Gas in a house refers to several different things: the fuel gas piped or delivered to your home for heating and cooking, the invisible byproducts those appliances produce when they burn that fuel, and a range of other gases that can accumulate indoors from soil, plumbing, and everyday household products. Some are harmless at normal levels, others are dangerous. Here’s what each one is, where it comes from, and what to watch for.
Fuel Gas: Natural Gas and Propane
The most common meaning of “gas in a house” is the fuel that powers your furnace, water heater, stove, or dryer. In most urban and suburban areas, this is natural gas, which is primarily methane delivered through underground utility pipelines. In rural areas without pipeline access, homes typically use propane (C₃H₈), a liquefied petroleum gas stored in tanks outside the house and refilled by a delivery truck.
Both gases are odorless in their pure form. To make leaks detectable, gas companies add sulfur-based chemicals called mercaptans that produce a strong rotten-egg smell. Federal regulations require that these odorants make gas detectable when it reaches just one-fifth of its lower explosive limit. For methane, the lower explosive limit is 5% concentration in air, so you should be able to smell a leak well before it becomes dangerous. Propane’s explosive threshold is even lower, around 2.1%, which is why propane leaks also carry a strong odor.
Beyond smell, signs of a gas leak include a hissing or whistling sound near pipes or appliances, bubbles forming when soapy water is applied to connection points, dead patches of grass or plants near an outdoor gas line, and visible dust blowing or a white vapor cloud near the source.
Carbon Monoxide From Combustion
Whenever fuel gas burns, it produces combustion byproducts. In a properly vented appliance, these exhaust safely outdoors. When something goes wrong (a cracked heat exchanger, a blocked flue, or an unvented appliance running too long), carbon monoxide can build up inside.
Carbon monoxide is colorless and odorless, which makes it far more dangerous than a fuel leak you can smell. The World Health Organization recommends indoor levels stay below 9 parts per million averaged over eight hours, and below 25 ppm for any single hour. Symptoms of exposure include headache, dizziness, nausea, and confusion. At high concentrations, it can be fatal. Every home with gas appliances or an attached garage should have carbon monoxide detectors on each level.
Nitrogen Dioxide From Gas Stoves
Gas stoves and unvented space heaters also release nitrogen dioxide, an irritant gas that affects the eyes, nose, throat, and lungs. In homes with gas stoves, indoor levels frequently exceed outdoor levels. There is no agreed-upon indoor standard for nitrogen dioxide, but the outdoor air quality limit set by the EPA is 0.053 ppm as an annual average.
Chronic exposure at elevated levels can contribute to bronchitis, reduced lung function, and increased respiratory infections, particularly in young children and people with asthma. Using a range hood that vents outdoors (not a recirculating filter model) while cooking significantly reduces nitrogen dioxide buildup.
Radon From the Ground
Radon is a naturally occurring radioactive gas that seeps into homes through cracks in the foundation, gaps around pipes, and porous concrete. It comes from the natural breakdown of uranium in soil and rock, and it’s present at some level in nearly every home. You can’t see, smell, or taste it.
The EPA recommends taking action to reduce radon if your home tests at 4 picocuries per liter (pCi/L) or above, and suggests considering mitigation for levels between 2 and 4 pCi/L. There is no known safe level of exposure. Long-term radon exposure is the second leading cause of lung cancer after smoking. Testing is inexpensive, with kits available at most hardware stores, and mitigation typically involves a venting system installed beneath the foundation slab.
Sewer Gas From Plumbing
Sewer gas is a mixture of gases that rises from your home’s drain system. Its most recognizable components are hydrogen sulfide (the rotten-egg smell, distinct from the mercaptan odor added to fuel gas) and ammonia. It also contains methane, carbon dioxide, sulfur dioxide, and nitrous oxides. In homes connected to municipal systems, traces of chlorine bleach, solvents, and other chemicals from the wastewater system can be present as well.
Your plumbing is designed to keep sewer gas out through water-filled traps in every drain. When a trap dries out, often in a floor drain, guest bathroom, or basement sink that hasn’t been used in weeks, sewer gas can flow directly into your living space. A blocked or leaking plumbing vent on the roof can also allow these gases inside. The fix is usually simple: run water in unused drains periodically to keep traps filled, and have a plumber check your vent stack if you notice persistent odors.
Volatile Organic Compounds (VOCs)
A wide range of gases classified as volatile organic compounds circulate in most homes at low levels. These aren’t fuel gases or combustion byproducts. They’re chemicals that off-gas from materials and products you use every day. Common sources include paint, adhesives, cleaning products, air fresheners, new furniture, flooring, pressed-wood cabinets, personal care products, and pesticides.
Some of the most frequently detected VOCs in homes include formaldehyde (from pressed wood and building materials), benzene (from attached garages, solvents, and tobacco smoke), toluene (from paints, adhesives, and synthetic fragrances), and limonene (from cleaning products and air fresheners). A recent review identified seven priority indoor air pollutants: acetaldehyde, acrolein, benzene, 1,3-butadiene, 1,4-dichlorobenzene, formaldehyde, and naphthalene. Many of these are classified as known or possible human carcinogens.
VOC levels tend to be highest in newly built or recently renovated homes, during and after cleaning, and in homes with attached garages where vehicle exhaust and stored chemicals migrate indoors. Improving ventilation, choosing low-VOC paints and building materials, and storing solvents and fuels outside the home’s living envelope are the most effective ways to reduce exposure.
How These Gases Compare in Risk
- Immediate danger: A natural gas or propane leak creates explosion and fire risk. Carbon monoxide at high concentrations can kill within hours. Both require immediate action: leave the house and call your gas utility or emergency services.
- Medium-term health effects: Nitrogen dioxide from gas stoves and hydrogen sulfide from sewer gas irritate the respiratory system and can worsen asthma or lung conditions with ongoing exposure.
- Long-term health effects: Radon and certain VOCs like formaldehyde and benzene increase cancer risk over years of exposure. These don’t cause obvious symptoms day to day, which is why testing and ventilation matter.
Most homes have some combination of these gases present at levels that are perfectly safe. The ones worth actively checking for are carbon monoxide (with detectors), radon (with a test kit), and fuel gas leaks (with your nose and, ideally, a combustible gas detector near appliances). For everything else, good ventilation is the single most effective protection.