The natural gas (mostly methane) that we use for heating and cooking is nearly odourless. A small amount of mercaptan (containing sulfur) is added so that any gas leaks will be noticed. This, in turn means that silver items will tarnish quickly in kitchens or rooms with open gas fires.

Talking of kitchens, if you regularly eat fried eggs your silver is probably doomed. A hot egg will give off a significant amount of sulfur-containing gas.