A rotten-egg smell can come from odorized fuel gas, sewer gas, or sulfur compounds in hot water. Because a gas leak is the most urgent possibility, a strong or sudden odor near a gas appliance or line should be treated by leaving first and calling for help from outside.
If fuel gas is possible, do not search for the source by switching things on or off
Avoid flames, switches, plugs, and any action that could create a spark. Get everyone out, move away from the building, and contact the gas utility or emergency services as appropriate for local guidance.
An odor localized to a drain points to a different cause
A dry trap, organic buildup, or plumbing-vent problem can allow sewer odor indoors. If the smell is limited to one sink or shower and there are no gas warning signs, investigate that drain separately.
If only hot water smells, investigate the water system
Sulfur compounds and reactions inside a water heater can create odor in hot water. Compare hot and cold water at several fixtures; do not open or service a gas or electric water heater without the appropriate procedure and qualifications.
A carbon monoxide alarm does not detect a fuel-gas leak
Carbon monoxide (CO) is odorless, and a CO alarm does not replace the response to a fuel-gas odor. Keep alarms where required, but if there is a strong gas smell, act on the odor and immediate safety rather than waiting for an alarm.
Sources
- U.S. Consumer Product Safety Commission — Winter Weather Safety Tips — If a gas leak is smelled or heard, leave the home and contact local gas authorities from outside without operating electronics before leaving.