Sick building syndrome - the collection of symptoms office workers have traditionally blamed on air-conditioning, photocopiers and dusty carpets - may have another cause: the computers at which they work.

Sick building syndrome - the collection of symptoms office workers have traditionally blamed on air-conditioning, photocopiers and dusty carpets - may have another cause: the computers at which they work.

A flame-retardant chemical used in the plastic casing of computers and VDU screens is emitted as a vapour when they heat up and can cause allergic reactions, scientists say.

Researchers in Sweden, who did tests on 18 computer VDU brands, found more than half produced significant levels of the chemical, triphenyl phosphate, which has been linked with a range of complaints, including itching, blocked noses and headaches.

The test involved taking samples of the air from in front of the computer screen where a user would be sitting and analysing it for presence of the chemical. The researchers found the amount was far higher than normal background levels.

Levels were highest in new computers on the first few occasions they were switched on. But even after 180 days of continuous use - equivalent to two years' work in an office - they were still ten times above the background level.

The chemical comes from a class known as organophosphate esters, which are widely used in electronic equipment and are known to cause the skin condition dermatitis.

The researchers, whose study is published in the journal Environmental Science and Technology, concluded: "Since triphenyl phosphate has a documented contact allergenic effect on humans, use of this compound as an additive in VDU covers may be considered as a risk to human health. The results of this study implicate that emission of triphenyl phosphate from computer VDU chassis may be one of the possible causes of reported skin problems related to VDU work."

To reduce the effects, the researchers recommend that new computers are run for ten days to "bake off" the flame retardant before being used.

The findings add to evidence that low levels of indoor fumes and gases may have a bigger impact than higher levels of pollutants outdoors - because we spend 80 to 90 per cent of our time indoors.

Research by the Institute of Environment and Health at Leicester University found wide variations in pollution levels between homes and in individual susceptibility to pollutants. People with respi-ratory and heart conditions were especially vulnerable.

Some scientists are sceptical on sick building syndrome, however. One study of 4,000 people working in 44 office buildings uncovered a clutch of symptoms, which had some link with the physical environment in which they worked. But there was a stronger link with job satisfaction.

The researchers, Dr Alexi Marmot, an architect, and her husband, Michael Marmot, professor of epidemiology at University College Hospital, London, said the problem was not so much sick buildings as sick organisations with poor management.

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