Carbon Monoxide Alarms: the Five-Year Rule Nobody Knows

A house fire announces itself. Carbon monoxide does the opposite — and the device that detects it wears out in half the time of the one next to it on the ceiling, which almost nobody knows.

Why can't you tell if carbon monoxide is in the house?

Because there is nothing to detect. The CDC's description is six words long and does the work: "Carbon monoxide (CO) is an odorless, colorless gas that kills without warning." No smell, no colour, no smoke, no sound.

Worse, the early warning your body gives is one you will almost certainly misread. The CDC lists the common symptoms as "headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion" and notes that they are "often described as 'flu-like'."

Consider what that means on a cold night. The heating has been on. Everyone feels headachy and a bit sick. The obvious conclusion is that something is going round — and the obvious response, going to bed, is the worst available option. Confusion is on that symptom list, which means the faculty you would need to reason your way out is itself among the things being impaired.

This is not a rare hazard. The CDC reports that "more than 400 Americans die from unintentional CO poisoning not linked to fires, more than 100,000 visit an emergency department" each year.

Isn't that what the smoke alarm is for?

No — and this is the misconception worth correcting first. A smoke alarm detects particles of combustion. A carbon monoxide alarm detects a specific gas with an electrochemical sensor. They are different instruments answering different questions, and a house full of working smoke alarms can have no CO protection whatsoever.

The confusion is understandable. They look alike, they mount in similar places, they beep in similar ways. Some units combine both, which is fine — but "we have alarms" is not the same as "we have a CO alarm", and the only way to know is to go and read the label.

Placement differs too. The CDC's guidance is to "install battery-operated or battery back-up CO detectors near every sleeping area in your home" — sleeping areas specifically, because the danger case is being unconscious while it accumulates.

When does a CO alarm need replacing?

Every five years, which is half the life of a smoke alarm. The CDC puts it plainly: "Replace your CO detector following the manufacturer's instructions or every 5 years."

That difference is the practical heart of this. Smoke alarms are replaced ten years from their manufacture date. If you bought both at the same time, in the same shop, on the same afternoon — and most people do — then the CO alarm reaches the end of its life while the smoke alarm is only halfway through, and nothing marks the occasion.

The reason is physical rather than arbitrary. An electrochemical CO sensor contains a chemical cell that degrades whether or not it ever encounters carbon monoxide. It is consuming its own working life sitting on your wall doing nothing. A unit at year seven may test fine on its button and detect nothing at all, because the button tests the circuit and the horn, not the cell.

Newer units often have an end-of-life chirp. That is a good backstop, not a plan — it is a different chirp from the low-battery one, it is easy to mistake for a dying battery, and replacing the battery in an expired alarm produces a quiet alarm that still does not work.

Which reminders are worth setting?

Three, and the middle one is the one nobody has:

• Twice a year: test the button and check the batteries. The CDC suggests doing it "when you change the time on your clocks each spring and fall", which is a reasonable anchor precisely because the clocks give you the cue.
• A one-off, five years from the date on the alarm. Take it down today, read the manufacture or replace-by date printed on it, and set a reminder for a month before. This is the reminder that protects against the failure you cannot see or test.
• Annually, before heating season: get fuel-burning appliances serviced. Boilers, furnaces, gas fires, wood burners. The alarm is the last line; a serviced appliance is the first.

A recurring call suits the twice-yearly one because it has no urgency attached — which is exactly the kind of task a phone notification gets swiped past. In ReminderIt you can set it by message — "remind me every 6 months on the 1st to test the CO alarm" — and it reads the schedule back before saving.

What the CDC says to do if it sounds

Leave, then call. The official guidance is to get outside into fresh air immediately and call emergency services from there — not to investigate, not to open windows first and wait to see whether anyone feels better.

I am not going to dress that up with advice of my own, because this is the one subject where improvising is genuinely dangerous and confusion is a documented symptom. Follow your own country's emergency guidance, and if anyone is unwell, treat it as an emergency rather than a debate.

What a reminder does here is narrower and still worth having: it makes sure the alarm above the bedroom door is one that still works, five years after somebody bought it and forgot about it.

Sources: U.S. Centers for Disease Control and Prevention, Carbon Monoxide Poisoning Basics — deaths and emergency department visits, symptoms, detector placement and the five-year replacement guidance.