Well, possessing a black belt in flatulence myself, I am happy to report no long term effects of second hand methane. Short term effects, including a malodorous effect, may include burning watery eyes, nausea, a desire to flee the effected area and an incredible urge to open windows.
Depending on food ingested, some of the malodorous effect can leave one believing that some fecal matter has been left someplace in a room. These symptoms will disappear with the presence of fresh air, or a good air freshener. A word of caution on air fresheners though; in some cases it intermixes with the fecal smell resulting in one questioning if someone crapped a christmas tree, or flowers.
Lighting a match to alleviate the odor should be treated with extreme caution. Larger volumes of methane are flammable and depending on the purity of the methane, can lead to explosion.