A study, carried out by German and Austrian researchers, found that working in an operating theatre may be as harmful to the immune system as smoking 11 to 20 cigarettes a day.

Doctors examined blood samples taken from 10 veterinary surgeons and 10 non-surgical vets who were all non-smokers.

The surgeons had been exposed to waste nitrous oxide and isoflurane anaesthetic over eight hours - levels equivalent to those experienced during operations on children.

Gas levels are higher in the operating room when children are operated on because surgeons cannot use the one-way valve device employed during adult surgery to minimise the release of exhaled gases. The device is thought to pose a risk because it could impair the breathing of small children.

The researchers found that chromosomal damage to the white immune cells of the surgeons was significantly higher than in the non-surgeons.

The results have implications for British operating theatre staff.

Recommended limits

Recommended limits for anaesthetic gases in the UK are more than double Germany's and three times higher than those in the United States.

DNA damage occurs in the body every day, but smoking, chronic illness and genetic susceptibility make it harder to repair.

Cumulative damage is associated with an increased risk of cancer.

Writing in the journal Occupational and Environmental Medicine, the study authors, led by Dr Klaus Hoerauf, from the University of Vienna, said: "Even exposure to low concentrations of waste anaesthetic gases leads to an increased risk of genetic damage.

"Whether the genetic damage found could result in cancer or other unfavourable health outcomes remains unclear, provided an otherwise healthy subject has sufficient DNA repair mechanisms and no other health risks."

Anaesthetists unconcerned

Professor Leo Strunin, president of the Royal College of Anaesthetists, said many large scale studies had been carried out into the impact on health of anaesthetic gases during the 1970s when controls over their use were not in place.

He said no firm evidence had been produced that breathing in the gases caused any significant damage to health.

Since the 1970s "scavenging" systems had been introduced to remove most of the gases from the operating room.

Professor Strunin said: "This is a reasonably well constructed study, but I think they have read more into the data than they can reasonably do.

"The study was based on a small group of people and the control group is not necessarily appropriate.

"It could also be that the genetic changes they recorded would occur in other situations, for instance where a job involves stress.