THE majority of childhood cancers are probably caused by exposure to pollution before birth, according to a report published today.

Professor George Knox, who has studied the birthplace of thousands of children who developed cancer, claims the inhaling of chemicals by pregnant mothers is the most likely trigger for the majority of such cases.

He said his findings highlighted the need to regulate emissions of carcinogenic chemicals and to consider where industrial sites and centres of populations were located.

Cancer cases among Scottish children have risen by more than 20% since the 1970s and significant increases in young sufferers have also been recorded across Europe. Campaign groups have expressed concern about a link with emissions from car exhausts and factories, and are calling for more research.

Cancer Research UK, however, described the evidence that pollutants trigger most childhood cancers as "very thin".

Professor Knox has been researching industrial and environmental pollutants in association with cancer for a number of years. He said that after initially being tentative about a link he was now convinced. "I expect my statements will be controversial," he said. "They will be disbelieved totally. People who do believe it will gradually increase and then something will be done about it."

For his research, Professor Knox, emeritus professor at Birmingham University, took chemical emissions maps of the UK produced by the UK National Atmospheric Emissions Inventory. These show concentrations of different pollutants such as carbon monoxide, which is produced by car exhausts, and 1,3-butadiene which is used in the production of rubber and is also formed as a by-product of combustion.

He then accessed data about 22,458 children who died from leukaemia and other cancers before their sixteenth birthday between 1953 and 1980. He examined postcodes to find out if they were living near pollution hotspots.

His report, in the Journal of Epidemiology and Community Health, one of the British Medical Journal's specialist magazines, suggests children born within a 1km radius of emissions hotspots of particular chemicals were between two and four times more likely to die of cancer before reaching 16 than other children.

Proximity to emissions of 1,3-butadiene and carbon monoxide carried the highest risks, according to his study, which examined numerous chemicals.

Professor Knox said in his conclusion: "Most childhood cancers are probably initiated by close perinatal encounters with one or more of these high emissions sources." However, he added: "Effective direct exposures in early infancy, or through breast milk, or even pre-conceptually, cannot be excluded."

Mary Taylor, campaigner with Friends of the Earth, said: "We have been calling for emissions to be cut drastically . . . This (research) probably vindicates our campaign."

Helen McDade, campaigns officer for the wildlife charity WWF Scotland, said: "WWF notes that childhood cancer incidence rates have increased across Europe over the last several decades and calls for more research into the health implications of environmental pollution. Anecdotal evidence has raised serious concerns about links between environmental pollutants and cancers in both wildlife and people."

Dr Lesley Walker, director of cancer information at Cancer Research UK, who said the evidence was "very thin" added that "a wealth of information suggests leukaemia, the most common type of cancer in children, may be a rare response to an unidentified but common infection". She also said some cancers may have an underlying genetic basis.