Diesel exhaust, which is common on construction sites due to the prevalence of heavy equipment and trucks, causes lung cancer and may increase the risk of bladder cancer, says the International Agency for Research on Cancer, which is part of the World Health Organization. The agency announced the new health risk on June 12.

Diesel advocates counter that modern regulations have brought emissions to near-zero levels. Though WHO recognizes this, it says that "while the amount of particulates and chemicals are reduced with these changes, it is not yet clear how the quantitative and qualitative changes may translate into altered health effects." WHO calls for further study.

In April, a study from the Health Effects Institute, sponsored jointly by the U.S. Environmental Protection Agency and private companies, reported that while modern clean diesels may present mild lung irritation, it found "no evidence of gene-damaging effects." Scientists exposed laboratory rats and mice to diesel exhaust from 2007-2010 on-road engines, which under EPA regulations were required to cut fine particles by more than 90%. HEI is still studying truck engines certified for 2010 and later, the most current on-road EPA regulation; the rule mainly cuts nitrogen-oxide levels.

An immediate concern, IARC notes, is in developing countries that have little or no regulations for tailpipe emissions.

Diesel fumes have been linked before to cancer. In 1998, California's Air Resources Board listed diesel particulate as a toxic air contaminant with potential to cause cancer.