Some of it's actually Hydrogen sulfide which, as anyone related to oil and gas knows, is incredibly toxic and our noses can pick it up in even the smallest concentrations! I was once an inspector on a construction site where we had to wear gas monitors/alarms (to warn us about Hydrogen sulfide among others) and was taking a lunch time nap in my car. Must have been farting in my sleep because my monitor went off and scared me half to death!