Heres one link, its more based on unvented heating elements, but the corrolation is the same, and a lot of the info is applicable.
http://www.epinions.com/content_5004370052?sb=1
Exert from above link: So in addition to the above mentioned normally anticipated by-products of combustion, we must add potential products of incomplete combustion, such as carbon monoxide, benzo(a)pyrene (a highly carcinogenic hydrocarbon), sulfur dioxide and something that varies from state to state and country to country, "other pollutants". Toluene is present in some natural gas and is known to cause reproductive harm.
Proposition 65 Warning: Fuels used in gas, woodburning or oil fired appliances and the products of combustion of such fuels, contain chemicals known to the State of California to cause cancer, birth defects and other reproductive harm. California Health and Safety Code Sec. 25249.6