A carbon monoxide (CO) level above 9 ppm in indoor air is considered elevated, and anything above 35 ppm requires immediate action. For context, a home without gas appliances typically has 0.5 to 5 ppm, so readings much above that range signal a problem. The danger of CO depends on both the concentration and how long you’re exposed, which means even a moderately high reading becomes serious if it persists for hours.
Normal vs. Elevated Levels in Your Home
Homes without gas stoves typically have CO levels between 0.5 and 5 ppm. If you have a properly adjusted gas stove, readings of 5 to 15 ppm nearby are common. A poorly adjusted gas stove can push levels to 30 ppm or higher, which is already in the range that causes headaches with prolonged exposure.
The EPA’s outdoor air quality standard is 9 ppm over 8 hours. The World Health Organization uses a similar threshold of about 9 ppm for 8 hours, noting that sustained exposure above that level starts to affect blood chemistry enough to cause health effects. If a CO detector in your home reads above 9 ppm for an extended period, something is producing more CO than your ventilation can handle.
The PPM Scale: From Mild to Fatal
CO danger scales sharply with concentration. Here’s how the numbers break down based on OSHA data:
- 35 ppm: Headache and dizziness after 6 to 8 hours of constant exposure.
- 50 ppm: The maximum level OSHA allows for an 8-hour work shift. This is also when most home CO detectors begin to alarm (though many won’t trigger until sustained for over an hour).
- 100 ppm: Slight headache within 2 to 3 hours.
- 200 ppm: Headache, impaired judgment, and vision problems within 2 to 3 hours. No one should remain exposed at this level.
- 400 ppm: Frontal headache and nausea within 1 to 2 hours. Life-threatening after 3 hours.
- 800 ppm: Dizziness, nausea, and convulsions within 45 minutes. Collapse within 2 hours, possible death.
- 1,600 ppm: Confusion, staggering, rapid heart rate within 20 minutes. Fatal in under 2 hours.
- 3,200 ppm: Unconsciousness in 10 to 15 minutes. Fatal within 30 minutes.
- 6,400 ppm: Convulsions and respiratory failure within 1 to 2 minutes. Fatal in under 20 minutes.
- 12,800 ppm: Unconsciousness after 2 to 3 breaths. Fatal within 3 minutes.
The pattern is clear: doubling the concentration roughly halves the time before serious symptoms appear. A level that feels tolerable for the first few minutes can become deadly if you stay in the space.
How CO Affects Your Blood
CO is dangerous because it binds to hemoglobin in your red blood cells about 200 times more readily than oxygen does. When CO molecules lock onto hemoglobin, they form a compound called carboxyhemoglobin (COHb), which prevents those blood cells from carrying oxygen to your brain, heart, and muscles. The percentage of your hemoglobin occupied by CO is what doctors measure to assess poisoning severity.
Non-smokers typically have COHb levels between 1% and 3%. Smokers run higher, around 5% to 8%, because cigarette smoke contains CO. When COHb climbs above 2.5% in a non-smoker, measurable effects start to appear. Studies have found that COHb levels of just 2% to 4% reduce exercise capacity in healthy people and shorten the time before chest pain begins in people with heart disease. At COHb levels above 25%, the risk of lasting cognitive problems (memory issues, confusion, difficulty concentrating) jumps to between 25% and 50%.
What Happens at the Hospital
If you go to an emergency room after CO exposure, a blood test measuring your COHb percentage is the primary diagnostic tool. Treatment centers on breathing high-concentration oxygen, which speeds up the process of displacing CO from your hemoglobin. On room air, it takes roughly 4 to 6 hours for your COHb level to drop by half. With high-flow oxygen through a mask, that drops to about 60 to 90 minutes.
For severe poisoning, particularly if you lost consciousness, showed confusion, or had a COHb level above 25%, hyperbaric oxygen therapy is often recommended. This involves breathing pure oxygen in a pressurized chamber, which forces CO off hemoglobin even faster and helps reduce the risk of long-term neurological damage. Research published in the New England Journal of Medicine found that cognitive problems lasting a month or longer occur in 25% to 50% of patients who lost consciousness or had COHb above 25%.
Low-Level Exposure Over Time
Not all CO poisoning is dramatic. Chronic, low-level exposure from a slow leak in a furnace, a blocked flue, or a malfunctioning water heater can produce symptoms that mimic the flu or general fatigue: persistent headaches, feeling unwell, poor sleep, and trouble concentrating. These symptoms tend to improve when you leave the space and return when you come back, which is one of the key clues.
Even after the source is found and removed, some people continue to experience neurological symptoms. A review of chronic low-level poisoning cases documented lasting problems with memory, balance, sense of smell, sleep, and anxiety that persisted after exposure ended. Epidemiological studies have also linked ambient CO levels to increased hospital admissions for heart failure in elderly populations, and higher CO exposure during the third trimester of pregnancy has been associated with lower birth weight.
Common Household Sources
Any appliance that burns fuel produces CO. Gas furnaces, water heaters, gas stoves, fireplaces, wood stoves, and attached garages with running vehicles are the most common indoor sources. A properly maintained furnace produces very little CO that enters living spaces. A cracked heat exchanger, blocked vent, or back-drafting chimney can send CO directly into your home at levels well above safe thresholds.
Running a car in an attached garage, even briefly with the door open, can push CO levels into the hundreds of ppm inside the garage and into adjacent rooms. Portable generators are another major source and should never be operated indoors, in a garage, or near open windows. Charcoal grills and camp stoves brought inside during power outages cause CO poisoning emergencies every year.
What Your CO Detector Actually Tells You
Most residential CO detectors are designed to alarm at 70 ppm after 1 to 4 hours, or at 150 ppm after 10 to 50 minutes. They are not designed to alert you to low-level, chronic leaks in the 15 to 30 ppm range, even though those levels can cause symptoms over days or weeks. If you suspect a low-level leak because you have recurring headaches that resolve when you leave home, a technician with a professional-grade CO meter (which reads down to 1 ppm) can check your appliances and airflow more precisely than a consumer detector.
Place detectors on every level of your home and near sleeping areas. CO mixes fairly evenly with air (it’s nearly the same density), so placement height matters less than location. If your detector alarms, open windows, get everyone outside, and call your fire department or gas utility before re-entering.