They're one letter apart, but they're different gases with different jobs. Carbon monoxide (CO) is a poison from fuel-burning equipment, and a CO alarm is safety equipment. Carbon dioxide (CO2) is what you breathe out, and a CO2 monitor tells you how much fresh air a room is getting. A CO2 monitor won't warn you about CO, and a CO alarm won't tell you anything about stale air. Most homes want both.
The difference at a glance
| Carbon monoxide (CO) | Carbon dioxide (CO2) | |
|---|---|---|
| What it is | A poisonous gas | The gas people breathe out; always in outdoor air too |
| Where it comes from indoors | Fuel-burning equipment: furnaces, gas water heaters, gas stoves and space heaters, wood stoves and fireplaces, cars in attached garages, generators | People breathing; it builds up when a room gets too little outside air |
| Normal level | None you'd want indoors | About 425 ppm outdoors; higher indoors with people in the room |
| The device | A CO alarm | A CO2 monitor with a true (NDIR) sensor |
| What it's for | Warning you of a dangerous leak | Showing how well a room is ventilated |
| If it reads high (or the alarm sounds) | If the alarm sounds or you suspect CO poisoning: get outside to fresh air, then call 911 | Bring in more outside air: the door, a fan, a window or a ventilation system |
Carbon monoxide: the one that needs an alarm
The US Consumer Product Safety Commission (CPSC) calls carbon monoxide the "Invisible Killer" because "it's a colorless, odorless, poisonous gas." You can't see or smell it, which is why an alarm is the only practical way to know it's there. It comes from burning fuel: the EPA lists leaking or back-drafting furnaces and gas water heaters, gas stoves, unvented space heaters, wood stoves and fireplaces, and car exhaust from attached garages. The CPSC singles out portable generators: "one portable generator can produce the same amount of carbon monoxide as hundreds of cars."
The CPSC's recommendations for alarms:
- "Install battery-operated CO alarms or CO alarms with battery backup on every level of the home and outside sleeping areas."
- "Interconnected CO alarms are best; when one sounds, they all sound."
- Don't install them in attics or basements unless there's a sleeping area there.
- Test them monthly and replace the batteries every year.
The CPSC lists the symptoms of CO poisoning as "headache, dizziness, weakness, nausea, vomiting, sleepiness, and confusion," and its advice is direct: "If you suspect CO poisoning, get outside to fresh air immediately, and then call 911."
Carbon dioxide: the one that tracks fresh air
CO2 is part of ordinary air: outdoors, it's about 425 ppm and rising slowly. Indoors, every person breathing adds to it, and outside air coming in carries it away. That makes it a handy stand-in for ventilation: when the number climbs well above outdoor air, the room isn't getting much outside air for the people in it.
ASHRAE, the engineering society behind US ventilation standards, calls indoor CO2 "at best an indicator of outdoor air ventilation rate per person." The UK Health and Safety Executive says CO2 measurements are "only a broad guide to ventilation rather than demonstrating 'safe levels'." For a bedroom, there's now a research-based goal: 800 ppm or less, on average, while you sleep, from a 2025 review of sleep studies for ASHRAE. Why 800.
A high CO2 reading isn't an emergency. It's a sign to bring in more outside air, and the place to start is a free change for one night. Free fixes to try first.
Why one can't stand in for the other
A CO2 monitor's sensor is built to detect CO2. The common true-CO2 type, NDIR, works by measuring how CO2 absorbs a particular band of infrared light. It isn't built to detect carbon monoxide. A CO alarm is built and tested for carbon monoxide, and it doesn't report CO2 at all. Neither one covers the other's job.
The overlap is in how a bad morning can feel. The CPSC's list of CO symptoms includes headache and sleepiness, which is also how people sometimes describe waking up in a stuffy room. So don't explain away morning headaches as stale air until you know your CO alarms are working, especially when the heat is on.
Which one do you need?
- A CO alarm, on every level and outside sleeping areas, as the CPSC recommends. It matters most if your home has fuel-burning equipment or an attached garage, and your local code may require it.
- A CO2 monitor if you want to know whether your bedroom gets enough fresh air at night. HouseExhale turns its overnight readings into one plain answer, with the goal of 800 ppm. Which monitors work.
Tonight: Check both
- Find your CO alarms. The CPSC recommends one on every level of the home and outside sleeping areas. If a level or the hallway outside the bedrooms has none, put one on your list.
- Press each alarm's test button. The CPSC recommends testing smoke and CO alarms monthly and replacing their batteries every year.
- Set your CO2 monitor in the bedroom at about head height, at least 2 ft (60 cm) from anyone's face.
- In the morning, before you open the door or window, write down the CO2 reading and compare it with the bedroom goal of 800 ppm.
Sources
- US Consumer Product Safety Commission: Carbon Monoxide Information Center
- US Consumer Product Safety Commission: CO alarms
- US EPA: Carbon monoxide's impact on indoor air quality (indoor sources)
- ASHRAE: Position document on indoor carbon dioxide
- UK Health and Safety Executive: Using CO2 monitors
- NOAA Global Monitoring Laboratory: Trends in atmospheric CO2
- Akimoto et al. (2025), ASHRAE 1837-RP: bedroom ventilation and sleep quality (the 800 ppm recommendation)