You can smell gas.

Get everyone out of the building immediately. Do not switch any light on or off, do not unplug anything, do not use a phone inside, and do not go looking for the source. Once you are outside and away, call your gas utility’s emergency number or the fire service.

Every gas utility runs a 24 hour emergency line, they attend as an emergency, and they do not charge for it.

The sequence, and it is short

1. Everyone out, including pets if they can be gathered without delay.
2. Leave the door open on the way out if it is easy, but do not go around opening windows. Getting out matters more than ventilating.
3. No switches, no phones, no vehicles started in an attached garage.
4. Get well away, at least across the street.
5. Call the gas utility or the fire service from outside.
6. Keep others away and wait for them.

If the smell is faint and clearly coming from an unlit stovetop burner that someone has just knocked, turning that burner off and opening a window is a reasonable response. Anything beyond that is a leave and call.

What gas smells like, and what else it can be

Natural gas and propane are odorless, so an additive is put in to make them smell. The smell is usually described as rotten eggs or sulfur.

Things that can produce a similar smell: a sewer trap that has dried out, some kinds of decaying organic matter, and certain chemical spills. A dry trap is common and harmless. How to check that in a minute.

But the order matters. Assume gas, leave, and call. Establishing that it was a dry trap from the sidewalk costs you an hour. Getting it the wrong way round costs more than that.

There is one important exception to the smell rule: propane is heavier than air and collects at low points, so in a basement or a crawl space it can be present at floor level and not detectable higher up. And a large leak can overwhelm the sense of smell.

If you also feel unwell

Headache, dizziness, nausea, or confusion alongside a smell means get out and call emergency services rather than the utility line, and say that people are symptomatic. Those symptoms overlap with carbon monoxide, which is a different and completely odorless hazard. What to do when a carbon monoxide alarm sounds.

What the utility will do

Attend, usually quickly, and check with a detector. Depending on what they find they will either clear it, shut off the supply at the meter and tag the appliance, or shut off and require repairs before it goes back on.

If they shut the supply off, they will not turn it back on until whatever they found is fixed by a qualified person and, in many cases, inspected. Plan for being without heating or hot water for at least a day. Keeping the house and its pipes safe in the meantime.

After it is dealt with

• Get the written report of what they found and what they did. It matters for any claim, and it matters if you are renting.
• Tell your landlord in writing the same day, if you rent. A gas safety issue is a landlord repair obligation everywhere and there are usually statutory timescales attached to it.
• Do not relight anything they tagged. A tag is a legal instruction, not a suggestion.
• Check the carbon monoxide alarms while you are thinking about it, and fit them if there are none.

Preventing the next one

• Have gas appliances serviced annually by a qualified technician, and keep the records.
• Fit carbon monoxide alarms on every level and outside sleeping areas, battery powered or with battery backup.
• Know where the gas shut-off is at the meter, and what it looks like when it is closed, so you could describe it to someone on the phone.
• Do not store anything against a boiler or a furnace, and do not block ventilation openings.

The short version

Out first, without touching anything. No switches, no phone inside, no searching. Call the utility or the fire service from outside and stay out until they say otherwise. Assume gas even if it turns out to be a dry drain trap, because getting that order wrong is the only version of this with a bad outcome.