California, known to be the nation's smoggiest state, already has a potential cancer risk to adults that is hundreds of times above levels seen as acceptable by the Environmental Protection Agency.

But the report said children were more vulnerable to pollutants than adults because, pound for pound, they breathe more air, drink more water, eat more food and play outdoors more than adults.

"A baby born in California will be exposed to such high levels of toxic air contaminants that the child will exceed the Environmental Protection Agency's (EPA) lifetime acceptable exposure level for cancer at a very early age, and will exceed the lifetime acceptable exposure level by many multiples by age 18," the Washington D.C-based environmental campaign group said.

The "Toxic Beginnings" study divided California into five geographical areas. It concluded that in Los Angeles an infant would have reached the EPA's one chance in one million limit of contracting cancer from contaminants in 12 days, and in Sacramento it would take 23 days.

It said diesel exhaust - from trucks and cars, school buses, and farm and construction equipment - was still the worst source of air pollution. But it also took into account chemicals emitted by dry cleaners and factories as well as pesticides, adhesives and lubricant oils.

The National Environmental Trust urged federal and state policy makers to make cleaning up the air a priority.

"The overwhelming policy implication of these findings can be reduced to one word: URGENCY", it said.