The findings are contained in a long-awaited EPA assessment of health risks from 32 toxic chemicals. The study is based on 1996 emissions data subjected to several years of internal analysis.
The assessment concludes that the accumulated exposure to the various toxic chemicals can be expected to cause 10 additional cancers over a lifetime of exposure for every 1 million people, or a 10 in 1 million cancer risk.
These risks can be found across virtually the entire country, said the study, which was reviewed by outside scientists.
"More than 200 million people live in census tracts where the combined upper bound lifetime cancer risk from these (chemical) compounds exceeded 10 in 1 million risk," said the study. It added that 20 million people live in areas where the risks are even higher - a risk of 100 additional lifetime cancers for every 1 million people.
"The risks are very much in line with what we expected all along," said Jeffrey Holmstead, head of the EPA's air office. He said in an interview the risks of cancer from toxic chemical exposure "are very, very small," compared with overall cancer risks from all sources.
The EPA considers a cancer risk of 1 in a million or greater as a matter of concern, although those levels do not always trigger regulatory actions.
Holmstead said the report was "designed to be a baseline" for further studies on risks posed by air toxins. He also emphasized the findings are based on 1996 data. "Since that time, the risks already have been reduced significantly," said Holmstead.
But environmentalists said the study's findings provide clear evidence that tougher measures are needed to reduce releases of toxic chemicals - such as benzene, mercury, formaldehyde and other carcinogens - from automobiles, power plants and industrial sources.
They show "a lifetime cancer risk at least 10 times greater than the level considered acceptable by the EPA," said Emily Figdor of the U.S. Public Interest Research Group.
"These findings are a wake-up call that EPA should take action to reduce this long overlooked public health threat" from toxic air releases, argued Figdor.
Among the study's conclusions is that automobiles and trucks contribute substantially to the public's exposure to cancer-causing air toxins.
It estimated that 100 million people live in areas where motor vehicles - both on and off-road - account for an additional lifetime cancer risk of at least 10 in a million.
These risks are largely the result of exposure to such chemicals as benzene, formaldehyde and butadiene - all components of motor fuels.
The study also concluded that toxic chemicals pose significant health hazards other than cancer to much of the U.S. population, especially problems with respiratory systems.
The report said the assessment was viewed as a "snapshot" that identifies the greatest health risks from toxic chemicals and the areas of most potential concern. It said the EPA will update the assessment with another report next year.
The authors of the internal EPA analysis cautioned that the risk analysis was subject to limitations "due to gaps in data or in the state of the science for assessing risk."
In some cases the shortcomings may have understated the risks, the authors suggested. For example, the study did not attempt to assess various dioxin compounds "that may contribute substantially to (cancer) risks," they wrote.
In addition, the study noted, the EPA is reassessing the health effects of the 32 toxic chemicals that were studied and that reassessment could show an increase in the overall risks that the chemicals pose.