Short definition
A carbon monoxide (CO) alarm is a small battery-, plug-, or hardwired device that monitors air for carbon monoxide and sounds an audible warning before levels reach a harmful concentration. Washington has required CO alarms in nearly every home since 2013 under RCW 19.27.530.
What it is
A CO alarm pulls room air past an electrochemical sensor that reacts to carbon monoxide (CO) — the colorless, odorless, tasteless gas produced by incomplete combustion of any carbon fuel: natural gas, propane, wood, gasoline, oil. When the sensor sees a CO concentration high enough to be dangerous over a defined exposure time, the alarm sounds at 85+ decibels.
The standard the device is tested to is UL 2034. A UL 2034 alarm is calibrated to alert before CO accumulates to a level that would impair an able-bodied adult — roughly 70 ppm sustained for an hour, faster for higher concentrations. The unit’s job is to wake a sleeping household before anyone loses the ability to react.
Most modern alarms have a sealed 7- to 10-year sensor and chirp when they reach end of life. After that point, replace the whole alarm — not just the battery.
Why it matters to a homeowner
CO is the leading cause of accidental poisoning death in the United States. It kills roughly 400 people a year and sends about 50,000 to the ER. The Pacific Northwest sees recurring fatal clusters during winter windstorms, when generators or unvented heaters get used inside garages or living spaces during power outages.
A working CO alarm is the single most effective device in the home for catching a slow back-drafting water heater, a cracked furnace heat exchanger, a blocked B-vent, or an idling vehicle in an attached garage — all of which can produce CO at lethal levels with no warning to the people inside.
When you’ll encounter this term
- Selling a home: Washington requires CO alarms before transfer to a new owner, even on homes that were previously exempt
- A home inspector flags missing alarms during a pre-purchase inspection
- A new gas appliance install — a tech recommends adding an alarm on that level
- Your alarm chirps every minute or so, often after 7+ years — sensor end-of-life
- A new bedroom or basement conversion triggers an alarm on that level
Common failure modes
- Sensor end-of-life. Sensors degrade after 5–10 years; older units made before 2014 typically had 5-year life.
- Dead battery. Battery-only models without low-battery chirp can fail silently.
- Painted-over device. Sealed shut during a remodel.
- Mis-siting. Next to a bathroom or stove the alarm trips on aerosols and cooking fumes; homeowners then disable it.
- Never tested. Most homeowners don’t press the test button monthly.
Washington note
RCW 19.27.530 — verified at the Washington State Legislature website — requires CO alarms in all residential buildings. Compliance dates were staged: rules adopted by July 1, 2010, new construction by January 1, 2011, and all other residential buildings by January 1, 2013.
The one carve-out: owner-occupied single-family homes legally occupied before July 26, 2009, were exempt during that occupancy. But on sale, the seller must equip the home with alarms before the new owner moves in. There is no longer an “old house” exemption that survives a transaction.
Placement follows IRC R315 as adopted by Washington: outside each separate sleeping area in the immediate vicinity of bedrooms, and on each level of the dwelling including basements (but not crawlspaces or uninhabited attics). New construction wires alarms with battery backup; retrofits use battery, plug-in, or hardwired units, all of which meet the rule as long as they’re UL 2034 listed.
For tenants and landlords: installation is the landlord’s responsibility, maintenance and battery replacement (per manufacturer instructions) is the tenant’s.
Cost: battery alarms run $20–$45. Hardwired or interconnected units run $40–$80, plus $100–$250 for an electrician if new wiring is required. Combination smoke/CO alarms run $30–$70.
FAQ
Where do I put a CO alarm in my house?
One outside each sleeping area in the immediate vicinity of the bedrooms, and one on each level of the home including basements. Crawlspaces and uninhabited attics do not need an alarm. If you have an attached garage or a furnace/water heater room, an additional alarm on that level near the appliance is good practice — Washington’s rule sets the floor, not the ceiling.
How often should I replace a carbon monoxide alarm?
Replace the entire unit, not just the battery, when the alarm reaches the end of its sensor life. Most current alarms last 7 to 10 years and chirp at end-of-life. If your alarm has no end-of-life indicator and is more than 5–7 years old, replace it on a schedule. Test the alarm monthly with the test button.
Does a CO alarm detect natural gas leaks?
No. CO alarms react to combustion exhaust, not to raw fuel gas. A natural gas or propane leak smells like rotten eggs because of an added odorant (mercaptan); for that you rely on your nose and, if you want a device, a separate combustible-gas detector. Don’t assume a CO alarm covers gas leaks — they’re different sensors and different threats.