A hospital has stopped using laughing gas as a painkiller for mothers in labour after staff were found to have been exposed to up to 12 times the safety limit. One obstetrician claimed the high levels present in the air could even cause some women to miscarry. Tests at the Prince of Wales Hospital confirmed suspicions voiced by staff in its obstetrics and gynaecology department that levels of laughing gas - nitrous oxide - in the labour wards were excessive. Pronouncing itself "very concerned", the Hospital Authority said the airborne concentration of the gas in all public hospital wards was now being assessed, although no similar problems had been found in any other labour wards. The Prince of Wales Hospital decided to carry out the tests after staff complained in early June about tiredness and headaches at work. The results showed that 42 out of 48 staff members were exposed to levels of the gas that exceeded the occupational exposure limit of 50ppm (parts per million of inhaled air) set by the Labour Department. The levels ranged from slightly above the limit to 12 times above the limit. Medical experts, including Chinese University medical professor Raymond Wong Siu-ming and former Medical Association president Dr Choi Kin downplayed the health risks. "According to previous studies and medical papers, the health risk that the gas poses to the staff should not be very high," Wong said. But one obstetrician and gynaecologist, Dr Kun Ka-yan, said a high intake of the gas for those in early pregnancy - 12 weeks or less - might put them at risk of miscarriage. Kun said it was possible for the painkiller to leak while being administered to a patient and accumulate in the room. A Hospital Authority spokesman said a panel of experts would be set up shortly to investigate the cause and advise on the best way to proceed. Other pain-relieving methods were now in use at the Prince of Wales, the spokesman said. The hospital had also installed extra extractor fans and adjusted the ventilation system to improve air flow in the places affected. The spokesman added that in an effort to stop gas leakage, the hospital would meet with its nitrous oxide supplier to find ways to improve the fitting of the face mask and tubing through which the gas is administered. The use of laughing gas would be resumed when test results showed it was safe, and the airborne concentration of the gas would be closely monitored. Experts in anaesthesiology quoted in a government statement said the use of laughing gas to relieve pain during labour was safe and effective for mothers and their foetuses. "Although some studies suggested that prolonged occupational exposure to high levels of nitrous oxide may increase incidence of spontaneous miscarriage of the staff in the workplace, there is limited supporting evidence," they said.