Sewer gas is dangerous to breathe mainly at high concentrations, which are uncommon inside a home. The faint rotten egg smell from a drain is usually a warning rather than a harmful dose, though it can cause headaches, nausea and irritated eyes. The real hazards are strong, lingering odors, enclosed spaces and a natural gas leak mistaken for it.
Sewer gas is a mix of gases from decaying waste, mainly hydrogen sulfide, methane and ammonia. Hydrogen sulfide makes the smell, and the CDC’s toxic substances agency says people can detect it at 0.0005 to 0.3 parts per million (ppm). Longer exposure at around 10 ppm or less can cause eye irritation, headache and fatigue. Methane is also flammable.
Two facts make a strong smell serious. At about 100 ppm, people lose the ability to smell hydrogen sulfide after 2 to 15 minutes, so a smell that fades is not proof the gas is gone. Above 500 ppm it can cause sudden unconsciousness or death. Levels like that are a hazard in sewers, pits and other enclosed spaces, not a normal household exposure.
One rotten egg smell needs different handling. If it hits suddenly and fills the whole house instead of one drain, treat it as a possible natural gas leak: leave, then call your gas company or 911 from outside.
For an ordinary drain odor, open windows and find the source. Anyone with headaches, dizziness, nausea or breathing trouble should get to fresh air and see a doctor. One honest limit: your nose cannot measure concentration, so only a gas meter tells you how much is present. The fixes, from a dry trap to a cracked line, are in the guides to sewer smell in the house and a drain that smells like rotten eggs.