Gas Leak vs CO Leak: Telling Them Apart in 60 Seconds

Advertising Disclosure: This site may receive compensation for service connections made through this page. Content is editorially independent.

You walk into your house at 11 p.m. on a Tuesday in January and something is wrong. Maybe there is a faint smell -- distinctive, like rotten eggs. Maybe everyone in the house has had a dull headache all evening and you are starting to feel nauseated. Maybe an alarm is going off that you are not sure what it is -- the smoke alarm sound is familiar; this is something else, a different beep pattern. In the next 60 seconds you have to make decisions: get everyone out, call the right number, avoid doing the specific things that make either emergency worse. The four signals below let you make those decisions correctly even when you are tired, scared, and dealing with people who are not yet evacuating.

This article is the safety-decision framework for distinguishing a natural gas leak from a carbon monoxide exposure in the first 60 seconds, the first-60-second response for each, and the specific homeowner mistakes that turn either emergency worse. The C2 carbon monoxide article (CO sources and prevention) covers the broader detector and inspection strategy. The C6 emergency-warning article (7 warning signs of HVAC failure) covers the slower-developing failure signals that precede an emergency. This article's unique territory is the distinction itself -- which emergency you have, in real time, with seconds to decide.

The Four Signals That Distinguish Gas From CO

Four specific signals tell you which emergency you are facing. Read them once now; you will recognize them faster if you ever need to.

Signal 1: Smell

Natural gas has to be odorized before it reaches your home. Under 49 CFR 192.625, part of the federal pipeline safety rules administered by PHMSA (the Pipeline and Hazardous Materials Safety Administration), gas in a distribution line must be odorized so that it is readily detectable by a person with a normal sense of smell at one-fifth of its lower explosive limit. In other words, the smell is designed to reach you well before the gas can ignite. Philadelphia Gas Works describes it plainly: natural gas smells bad on purpose, and that "rotten egg" smell is the smell of natural gas.

Carbon monoxide is the opposite. CPSC describes it as a colorless, odorless, poisonous gas, and CDC warns that people who are asleep can die from CO poisoning before they have symptoms. Any smell you notice is not coming from the CO itself. A CO emergency shows up as symptoms or a sounding CO alarm.

One important nuance: a faint smell still counts. Do not wait for it to get stronger, and do not go looking for where it is coming from. If a CO alarm is going off and you also smell gas, treat both as emergencies and get out.

Signal 2: Symptoms

The two emergencies show up differently. A natural gas leak is dangerous mainly because it can ignite; PGW warns that electrical devices, including cell phones, may cause a spark and ignite the gas. With gas, the smell is your signal, not how you feel. CO exposure produces headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion, symptoms CDC says are often described as "flu-like." CDC adds that breathing in a lot of CO can make you pass out or kill you.

The CO pattern to watch for is a headache or flu-like feeling that affects several people in the home at once. CPSC notes that with slowly developing CO problems, occupants and even physicians can mistake mild to moderate CO poisoning for the flu. If several people feel unwell for no clear reason, and it gets better when you go outside, treat CO as a possibility even if no alarm is sounding. CPSC’s advice for suspected CO symptoms is to get outside to fresh air immediately and call the fire department from a neighbor’s home. A silent alarm does not mean the air is CO-free: according to a 2014 CPSC test report on the UL 2034 standard, home CO alarms must not sound when exposed to 30 ppm of CO for 30 days or 8 hours, depending on the test.

Signal 3: Detector Type

A CO alarm detects carbon monoxide. CPSC recommends battery-operated or battery-backup CO alarms that meet the UL 2034 safety standard on every level of the home and outside sleeping areas, installed according to the manufacturer’s instructions. Interconnected alarms are best: when one sounds, they all sound.

A natural gas (methane) alarm is a separate device that detects a different gas. Combination units that detect both exist. The two devices are not interchangeable: a CO-only alarm will not warn you about natural gas, and a gas-only alarm will not warn you about CO. Knowing which device is sounding tells you which gas it detected.

Signal 4: Source

Gas leaks come from the natural gas supply chain into the home: the meter, the supply line, valves, fittings, and connections at gas-burning appliances (furnace, water heater, range, dryer, fireplace). On HVAC equipment, possible leak points include the gas supply connection at the furnace, the gas valve assembly, and the fittings inside the burner compartment. All of them are technician territory.

Carbon monoxide is produced by the incomplete burning of fuels, including natural gas, per CPSC. EPA lists HVAC-related sources such as leaking chimneys and furnaces, back-drafting from furnaces and gas water heaters, and combustion equipment whose flue is improperly sized, blocked, disconnected, or leaking. When a technician investigates, the checks include the furnace heat exchanger (a crack can let combustion gases into the supply air), the exhaust vent (snow, a bird nest, or debris can block it), and whether a water heater’s exhaust is being pulled back into the house. Identifying the source is technician work; your job is to get out first.

The First 60 Seconds: What to Do

The response is largely the same for both emergencies but with specific differences in what NOT to do.

Step 1 (0-15 seconds): Evacuate everyone. Get every person and every pet out of the home. Do not stop to open windows, gather belongings, or switch anything on or off. Speed matters more than anything else in the first 15 seconds.

Step 2 (15-30 seconds): Get away from the home. Walk to the curb, a neighbor’s driveway, or farther. PGW’s guidance is to leave the area and make the call from a safe location, and not to start vehicles.

Step 3 (30-60 seconds): Call 911 or the gas utility from outside. For a gas smell, call 911 or your gas utility’s emergency line once you are at a safe distance. For a CO alarm or CO symptoms, call 911 or the fire department; CPSC also says to do a head count after calling to check that everyone is accounted for. Save your gas utility’s emergency number in your phone now so you are not searching for it later. Tell the dispatcher what you observed (smell, CO alarm, symptoms), how many people and pets are out, and whether anyone has symptoms.

What NOT to do in the first 60 seconds:

• Do NOT operate any electrical switches inside the home -- including the doorbell, light switches, garage door opener, ceiling fans, range hoods, and bath fans. PGW warns that electrical devices may cause a spark and ignite the gas.
• Do NOT use a cell phone inside the home. PGW lists cell phones among the electrical devices that may cause a spark; make the call once you are outside at a safe distance.
• Do NOT shut off any gas valve yourself, at the meter or at an appliance. PGW’s guidance is explicit: do not try to shut off any natural gas valves or gas appliances. Leave that to the gas utility or emergency responders.
• Do NOT try to find the leak source, and never use an open flame (matches, lighters) near a suspected leak. PGW says not to try to locate the source of a gas leak; for a CO alarm, CPSC says not to try to find the source of the CO. The warning tells you to leave, not where to look.
• Do NOT go back in for pets after evacuation. Pets are part of the initial evacuation; if a pet is left behind in error, tell the dispatcher and let responders retrieve.
• Do NOT go back inside until the gas utility or emergency responders say it is safe. Both PGW (for gas) and CPSC (for CO) say to wait for that permission.
• Do NOT relight any pilot light or attempt to restart any equipment. Pilot work is a technician task; relighting after a gas event without isolation is hazardous.

When the Source Is HVAC Equipment

Gas-fired HVAC equipment has gas connections that can leak, and furnaces and gas water heaters appear on EPA’s list of CO sources. Recognizing the equipment-side patterns helps you anticipate risk.

Furnace as CO source: EPA lists leaking furnaces, back-drafting furnaces, and blocked or disconnected flues among CO sources. On a furnace, the two failures a technician looks for are (a) a cracked heat exchanger -- the metal partition between combustion gases and supply air -- which can let combustion gases into the home’s supply air, and (b) blocked exhaust venting, such as a snow-covered vent termination, a bird nest, or debris in the vent connector. PGW notes that snow, ice, bird nests, and debris can block outdoor appliance vents. In snowy markets like Bridgeport, Connecticut and Buffalo, New York, keep snow cleared away from outside vent openings without touching or detaching the vent itself. The annual professional inspection described in the C6 maintenance pillar covers both failure modes.

Water heater as CO source: EPA lists back-drafting from gas water heaters as a CO source. Atmospheric-vented water heaters (the older style with a draft hood and a natural-draft flue) rely on the flue drawing exhaust up and out; strong exhaust fans (kitchen range hoods, bath fans) in a tightly sealed house can work against that draft. If your home has had air-sealing or insulation work, including homes in mixed-humid markets like Knoxville, Tennessee, ask a technician to check the water heater’s venting.

Furnace and water heater as gas leak source: Possible leak points at HVAC equipment include the gas supply connection at the unit, the appliance shutoff valve, the gas valve assembly, and the fittings inside the burner compartment. Aging pipe and fittings, or connections that have shifted, can leak. Hot-dry markets like Mesa, Arizona use furnaces less, but a gas water heater, range, or dryer still has gas connections all year.

Prevention: What to Set Up Before an Emergency

Three things to set up before you ever need them.

CO alarms on every level and outside sleeping areas. CPSC recommends this placement. Replace each alarm at the age its manufacturer specifies; CPSC notes the recommended replacement age is in the product literature or available from the manufacturer. CDC suggests checking CO detector batteries when you change your clocks each spring and fall.

Annual professional furnace inspection. EPA advises having a trained professional inspect, clean, and tune up the central heating system (furnaces, flues, and chimneys) annually. Ask the technician to include the heat exchanger, the flue and exhaust vent (clearance, condition, blockage), and a combustion check with an analyzer. The C6 pillar (year-round HVAC maintenance playbook) covers the methodology, and our cost guide lists current tune-up price ranges. In cold-climate markets like Bridgeport and Buffalo, schedule the inspection before the heating season starts.

Know where your gas meter is BEFORE an emergency. The main gas shutoff is at the meter, which may be outside or, in some homes, indoors; some homes also have shutoffs at individual appliances such as the furnace, water heater, range, and dryer. Knowing the location only helps you tell the dispatcher where to look. Knowing the location does not mean you should operate it yourself: PGW says not to try to shut off any natural gas valves, and not to tamper with the meter.

Trusted Industry Sources

The safety guidance, federal citations, and detection-equipment recommendations in this article are consistent with published guidance from:

• CDC — Carbon Monoxide Poisoning
• 49 CFR § 192.625 — Odorization of Gas
• U.S. CPSC — Carbon Monoxide Questions and Answers
• EPA — Carbon Monoxide’s Impact on Indoor Air Quality
• Philadelphia Gas Works — Natural Gas Safety
• EPA — Section 608 Technician Certification
• U.S. Department of Energy — Purchasing Energy-Efficient Residential Furnaces

Frequently Asked Questions

Local HVAC Service Areas

Cool Call Pro connects homeowners with independent HVAC technicians nationwide. Find a pro in Bridgeport (CT), Buffalo (NY), Knoxville (TN), or Mesa (AZ), or browse by state: Connecticut, New York, or all locations.