The carbon monoxide alarm went off.

Get everyone outside into fresh air immediately, including pets, then call emergency services or your gas utility from outside. Do not stop to open windows, do not look for the source, and do not go back in for anything until you are told it is safe.

Carbon monoxide is colorless and has no smell. The alarm is the only warning there is.

The sequence

1. Everyone out, pets included. Count heads.
2. Do not ventilate first. Opening windows before leaving can clear the air enough that responders cannot find the source, and it delays the thing that matters, which is getting people out.
3. Call from outside. Emergency services, or the gas utility, or both.
4. Stay out. Not the porch, not the garage. Away from the building.
5. Do not re-enter for medication, a phone, or a pet, until responders say so.

Where it comes from

Anything that burns fuel and is not venting properly:

• A furnace, boiler, or water heater with a blocked or damaged flue.
• A gas stovetop or oven used for heating a room, which is a common and dangerous improvisation in a cold snap.
• A blocked chimney, including by a bird nest.
• A generator run in or near the building. This is the single largest source of fatal poisoning during power outages. Where a generator has to be.
• A car left running in an attached garage, even with the door open.
• An unvented portable heater in a closed room.
• A charcoal or gas grill used indoors or in a garage.
• A neighboring property, in an apartment or an attached house.

Why alarms matter more in specific circumstances

The risk goes up when the heating is under strain or improvised, which is exactly when people are least likely to be thinking about it: a cold snap, a power cut, a heating system that has just failed, or the first firing of a system in fall.

That means the two situations most likely to produce a poisoning are the two covered elsewhere on this site: a furnace that will not start and gets improvised around, and a power outage where someone runs a generator too close to the house.

Fitting them properly

• On every level, and outside every sleeping area.
• Battery powered, or mains with battery backup. A mains only alarm is not working during a power cut, which is when the risk is highest.
• Test monthly, and replace batteries on a schedule you will actually keep.
• Replace the alarm itself at the interval on the back of it, usually seven to ten years. The sensor degrades whether or not the alarm has ever sounded.
• Not directly above a fuel burning appliance, and not in a bathroom or a very drafty spot. Follow the manufacturer’s positioning.

They are inexpensive. There is no version of this where the cost is the reason not to.

After it has sounded

Even if everyone feels fine and the alarm stops:

• Do not simply reset it and go back to bed. An alarm that sounded, sounded for a reason.
• Have the appliances checked by a qualified technician before they are used again, and get the finding in writing.
• Ask about the flue and the ventilation, not just the appliance.
• If you rent, report it in writing the same day. Carbon monoxide and gas safety are landlord obligations with statutory timescales. The wider split between landlord and tenant.

If the alarm is old, replace it as well as investigating. An alarm at the end of its life can produce both false alarms and, more dangerously, no alarm.

The one thing not to do

Do not use a gas oven, a stovetop, a grill, or an unvented heater to warm a room because the heating has failed. It is the most common route to a poisoning in a cold snap, and there are safe alternatives. Getting through without heat.

The short version

Out, into fresh air, with everyone and every pet. Call from outside. Do not ventilate first and do not hunt for the source. Anyone with symptoms needs medical assessment. Afterwards, have the appliances and the flue checked before anything is used again, and never heat a room with a stove or an unvented heater.