Polycyclic aromatic hydrocarbons (PAH's) such as benzo[a]pyrene are relatively stable due to their aromaticity (delocalized pi systems). When alkanes are partially burned (oxidized) they tend to form the most stable possible species, namely aromatics. When meat is cooked over an open flame or hot coals, the fat that drips down is largely oxidized to CO2 and H20, but some is only partially oxidized and polycyclic aromatic hydrocarbons are made in small quantities. Some of these get back into the meat in the smoke rising from the fire or coals. Well-done meat has an espcially large amount.

Benzo[a]pyrene and related polycyclic aromatic hydrocarbons are not harmful by themselves. However, in the body, the liver oxidizes them to various species including epoxides. The epoxide diol from benzo[a]pyrene is particularly dangerous because the large flat portion of them molecule likes to slide inbetween the bases of double stranded DNA (a process called intercalation), facilitating attack by nucleophilic sites on DNA at the electrophilic epoxide. This leads to a covalent bond between the DNA and the benzo[a]pyrene diols as shown in the rotating structure above to the right.

It takes a lot of these to cause a problem, because our body has special enzymes that remove these as fast as they can. However, if too many benzo[a]pyrene molecules do accumululate in exactly the wrong places in our DNA, then cancer can result. Note: having one plate of Bar-BQ ribs is not a problem. It takes long term exposure to significantly increase your risk of cancer from eating flame cooked meats. To avoid the problem altogether, it is best to Bar-BQ using a grill that has a pan of water between the meat and the coals (sometimes called a Louisiana cooker or smoker). This keeps the meat moist from the steam, you get all the smoke flavor, but the smoke does not have any dangerous PAH's in it because the fat drips into the pan, not onto the hot coals. The George Foreman grill is also pretty good because the fat drips away quickly, and does not sit there getting heated.

Note that PAH's are major cancer causing molecules in cigarette and cigar smoke!

Interesting articles on benzo[a]pyrene including the link between PAH's in cigarette smoke and cancer, the harmful effects of mothers smoking on unborn children, and analysis of PAH's in foods cooked in different ways.