Federal rules require distribution natural gas to be odorized so people can detect it well below explosive levels; the regulation specifies adding an odorant so the gas is readily detectable at one-fifth of the lower explosive limit[1].
Utilities add sulfur-containing odorants (mercaptans/thiols) such as tert-butyl mercaptan (TBM) or related organosulfur compounds; these are described by utilities and regulators as producing a rotten-egg, sulfur, or skunk-like smell used to signal leaks[4][7].
Tert-butyl mercaptan has a very low odor threshold and is detectable at sub-ppb levels in air, which is why small releases are often noticeable by smell[7].
How to recognize a possible leak
A persistent, strong rotten-egg or sulfur-like smell inside a home is a primary sign of a possible gas leak and should be treated seriously[4][5][2].
Other warning signs include audible hissing, whistling, or roaring sounds at pipes or appliances; continuous bubbling in standing water near piping; dead or dying vegetation over a buried line; and visible dense white clouds or plumes near the ground or mains[2][5].
Brief, very short odors that occur only when an appliance starts can be normal; persistent smells when appliances are off or odors that continue after appliance shutdown are suspicious and warrant action[5][4].
Immediate safety steps (do this now)
Evacuate everyone from the building to a safe location outside immediately — do not try to investigate from inside[2][5].
From a safe location (off the property or a neighbor's phone) call 911 if there is immediate danger, then call your gas utility emergency number or 911 as directed by the utility[2][3][5].
Do not operate electrical switches, phones, garage door openers, or any device that could create a spark. Do not light matches, lighters, or other open flames while inside or near the building[2][5].
Await trained responders and your utility; utility crews will investigate and take steps to make the scene safe before you return[2][5][3].
If you know how and can do so safely, you may shut off gas at an appliance or the main shutoff, but many utilities will isolate and shut off service at the meter during emergencies—do not attempt major repairs yourself[5][4][2].
What utility crews and emergency responders will do
When you report a smell, expect the utility or emergency responders to inspect the meter, service line, and nearby distribution pipe; they will perform visual and auditory checks, use instruments to monitor air, and isolate or shut off gas service if necessary to make the area safe[2][5][4].
Utilities commonly have authority and responsibility for the distribution main and often the service line up to the meter; homeowners typically own piping downstream of the meter, but ownership and responsibility vary by utility and state—confirm the local rule with your utility[12][4][2].
If a leak is on the homeowner side, the utility will often instruct you to hire a licensed plumber or gas fitter to perform repairs and may require permits and inspection before gas service is restored[4][12][2].
What contractors will typically do and what repairs cost
A typical contractor workflow for customer-side leaks is: locate and isolate the leak, replace or repair piping or fittings, pressure-test the system, obtain required permits and inspections, and restore appliances and gas service after inspection[4][11].
Consumer-facing repair cost ranges are broad: minor appliance or fitting repairs can be relatively inexpensive while underground service-line replacement or major repairs can run into the thousands. HomeGuide summarizes national examples ranging from the low hundreds to several thousand dollars depending on the repair type and site conditions; actual cost depends on trenching vs. directional drilling, length of pipe, local labor and permit costs, and whether the utility covers the utility-owned portion[11][12][4].
Always obtain a written estimate, confirm ownership/responsibility with your utility, and verify required permits and inspections before work begins[11][12].
Health and odorant notes
Mercaptan odorants are what you smell — methane itself is effectively odorless; the rotten-egg/sulfur smell comes from the added mercaptans rather than the natural gas molecule[7][4].
Published toxicology and review literature indicate odorants primarily produce strong odor and irritation (nose, throat, eyes) at community exposure levels; more serious toxic effects occur at much higher concentrations, and the literature notes gaps and variability in formal exposure limits for some mercaptans[8][7].
Gas detection devices and alarms
Homeowners can install listed combustible-gas ("explosive gas") alarms to detect the presence of natural gas; NFPA has introduced NFPA 715 addressing residential fuel-gas alarms and research discusses placement and dispersion characteristics to guide detector siting[6][13].
Manufacturers sell combined natural-gas/propane/explosive-gas and carbon-monoxide alarms in plug-in and battery-backup formats; check product listings for placement guidance and UL/listing details when selecting devices[9][6].
For general detector planning and overlap with other indoor alarms, see our guide on home carbon monoxide detectors for placement and testing best practices that often align with combustible-gas detector guidance.
Practical homeowner tips and final reminders
Evacuate first and call from a safe location — do not assume a faint odor is harmless[2][5].
Do not perform complex gas repairs yourself; contact the utility and then a licensed contractor if the issue is on your side of the meter[5][4][12].
Keep written estimates and confirm whether the utility or the homeowner is responsible for the repair before paying for major work[11][12].
Install and test listed explosive-gas alarms per manufacturer and NFPA guidance to provide additional early warning in occupied spaces[6][9].