Carbon Monoxide Symptoms: 7 Silent Signs You Must Never Ignore

TL;DR: Carbon monoxide poisoning causes more than 100,000 ER visits and roughly 400 deaths in the US every year, according to the CDC, and its early symptoms (headache, dizziness, nausea) are almost identical to the flu. The difference: CO symptoms improve when you leave the house and return when you go back inside. Install a CO detector on every level of your home, test it monthly, and replace the unit every 5-7 years.

_Last reviewed: July 2026 · 7 min read_

You feel foggy, a little nauseous, maybe a headache that won't quit, and you assume it's a cold coming on. That guess is exactly what makes carbon monoxide dangerous. It's odorless, colorless, and its symptoms look like a dozen ordinary illnesses until someone doesn't wake up.

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What are the early symptoms of carbon monoxide poisoning?

The early signs are headache, dizziness, weakness, nausea, vomiting, and confusion, and they show up in that rough order as exposure builds. At low concentrations, around 70 parts per million (ppm), most healthy adults notice a dull headache within 1-2 hours. At 150-200 ppm, the headache sharpens, dizziness sets in, and nausea follows within 45 minutes.

The tell that separates CO poisoning from a stomach bug or flu: everyone in the house gets sick around the same time, symptoms ease when you step outside, and pets act strange too, birds especially, since they're more sensitive to CO than humans. If two or more people in the same home develop identical flu-like symptoms simultaneously and there's no fever, treat it as a CO emergency until proven otherwise.

Children, older adults, and anyone with heart or lung conditions absorb CO faster and show symptoms sooner. A furnace with a cracked heat exchanger is one of the most common indoor sources, which is why keeping your filter changed and your system serviced matters more than people think. 1-inch vs 4-inch furnace filters explains how filter choice affects airflow and, indirectly, combustion efficiency.

What causes carbon monoxide to build up inside a home?

CO builds up when fuel-burning appliances don't vent properly or when combustion is incomplete. Furnaces, water heaters, gas ranges, fireplaces, generators, and even attached garages are the usual sources. A blocked chimney flue, a corroded heat exchanger, or a flue pipe knocked loose can turn a normal appliance into a CO source overnight.

Running a generator in a garage, even with the door open, is the single most common cause of fatal CO poisoning after a power outage. The CDC recorded a spike of over 900 generator-related CO deaths in the decade following widespread hurricane and winter storm outages. Generators need to run at least 20 feet from any window, door, or vent, with the exhaust pointed away from the house.

Poor attic or crawlspace ventilation can also trap combustion byproducts that backdraft into living spaces, particularly in tightly sealed newer homes. If your attic isn't venting correctly, it changes the pressure balance in the whole house and can pull flue gases back down the chimney. Signs your attic ventilation is failing covers what to look for before it becomes a combustion safety issue.

How do you know if your carbon monoxide detector is working?

Test it monthly by pressing the test button and confirming the alarm sounds at full volume. Most CO detectors also have a digital readout or a status light that blinks green when everything's normal. If the light is off, blinking amber, or the unit chirps every 30-60 seconds, the battery is low or the sensor has expired.

CO sensors degrade over time regardless of battery type. Manufacturers rate most units for 5-7 years of accurate sensing, after which the alarm may still "work" mechanically but fail to detect actual CO levels. Check the manufacture date printed on the back of the unit, not the install date, since a detector that sat in a warehouse for two years before you bought it has less useful life left than the box suggests.

| Detector Type | Typical Lifespan | Best For | |---|---|---| | Battery-powered standalone | 5-7 years | Rentals, quick installs, no wiring needed | | Hardwired with battery backup | 7-10 years | Whole-home systems, new construction | | Combo smoke/CO alarm | 7-10 years | Simplifying multiple alarms into one unit |

Placement matters as much as testing. Put one detector on every level of the home, including the basement, and one within 10 feet of each sleeping area. Avoid placing units directly above or beside fuel-burning appliances, since normal startup combustion can trigger false alarms there.

What should you do the moment a CO alarm goes off?

Get everyone outside immediately and call 911 or your local fire department from outside the house. Do not open windows first, do not go looking for the source, and do not re-enter to grab belongings. CO alarms are calibrated to trigger before symptoms become severe, so treat every alarm as real until the fire department confirms otherwise with a CO meter.

Once you're cleared to re-enter, have a licensed HVAC technician inspect every fuel-burning appliance in the home before you use them again, especially the furnace heat exchanger and water heater flue. A cracked heat exchanger is invisible to the eye in most cases and requires a combustion analyzer to detect properly.

If the alarm sounds again after appliances are checked and cleared, don't assume it's a false alarm from cooking or a shower. Call the gas company's emergency line separately, since they can test for leaks that an HVAC tech's equipment won't catch. Old wiring issues can complicate an emergency response too, since breakers and outlets sometimes get blamed for smoke when the real issue is gas-related; if your panel or outlets are outdated, signs you still have knob-and-tube wiring is worth a read separately from CO concerns, since older electrical systems often accompany older, poorly maintained heating systems in the same home.

How often should fuel-burning appliances be inspected?

Have your furnace, water heater, and any gas fireplace inspected once a year, ideally before heating season starts in early fall. Annual inspections catch cracked heat exchangers, corroded vent pipes, and blocked chimneys before they become CO sources. A basic furnace tune-up runs $80-$200 depending on region, which is a fraction of the cost of an emergency room visit or a house fire.

Bathroom exhaust fans and other ventilation points also affect indoor air pressure and combustion safety indirectly. A home that's too tightly sealed without adequate makeup air can backdraft appliances that would otherwise vent fine. Signs your bathroom fan isn't doing its job is a quick check worth adding to the same fall maintenance pass as your furnace inspection.

FAQ

Can carbon monoxide poisoning happen without a gas leak smell?

Yes, carbon monoxide has no smell, color, or taste at all. The rotten-egg odor people associate with gas leaks comes from mercaptan added to natural gas for detection, but pure CO produced by incomplete combustion has none of that, which is why a detector is the only reliable warning.

How quickly can carbon monoxide become fatal?

At very high concentrations, around 1,600 ppm, exposure can be fatal within 1-2 hours, and at 12,800 ppm death can occur in 1-3 minutes. Lower level exposure in the 200-400 ppm range typically causes severe symptoms within a few hours but is rarely immediately fatal for a healthy adult who leaves the space.

Do carbon monoxide detectors expire even if the battery is fine?

Yes, the sensor inside degrades chemically over roughly 5-7 years regardless of battery condition. Check the manufacture date on the back of the unit and replace it on schedule even if it still passes the test button check.

Is it safe to run a generator in an open garage during a power outage?

No, an open garage door is not enough ventilation and CO can still accumulate to fatal levels inside the attached house within an hour. Generators need to run at least 20 feet from the home with the exhaust directed away from any windows, doors, or vents.

Should renters install their own CO detector if the landlord hasn't?

Yes, in most states landlords are required to provide working CO detectors, but a battery-powered unit from a hardware store costs under $30 and is worth installing immediately if you're unsure your unit is current or working. Check your state's specific requirement, since some mandate CO detectors only in units with fuel-burning appliances or attached garages.

This is educational information, not medical or safety advice. Call 911 or your local fire department immediately if a CO alarm sounds, and have a licensed HVAC technician inspect any fuel-burning appliance before further use.