Smelling a small whiff of natural gas briefly, like when you light a stove, is not harmful. But if you notice a persistent gas smell in your home, that’s a different situation. Even at concentrations too low to cause an explosion, a gas leak can displace oxygen, irritate your airways, and expose you to toxic chemical additives. The stronger the smell and the longer the exposure, the greater the risk.
What You’re Actually Smelling
Natural gas itself is odorless. What you detect is a chemical called mercaptan, a sulfur-based compound that gas companies add specifically so you can notice leaks. Your nose is remarkably sensitive to it, picking up mercaptan at concentrations as low as 0.7 to 35 parts per billion. That rotten-egg smell is an intentional safety feature, designed to alert you well before gas reaches dangerous levels.
There’s a catch, though. Mercaptan can cause something called olfactory fatigue: the longer you’re exposed, the less you notice the smell. So a gas leak that seemed faint at first may still be present (or worsening) even after your nose stops registering it. This is one reason why persistent low-level leaks can be particularly dangerous.
Short-Term Health Effects
Natural gas is roughly 60% to 90% methane, mixed with other volatile organic compounds. Methane works as a “simple asphyxiant,” meaning it doesn’t poison your cells directly but pushes oxygen out of the air you’re breathing. In a documented case, a 21-year-old man exposed to concentrated methane for just one minute lost consciousness, and his blood oxygen dropped to 75% (normal is above 95%). He arrived at the emergency department cyanotic, meaning his skin had turned blue from oxygen deprivation, and needed immediate respiratory support.
That’s an extreme scenario involving a high concentration in a confined space. At lower levels, the symptoms are more subtle but still worth taking seriously:
- Headache and dizziness
- Nausea
- Eye, nose, and throat irritation
- Shortness of breath or coughing
- Fatigue or feeling unusually drowsy
The mercaptan additive itself is an irritant. At higher concentrations, it can cause wheezing, tearing eyes, and skin irritation on contact with moist tissue. The workplace safety threshold set by NIOSH is just 0.5 parts per million, and concentrations above 150 ppm are considered immediately dangerous to life.
The Hidden Risk: Carbon Monoxide
A gas leak doesn’t just mean unburned gas floating through your home. Malfunctioning gas appliances often produce carbon monoxide (CO), a colorless, odorless gas that binds to your red blood cells far more aggressively than oxygen does. Unlike methane, CO is genuinely toxic even at low concentrations.
Research on prolonged low-level CO exposure suggests it can produce mild neurological effects over time. The evidence is still accumulating, but case reports describe memory problems, difficulty concentrating, and persistent headaches in people exposed to small amounts of CO over weeks or months. The challenge is that these symptoms are vague enough to be blamed on stress, poor sleep, or other causes, so many people don’t connect them to a gas issue until someone else in the household develops the same complaints.
Burning Gas Produces Indoor Pollutants
Even when everything is working correctly, gas stoves release pollutants during normal use. Research from Stanford found that both natural gas and propane stoves generate benzene, a known carcinogen, at levels that can exceed EPA and World Health Organization benchmarks. This isn’t about a leak. It’s about regular cooking with gas in a kitchen that doesn’t have strong ventilation.
So the smell of gas while your stove is running may signal incomplete combustion or a small leak at the burner, both of which increase your exposure to these byproducts. If you consistently smell gas when your stove is on, that’s worth investigating rather than ignoring.
Propane Carries Extra Risk
If your home uses propane instead of natural gas, the stakes are slightly different. Propane is heavier than air, so when it leaks, it sinks and pools in basements, crawl spaces, and low-lying areas rather than dispersing upward. This makes it easier for propane to accumulate to dangerous concentrations in enclosed spaces without being noticed. Natural gas is lighter than air and tends to rise and dissipate more quickly.
Propane is typically a blend of propane and butane with trace amounts of other compounds. It carries the same asphyxiation and explosion risks, but the pooling behavior means a leak in a basement or garage can become hazardous faster.
Who Is Most Vulnerable
Children, older adults, and people with existing respiratory conditions like asthma are more sensitive to both the oxygen displacement and the irritant effects of gas exposure. Smaller bodies mean less lung capacity and faster oxygen depletion at the same concentration.
Household pets are also affected. During the 2016 Aliso Canyon gas leak in Los Angeles, the county health department documented irritation to eyes, nose, and throat in pets, along with coughing, lethargy, nausea, and vomiting. Birds and fish are notably more sensitive than dogs and cats. If your pets seem unusually lethargic or are showing respiratory symptoms alongside a gas smell, take it seriously.
When a Gas Smell Becomes an Emergency
Natural gas becomes explosive when it reaches 5% to 15% of the air in a space. You’ll typically smell mercaptan long before concentrations get that high, which is exactly the point. But acting quickly matters because you can’t gauge the concentration with your nose alone, especially as olfactory fatigue sets in.
If you smell gas and the odor is strong or persistent, take these steps:
- Don’t flip light switches, use electronics, or create any spark. Even a light switch can ignite gas at explosive concentrations.
- Turn off any open flames including stove burners and candles.
- Open windows and doors as you move through the space.
- Leave the building and keep moving until you can no longer detect the smell.
- Call 911 or your gas company from outside. Don’t re-enter until emergency responders have cleared the building.
A brief, faint smell near a gas stove that disappears quickly is usually just a small amount of gas escaping before ignition. A smell that lingers, that you notice in rooms without gas appliances, or that seems to come from walls or floors suggests a leak in the piping and warrants immediate action. The fact that you can smell it at all means the system is doing its job. What matters is what you do next.