Air pollution increases the risk of miscarriage by as much as smoking, a study shows.
Researchers found raised levels of nitrogen dioxide – produced by burning fossil fuels, particularly diesel – raised the chances by 16 per cent.
Long-term studies have already shown air pollution harms foetuses by increasing the risk of premature birth and low birth weight.
But the new US study is the first to assess the impact of short-term exposure to air pollution on unborn children.
Dr Matthew Fuller, who led the research at the University of Utah, said: ‘It’s pretty profound.
'Many of us think there is an effect on our health, but to find out there are actual effects on unborn children is very upsetting.
‘If you compare that increase in risk to other studies on environmental effects on the foetus, it’s akin to tobacco smoke in first trimester pregnancy loss.’
Dr Fuller told The Guardian he started his research after a family member miscarried during a poor period of air quality in 2016.
He said: ‘That triggered the question in my mind and then I started noticing anecdotally that I was seeing spikes in miscarriage numbers in the emergency department during and after pollution spikes.’
Dr Fuller advised pregnant women to avoid exertion on polluted days and consider buying indoor air filters.
The research was conducted in Salt Lake City, US, where nitrogen dioxide levels are similar to those in cities such as London and Paris.
The report, published in the Fertility and Sterility journal, analysed the records of more than 1,300 women who miscarried from 2007 and 2015. Their exposure to air pollution at miscarriage was compared with times when they did not miscarry, meaning that age, weight, income and other factors were taken into account.
The strongest link with a miscarriage was the level of nitrogen dioxide in the seven days before the miscarriage.
It is not known how air pollution harms a foetus but scientists believe it can cause inflammation and other problems in the womb. Separate recent research has also found particles of pollution within the placenta.
In 2016 a report by the Royal College of Physicians and the Royal College of Paediatrics and Child Health said toxins emitted by diesel cars were fuelling a health crisis that kills 40,000 Britons a year.
Smog from busy roads and toxins from industrial emissions were linked to premature births, stillbirth, miscarriage, low birth weight and organ damage, it found.