You don't need to worry about contributing to pollution.

The flatulence of cattle and other ruminants is thought to contribute to global warming from its methane content. Ruminants produce methane because the bacteria in their more-complex digestive systems break down cellulose, forming methane as a byproduct. It's why cows can eat grass and hay. Human digestive systems can't do that, so we don't produce significant amounts of methane.

If I had to guess, I'd bet the taboo on human farting came from our hunter-gatherer days, as it could spook the tribe's dinner whilst your hunting group was sneaking up on it with spears.