Exposure to air pollution has been linked to a lower chance of IVF success, a study has found. Nitrogen dioxide, fine particulate matter and ozone posed a particular risk, according to the researchers from Pennsylvania State University College of Medicine, US.

Dr Richard Legro and colleagues followed 7403 women who were undergoing their first IVF cycle, between 2000 and 2007, at rural, suburban and urban fertility clinics in Hershey, Rockville, Maryland or New York City. They used local air quality monitoring data from the US Environmental Protection Agency to estimate the daily air pollution levels near the women's homes and fertility clinics. They then related the effects of these pollutants at each stage of the IVF cycle and during pregnancy to the women's pregnancy outcomes.

Overall, 36 per cent of the women had a successful pregnancy after their first IVF treatment. Higher levels of nitrogen dioxide near a woman's home or the fertility clinic, and at any stage of the IVF cycle or pregnancy, were always associated with a lower chance of pregnancy and live birth.

Higher levels of ozone at the time of embryo implantation into the womb decreased the chances of a successful pregnancy, but high levels at the time of ovulation were found to increase the chances of pregnancy and live birth. The authors speculated that, because ozone levels are typically high when nitrogen dioxide levels are low, the latter result might reflect the fact that lower nitrogen dioxide levels during ovulation have a positive effect on IVF success rate. In fact, when they analysed the combined effect of nitrogen dioxide and ozone on pregnancy outcomes, the researchers found that nitrogen dioxide still had a strong negative effect, but the effect of ozone was no longer significant.

Higher levels of fine particulate matter (PM2.5) at the fertility clinic site were also associated with lower rates of conception, but not with live birth rates. Larger particulate matter (PM10) and sulphur dioxide did not have an effect on pregnancy outcome.

Other studies have demonstrated a link between exposure to air pollution during pregnancy and risk of preterm birth and low birthrate. By focusing on IVF pregnancies, this study, published in the journal Human Reproduction, was able to link the daily air pollution data to specific stages of the pregnancy process - from ovulation to fertilisation, embryo implantation into the uterus, conception and pregnancy.

Dr Legro stressed that the study didn't demonstrate a mechanism of action of air pollution. Duanping Liao, professor of epidemiology at Pennsylvania State College of Medicine, hypothesised that air pollution might affect pregnancy by causing chronic inflammation in the body or by increasing the risk of blood clotting - factors that the researchers linked to air pollution in their earlier research.

The main source of air pollution is fuel combustion for motor transport, which produces nitrogen dioxide, fine particulate matter, carbon dioxide and other pollutants. Ground-level ozone, the primary constituent of smog, is formed from in a chemical reaction with sunlight from nitrogen dioxide and volatile organic compounds (VOC).