Gas leak and carbon monoxide warning signs: what to do

This is general safety information, not a substitute for professional inspection or emergency response. If you suspect a gas leak or carbon monoxide exposure right now, leave the home and call for help before reading further.

Signs of a gas leak

A rotten-egg or sulfur smell near an appliance, meter, or gas line is the most recognizable sign, since utilities add that odorant specifically so leaks are noticeable. Other signs include a hissing sound near a gas line or appliance, dead or discolored vegetation in an otherwise healthy patch near an outdoor line, and bubbles in standing water above a buried gas line. Indoors, a pilot light that won’t stay lit or an unusual increase in your gas bill without a clear reason can sometimes point to a slow leak too, though these are less reliable signs on their own.

Signs of carbon monoxide exposure

Carbon monoxide is odorless, so the signs are physical rather than something you smell or see in the air. Watch for headache, dizziness, nausea, confusion, or flu-like symptoms that improve when you leave the house and return when you’re back inside, especially if multiple people or pets in the home are affected around the same time. This pattern, symptoms tied to being in the house, is one of the more reliable clues that something is wrong with an appliance rather than a passing illness.

Sign	Gas leak	Carbon monoxide
Detectable by smell	Yes, rotten-egg odor	No, odorless
Physical symptoms	Possible, from prolonged exposure	Headache, dizziness, nausea, confusion
Source	Leaking line, fitting, or appliance	Incomplete combustion in a gas appliance
Immediate action	Leave, don’t use switches or flames, call from outside	Leave, get to fresh air, call emergency services

What to do immediately

Leave the home. Don’t turn light switches, appliances, or anything electrical on or off on your way out, since a spark near a gas leak is a real risk. Don’t light a match or use a lighter to check for a leak; that sounds obvious written down, but it’s a documented cause of injury. Once outside, call your gas utility’s emergency line or 911, and don’t go back in until you’re told it’s safe.

For suspected carbon monoxide, the response is the same: get everyone, including pets, into fresh air immediately, and call emergency services. If anyone is showing symptoms, seek medical attention even after leaving the home, since CO exposure can have effects that don’t fully resolve just by getting outside.

Where problems tend to start

Aging appliances are the most common source of both risks. A furnace or water heater nearing the end of its service life is more likely to develop a cracked heat exchanger or a venting problem that lets exhaust gases spill back into the home instead of escaping outside. Blocked or damaged venting, sometimes from bird nests, ice buildup, or a disconnected pipe, is another common cause, and it can develop gradually without any obvious sign until a detector goes off or someone starts feeling unwell. Homes that have had renovation work done, particularly anything that touched ductwork, chimneys, or venting, are worth a follow-up inspection to confirm nothing was disturbed or sealed incorrectly.

Reducing the risk before it happens

Working carbon monoxide detectors on every level of the home, especially near sleeping areas, are the single most effective early warning system available to homeowners, since they catch what people can’t smell. Have gas appliances, furnaces, and water heaters serviced on a regular schedule, and pay attention to any appliance that seems to be burning with a yellow, flickering flame instead of a steady blue one, which can indicate incomplete combustion. Never use an outdoor appliance, like a barbecue or generator, inside a garage or enclosed space.

A licensed gas fitter can inspect lines, appliances, and venting for issues that aren’t obvious from the signs above, which is worth doing periodically even without a specific concern. How gas fitting listings on this directory are scored for safety-related review signals is covered on the methodology page, and the full directory lists plumbers across every Ontario category covered here.