Children whose mothers were exposed to traffic pollution during pregnancy may be more likely to develop asthma before the age of five, research suggests.
A study in the European Respiratory Journal of more than 65,000 children is one of the largest to look at the link between air pollution and asthma.
Children were tracked until the age of 10 and cases of doctor-diagnosed asthma were noted.
Mothers' postcodes were analysed and their exposure to air pollutants was determined by several factors, including the presence of major roads and monitored levels of pollution.
The measurements focused mainly on traffic-related pollutants, including black carbon, fine particulate matter, nitrogen oxide and nitric oxide.
The researchers suggested that exposure to traffic pollution during pregnancy increased the risk of developing asthma during the first five years of life, even in urban areas with relatively low levels of pollution.
Children whose mothers lived close to highways during pregnancy had one of the highest risks, and premature babies were among those at highest risk.
Lead author, Dr Hind Sbihi, from the University of British Columbia in Vancouver, said: "Our study results highlight the importance of exposure to pollution while babies are still in the womb.
"Air pollution from traffic sources increased the risk of developing asthma during early years before children reach school age, even in an urban area with relatively low levels of air pollution.
"There are some measures individuals can take to reduce this risk. First we would suggest installing high-efficiency particulate air (HEPA) filters in the home and avoiding busy routes when taking a stroll. You can also check air quality levels online and postpone high-intensity physical activity until conditions improve."
Chair of the European Lung Foundation, Dan Smyth, added: "Air pollution affects 100% of the population and this study highlights the harmful effects of air pollution right from the moment we are created.
"A large proportion of Europe's population live in areas with unhealthy outdoor air and it is essential that we increase awareness of the dangers of polluted air."
Dr Penny Woods, chief executive of the British Lung Foundation, said: "These research findings suggest pollution isn't only harming people alive today, but is impacting on the future health of the country.
"We think air pollution should be measured near schools, reported on by Ofsted and reduced, through filtration systems and anti-idling measures. Given the severity of the problem, immediate action must be taken by the Government."