Q. Why is it that lighting a match seems to dispel the odor of flatulence?

``I think lighting a match covers up the smell more than it actually dispels the odor,'' he says. He believes the combustion does change some of the offending molecules so they no longer bind with the same smell receptors in the nose, but that mostly it provides olfactory camouflage until the flatus dissipates.

Levitt says most of the gas we produce (on average, up to 2,000 milliliters, or about 120 cubic inches, in 13-14 episodes daily) is non-aromatic, made up of nitrogen, oxygen, carbon dioxide, hydrogen, and methane. (Methane is natural gas, the kind we cook with.)

Most of these gasses are produced when bacteria in our intestines feast on food not fully broken down. That's why some fruits and vegetables with complex carbohydrates that we can't fully digest are so gaseously productive. A small portion of the gas we pass is plain old air we swallow as we eat.

Levitt says less than 1 part in 10,000 of flatus is odiferous. Hydrogen sulfide, methanethiol, and dimethyl sulfide are the principal nasal nasties. Sulfur is the key ingredient in all of them. Half of it comes from what we eat. Half is naturally produced, in our bile, and in the mucus and linings of the intestine.

These gases are highly toxic. We've evolved to detect them at extremely low levels, so it doesn't take many molecules to trigger a response. (They're chemically similar to the ingredients in skunk spray and the odorant added to natural gas so we can detect leaks.)

To confirm what makes flatus smell unpleasant, Levitt fed study participants pinto beans and collected the resulting gas before passage. The gaseous components were analyzed and separated. Two ``smell experts'' said the most unpleasant were the hydrogen sulfide, methanethiol, and dimethyl sulfide.

Maybe we should send Levitt an air freshener for his lab.