The most reliable way to test for carbon monoxide in your home is with a dedicated CO alarm or a handheld digital CO meter. Because the gas is colorless and odorless, there’s no way to detect it with your senses alone. Indoor air is considered safe at or below 9 parts per million (ppm) over eight hours. At 200 ppm, you’ll develop headaches, dizziness, and nausea. At 800 ppm or higher, exposure is fatal within minutes.
Using a Home CO Alarm
A standard battery-operated or plug-in CO alarm is the baseline protection every home should have. These devices are certified under the UL 2034 safety standard, which sets specific response times: the alarm must sound within 60 to 240 minutes at 70 ppm, within 10 to 50 minutes at 150 ppm, and within 4 to 15 minutes at 400 ppm. That built-in delay is intentional. Low-level spikes from a gas stove or fireplace are normal, and the alarm is designed to ignore brief, harmless fluctuations while still catching sustained, dangerous concentrations.
Most alarms have a test button you can press to confirm the unit is working. On digital-display models, pressing it cycles through a series of programmed numbers (typically starting at 888, then a number in the 200 range, back to 888, and finally zero). These are not actual CO readings. They’re internal diagnostics confirming the circuitry and display are functional. If the alarm chirps or beeps in the expected pattern, the electronics are working.
One important limitation: the test button only checks that the alarm’s electronics can still sound. It does not verify that the electrochemical sensor inside can actually detect carbon monoxide. For that, you need a CO test gas.
Testing the Sensor With Aerosol CO Gas
Aerosol CO test sprays (sometimes called “canned CO”) deliver a controlled burst of carbon monoxide directly to the sensor. You place a delivery hood over the alarm, press the aerosol for about 3 to 5 seconds, then leave it in place for 5 to 10 minutes. If the sensor is functioning, the alarm will sound. Some detectors have a speed-up mode slot where you can spray directly with a straw attachment, and a working sensor will typically respond within about 12 seconds.
Fire departments and inspection companies use these sprays routinely. They’re available to consumers as well, though they cost more than a replacement alarm. If your CO alarm is older than five or six years, replacing it outright is often more practical than buying test gas.
Where To Place CO Detectors
Carbon monoxide is slightly lighter than air and tends to rise with warm air currents from furnaces and water heaters. The EPA recommends mounting detectors on a wall about 5 feet above the floor, or on the ceiling. If you’re only getting one detector, place it near your sleeping area and make sure the alarm is loud enough to wake you. Ideally, you’d have one on every level of the home and near any room with a fuel-burning appliance.
Handheld CO Meters for Active Testing
A handheld digital CO meter gives you a real-time ppm reading rather than waiting for a threshold alarm. This is useful for investigating a suspected leak, checking air quality around a furnace or garage, or confirming that your home returns to safe levels after ventilating. Professional-grade meters cost $100 to $300, while basic consumer models start around $30 to $50.
Here’s how to read the numbers:
- 0 to 9 ppm: Normal indoor air. The World Health Organization considers up to 9 to 10 ppm safe for exposures lasting eight hours.
- 10 to 35 ppm: Warrants investigation. The WHO limit for a one-hour exposure is 25 to 35 ppm. You should ventilate and identify the source.
- 35 to 70 ppm: Potentially harmful with prolonged exposure. OSHA’s workplace limit is 50 ppm averaged over eight hours.
- 70 ppm and above: Your home CO alarm should trigger within one to four hours at this level. Open windows, leave the house, and call your gas utility or fire department.
- 200 ppm or above: Immediately dangerous. Physical symptoms begin quickly and the concentration is fatal within hours.
Visual Signs of a CO Problem
You can’t see or smell carbon monoxide itself, but the appliances producing it often give visual clues. A healthy pilot light on a furnace or water heater burns with a small, steady blue flame. If that flame turns yellow or orange, the appliance is burning fuel incompletely, which raises carbon monoxide production. Other warning signs include black soot or streaking around furnace vents, excessive moisture on windows near gas appliances, and a stale or stuffy smell in a room that should be well-ventilated. None of these are a substitute for a CO detector, but they can prompt you to test more carefully.
Testing Your Body for CO Exposure
If you suspect you’ve been exposed, testing the air in your home is only half the picture. Carbon monoxide binds to hemoglobin in your blood about 200 times more readily than oxygen does, forming a compound called carboxyhemoglobin (COHb). A simple blood draw can measure your COHb level and tell you how much CO your body has absorbed.
Normal COHb in a nonsmoker is below 3%. Smokers can run as high as 12% due to the carbon monoxide in cigarette smoke. Levels above 20% are considered critical and indicate significant poisoning. Symptoms like headache, confusion, and nausea generally appear well before levels reach 20%, but some people, especially during sleep, won’t notice mild symptoms until exposure has become dangerous.
One thing to know: a standard fingertip pulse oximeter cannot detect carbon monoxide poisoning. These devices measure oxygen saturation by shining two wavelengths of light through your finger, and CO-bound hemoglobin looks almost identical to oxygen-bound hemoglobin at those wavelengths. In one study, pulse oximeters showed oxygen saturation above 96% even when patients had COHb levels as high as 44%. Emergency departments use a specialized device called a CO-oximeter, which reads additional wavelengths of light to distinguish between oxygen and carbon monoxide on hemoglobin. If you go to an ER with suspected CO exposure, this is the test they’ll run alongside a standard blood draw.
When To Replace Your CO Alarm
CO alarms have a life expectancy of about seven years. The electrochemical sensor gradually degrades, and after that point even a passing test-button check doesn’t guarantee the sensor can still detect real CO. All alarms manufactured after August 2009 are required to have an end-of-life notification. When yours reaches expiration, it will chirp every 30 seconds or display “ERR” or “END” on the screen. This chirp pattern is distinct from a low-battery warning, which is typically a single chirp every 60 seconds.
Check the manufacture date printed on the back of your alarm. If it’s older than seven years, or if you can’t find a date at all, replace it. CO sensors don’t fail dramatically. They just quietly stop responding, which is exactly the kind of failure you won’t notice until it matters.