AN anaesthetic gas used in many dental surgeries may reduce fertility in female dental assistants. Nitrous oxide, also known as laughing gas, appears to reduce the probability of conceiving in each menstrual cycle by more than half.

The study, by the National Institute of Environmental Sciences and the National Institute of Dental Research, involved more than 400 female dental assistants in California between 18 and 39 who had become pregnant within the preceding four years. The women were asked how long it took them to become pregnant once they began trying to conceive, and about their exposure to nitrous oxide.

Women not exposed to nitrous oxide took an average of six menstrual cycles to become pregnant. Women who worked five hours or more a week in surgeries where there was poor protection from exposure to the gas took an average of 32 cycles.