Exposure to environmental pollution while in the womb might increase a child's risk of cancer, a study suggests.
Emission levels are going down
Children born near emission "hotspots" were more likely to die of cancer before their 16th birthday than others.
Although not conclusive, the author of the study in the Journal of Epidemiology and Community Health believes the threat is real.
But cancer experts believe the research is heavily flawed and urge caution.
Professor George Knox, emeritus professor at the University of Birmingham, looked at a chemical emissions map for the UK for 2001 and details of children under 16 who had died from leukaemia and other cancers between 1966 and 1980.
He suggested that children born within a 1km radius of emissions hotspots of particular chemicals were two to four times as likely to die of cancer before reaching the age of 16, as other children.
Proximity to emissions of 1,3-butadiene and carbon monoxide, which are produced by vehicle exhaust, carried the highest risks.
Early life exposure
Professor Knox believes the mothers can inhale environmental toxins and pass them to the foetus across the placenta.
"It's a fair assumption that one or more of these substances are harmful.
"We know several are carcinogenic [cancer causing] in animals," he said.
Professor Knox acknowledged that emissions had gone down over the years and that there was a gap spanning decades between the deaths he looked at and the pollution data.
He also said it was more of a concern on the population level, and that individual people should not be alarmed by the findings.
"The risk of a random child having a cancer is about one in 1,000. In the hotspots it is two to four in 1,000 so it's still a low risk."
However, he does think that emissions should be reduced further and that further research is warranted.
A spokesman from the Leukaemia Research Fund said: "The Leukaemia Research Fund does not consider that this paper demonstrates that atmospheric pollution plays any key role in causing childhood leukaemia.
"It uses 2001 airborne emissions data but is relating this to births up to 40 years earlier."
"We would not wish parents to be made to feel that they may, in any way, have been to blame for their child's illness."
Dr Anthony Michalski, from the Institute of Child Health, said: "The rates of most paediatric tumours are relatively similar in industrialised and non-industrialised countries and that would not be expected if this hypothesis was correct."
Dr Lesley Walker, of Cancer Research UK, called the evidence "very thin".
A spokeswoman from the Department of Health said: "The Committee on Carcinogenicity set up a review of cancer in children at its November 2004 meeting.
"It has recommended a number of follow-up pieces of work which includes a review of published papers on environmental exposures and childhood leukaemia.
"The new paper from Professor Knox will be considered as part of this review."