Air pollution linked to lower IVF success

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).