Does barbecuing cause cancer? Let's put it this way. Grilled meat contains known and suspected carcinogens. Whether it contains enough to significantly increase your risk of cancer hasn't been firmly established. Grilling meat produces at least two types of potentially dangerous chemicals: polycyclic aromatic hydrocarbons (PAHs) and heterocyclic amines (HCAs). PAHs are products of imperfect combustion found in smoke and burned matter. In large enough quantities they will definitely cause cancer in humans--to cite one famous example, scrotal cancer in chimney sweeps. In barbecue grills they're commonly formed when dripping fat flares up, charring the underside of the meat. For that matter, you make PAHs when you burn a piece of toast.
HCAs, whose possible role in cancer was more recently discovered, are the result of reactions between chemicals in muscle meat produced by high heat or prolonged cooking. In contrast to PAHs, HCAs are found inside the meat, not just on the surface, and can't be easily scraped or trimmed off. Also in contrast to PAHs, HCAs aren't necessarily more likely to be produced during grilling--they can be created when cooking in an ordinary oven or frying pan if you turn the heat up high enough. Researchers have found that HCAs are potent carcinogens when fed to rodents, but the link to cancer in humans is less clear; the rodents get thousands of times the dose you get from the occasional grilled burger. I won't bore you with the numbers, but in most of the epidemiological studies I've seen, the tests of statistical significance are barely out of the weeds.
Are all types of barbecuing equally risky? No. Mainly we're talking about grilling over open flame at high heat--backyard barbecuing, in other words. Slow barbecue, which generally involves lower temperatures and minimal flame exposure, seems to be less problematic. When grilling, cooking meat till it's well-done supposedly generates the most HCAs and PAHs, but the write-ups I've seen don't distinguish degrees of doneness adequately. Are we talking burned to a cinder or a little toothsome char? Not clear.
How serious is the threat? No one knows. The National Cancer Institute, a federal agency, states, "There is no good measure of how much HCAs would have to be eaten to increase cancer risk, and there are no guidelines concerning consumption of foods with HCAs." No thresholds have been established for PAHs either.
Are gas grills safer than charcoal? From what I can tell, there's nothing inherent in gas or charcoal that makes one safer than the other. You can make the case that since (a) production of dangerous chemicals is linked to high heat, and (b) the heat level is easier to control in a gas grill by adjusting the gas flow and rack height (in some grills, anyway), gas grills are safer. But that's speculation.
What if anything should you do to reduce risk due to barbecuing? Most dispensers of health advice, taking a better-safe-than-sorry approach, offer a grab bag of tips. Some are easy: don't incinerate your food, avoid flare-ups by not placing meat directly over coals, cook at lower temps, cut off blackened bits. Some are more hassle but not unreasonable: microwave meat beforehand for one to two minutes to reduce levels of chemicals that form HCAs and to cut down grill time. Others sound, well, half-baked, and aren't adequately supported by research: add vitamin E to the meat, drink tea with your barbecue (antioxidants supposedly detoxify HCAs). A few skeptics, notably Steven Milloy of junkscience.com, think the link between well-done meat and cancer is a crock and rightly note that undercooking hamburger increases the risk of food poisoning due to E. coli bacteria. Me? All things in moderation, bubba. If you're a cave dweller grilling the wily wildebeest on a daily basis, maybe you should worry. A couple steaks once a week during the summer? Hey, I'll bring the beer.
