak of cholera which affected more than 500,000 people and caused 5,000 deaths.

However, organochlorines are not the reason why bleach is discouraged in schools, hospitals and workplaces. Stephen Bailey, chief executive of the National Occupational Hygiene Service, based at Manchester, explains that bleach can be a serious hazard because it may release chlorine gas if used wrongly.

Several people are rushed to hospital each year because of such accidents. Bleach is covered by the COSHH regulations (Control of Substances Hazardous to Health) and is often cited as a common chemical that can be dangerous. Cleaning staff are most likely to be affected if they use a bleach and a descaler together.

Descalers are strong acids that work by dissolving the calcium carbonate that builds up on surfaces, sinks and toilets in hard-water areas. As well as neutralising the scale, they can neutralise the alkali in bleach and make it acid enough to convert hypochlorite back to hazardous chlorine gas. Some descalers even contain hydrochloric acid, which is doubly dangerous because it releases some of its own chloride as chlorine gas. Here the real culprit is not bleach, which has saved millions of lives in its time, but the acid descalers.

Health and safety bureaucrats should learn a little chemistry and ban these, whose use, in any case, is merely cosmetic.

Dr John Emsley is science writer in residence at the department of chemistry, Imperial College, London.

(Photograph omitted)