Know Before Buying

We've used, tested, and calibrated carbon monoxide detectors for more than 25 years. Along the way we've learned that CO detectors differ a great deal, and most buyers never find out how much until it matters.

See it for yourself

How much CO is too much?

The answer depends on who you ask, but the major health and safety organizations set their limits far below what most home CO alarms are designed to detect:

- U.S. EPA (outdoor air): 9 ppm averaged over 8 hours
- World Health Organization: 9 ppm over 8 hours; about 3.5 ppm over 24 hours
- ASHRAE (indoor air): 9 ppm
- NIOSH (workplace): 35 ppm averaged over 8 hours
- OSHA (workplace): 50 ppm averaged over 8 hours

Workplace limits are higher because they're set for healthy adults on a work shift, not for children, pregnant women, the elderly, or people with heart or lung conditions who may be breathing the air at home around the clock. Those groups are the most vulnerable to long-term, low-level exposure.

UL 2034 alarms vs. low-level CO monitors

Most CO alarms sold at big-box stores are listed to UL 2034. In the U.S. we've learned to trust the UL mark, and for good reason. But it helps to know what UL 2034 actually requires, because the standard is designed to prevent nuisance alarms, not to warn you about low-level CO.

What a UL 2034 alarm is required to do:

- Below 30 ppm: The alarm must stay silent. Models with a display don't automatically show readings below 30 ppm.
- 30 ppm: The alarm must not sound, even after 30 days of continuous exposure.
- 70 ppm: The alarm must sound within 60 to 240 minutes.
- 150 ppm: The alarm must sound within 10 to 50 minutes.
- 400 ppm: The alarm must sound within 4 to 15 minutes.

Between 30 and 69 ppm, a UL 2034 alarm has no obligation to alarm at all. That's more than three times the EPA and WHO limits, and a household could live at those levels for weeks without a single alert.

The standard was written this way on purpose. High alarm thresholds keep the number of 911 calls down in areas where background CO is naturally higher, such as street-level apartments in dense, high-traffic cities. If that describes your home, a UL 2034 alarm may be all you want.

If anyone in your home is pregnant, very young, elderly, or has a heart or respiratory condition, consider a low-level CO monitor. These units alert at much lower concentrations. Because they're designed to alarm earlier than UL 2034 permits, they can't carry the UL 2034 listing. That's by design, not a quality shortcut.

Tip: The Defender CA6150 is UL 2034 listed but has a one-button readout that shows CO levels as low as 10 ppm, giving you a look at low-level readings without changing how the alarm behaves.

The three main types of CO detectors

1. Home and light-commercial detectors: Wall-mounted or tabletop units for houses, apartments, offices, and small businesses.
2. Personal (wearable) detectors: Compact units you carry with you indoors, outdoors, in vehicles, or while traveling.
3. Professional and industrial detectors: Instruments built for HVAC technicians, inspectors, and industrial safety teams.

How fast do CO detectors respond?

It depends on the type. Personal monitors and pumped professional instruments respond the fastest. A good fixed-mount detector should register a change in CO within about a minute. Response time and alarm timing are two different things, though. A UL 2034 alarm can detect CO quickly and still wait hours to sound. Always check the manufacturer's specifications before you buy.

Can CO detectors be calibrated?

Most home and office CO alarms can't. Personal detectors, such as Sensorcon and BW models, and professional HVAC and industrial instruments usually can be calibrated and serviced.

How long do CO detectors last?

Lifespans and warranties range from 1 to 10 years, depending on the type of detector and the quality of its sensor:

- Personal / wearable detectors: typically a 1-3 year warranty. They're built for rough use, which shortens sensor life.
- Wall-mount home and commercial detectors: typically a 5-10 year warranty.

In our experience, North American-made detectors have proven more reliable than many low-cost imports.

What is carbon monoxide?

Carbon monoxide (CO) is a colorless, odorless, poisonous gas. It forms when fuel doesn't burn completely, whether that fuel is natural gas, propane, gasoline, diesel, wood, or charcoal. You can't smell it, taste it, or see it. A detector is the only way to know it's there.

Where does CO come from?

Any fuel-burning appliance or engine can produce dangerous levels of CO, including:

- Furnaces and space heaters
- Water heaters
- Gas stoves and ovens
- Fireplaces and wood-burning stoves
- Gas clothes dryers
- Cars, trucks, and motorcycles, especially in attached garages
- Generators, lawnmowers, and gas-powered tools
- RV and boat engines and generators

How many CO detectors do you need, and where?

The minimum: one CO alarm outside each sleeping area and one on every level of the home. Add one inside any bedroom where someone sleeps with the door closed.

Typical homes:

- Single-story home, bedrooms down one hallway: 1
- Single-story home, bedrooms at opposite ends: 2
- Two-story home, bedrooms upstairs: 2 (upstairs hallway plus main level)
- Two-story home with a finished basement: 3

What the manufacturers and codes say:

- Defender: at least one outside each sleeping area and one on every level, plus one inside any bedroom where the door stays closed. For the most complete coverage, Defender recommends detection in all rooms and areas of the home.
- CO Experts: if you have only one, install it in an occupied bedroom or the hallway within 10 feet of all bedrooms. Recommends at least one low-level monitor on each level.
- International Residential Code (IRC R315): required in homes with a fuel-burning appliance or an attached garage. Install outside each separate sleeping area, near the bedrooms, and inside any bedroom that contains a fuel-burning appliance.
- NFPA 72 (formerly NFPA 720): outside each sleeping area near the bedrooms, and on every occupiable level, including basements.
- U.S. Consumer Product Safety Commission: on each level of the home and outside sleeping areas.

State and local codes can add requirements, such as hardwired or interconnected alarms in new construction, so check yours before you install.

Where not to put them:

- Within 5 to 6 feet of stoves, furnaces, water heaters, fireplaces or other fuel-burning appliances
- In garages, bathrooms or laundry rooms
- Outdoors or in unheated spaces
- Near air vents and ceiling fans, or tucked into corners where air doesn't move

Our recommendation: For most homes, install a listed CO alarm, such as the Defender CA6150, outside the bedrooms and on every level to meet code, then add a low-level monitor where people sleep. If anyone in the home is pregnant, very young, elderly, or has a heart or lung condition, put a low-level monitor on every level.

Recommended low-level protection

The detectors we recommend for homes with infants, older adults, or anyone with heart or lung conditions.

Questions? Email us at moreinfo@codetectors.com.