I don't have any raw data, but I'd guess that the number of sewer-gas casualties worldwide is fairly low. But they're not unheard of. Just last week, I read about a man who literally farted himself to death. As this poor soul slept in a small, poorly ventilated room, he loaded the atmosphere over his bed with his own nocturnal toxic-fart fumes. The mighty funk even downed his rescuers and put one of 'em in the hospital.
Investigators blame the man's diet of mostly beans and cabbage.
Sewer gas isn't confined to exotic locales like Hong Kong and the air above farting men. You've got a load of it in your very own house. If you're hooked up to the city sewer, only the water in your traps stands between you and the funk of your fellow citizens. If your plumbing has been exposed to buckethead plumbers or misguided handy types, you might even have some free-floating sewer gas in your house. A while back, I had an e-mail correspondence with Bill Coull, a fellow home inspector who works in New York City.