This story is a supplement to the feature "Is China's Pollution Poisoning Its Children?" which was printed in the August 2008 issue of Scientific American.
PAHs can affect genes by causing mutations in the DNA code letters. They may also alter gene activity through what are termed epigenetic changes. For example, PAH exposure appears to increase DNA methylation, in which methyl groups (CH3) attach to DNA. Methylation tends to silence genes and can shut off ones needed to suppress certain diseases, including cancer.