It Looks Exactly Like Flu, Except for the Fever

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Every winter, families come down with what they take to be flu. Headache, tiredness, nausea, a bit foggy. It clears up when they go out and comes back when they are home for the evening, which nobody notices because that is also what a virus feels like when you are resting.

Carbon monoxide has no smell, no colour and no taste. The only thing it has is a symptom set that impersonates the most common illness of the season, and a detector that costs about twenty dollars.

The scale, and it is not fires

Unintentional carbon monoxide poisoning in the U.S., per year

CDC figures. These exclude poisonings linked to fires.

400+deaths a year, not fire-related

100,000+emergency department visits

14,000+hospitalisations

$20roughly what an alarm costs

Source: U.S. Centers for Disease Control and Prevention, Carbon Monoxide Poisoning Basics.

Carbon monoxide binds to haemoglobin far more readily than oxygen does. Your blood keeps circulating and stops carrying what it is for. That is why the early symptoms are so vague — every tissue is mildly starved at once, and the brain complains first.

Flu, or carbon monoxide?

The symptoms overlap almost completely. The pattern does not.

	Points to flu	Points to carbon monoxide
Fever	Usually present	Absent — CO does not cause fever
Who is ill	One person, then others over days	Several people at once, including pets
Location	Follows you everywhere	Improves when you leave the building, returns when you come home
Timing	Any season, runs its course	Starts when heating season begins, or after a new appliance or a generator
Other signs	Sore throat, cough, aches	Confusion, unsteadiness, unusual sleepiness

Source: CDC carbon monoxide poisoning guidance; symptom presentation as described in clinical toxicology references.

One sentence from the CDC deserves to be read twice: people who are sleeping, drunk, or under the influence of other substances can die from CO poisoning before they have symptoms. The warning signs are only useful to someone awake enough to notice them, which is precisely why the alarm has to do the noticing.

Where it comes from

Anything that burns fuel produces some carbon monoxide. In a working appliance it goes up a flue. The danger arises when combustion is incomplete, when the flue is blocked, or when the burning happens somewhere it was never meant to.

Sources, ranked by how often they kill people

The top two are not appliances.

PORTABLE GENERATORSThe single largest cause of consumer CO deaths — around 100 a year. Twenty feet from the house, outdoors. An open garage door is not ventilation.

CARS IN GARAGESWarming a car up with the garage door open still fills the house. Modern keyless cars add a failure mode: one left running by accident, unheard, all night.

FURNACES AND BOILERSCracked heat exchangers and blocked flues. The reason for an annual service — it is a CO check as much as a reliability check.

BLOCKED CHIMNEYS AND VENTSBirds’ nests, leaves, and snow drifting over an outside vent after a storm. Worth checking after heavy snowfall.

CHARCOAL AND CAMPING STOVESNever indoors, in a tent, a garage, or on a balcony under an overhang. Charcoal is a notably heavy CO producer.

GAS OVENS USED FOR HEATA recurring cause in cold snaps among households that cannot afford heating. Dangerous, and it comes from being cold rather than careless.

Sources: CDC carbon monoxide guidance; U.S. Consumer Product Safety Commission portable generator hazard reporting.

Getting the alarm right

This is one of the few risks on this site where a single cheap object genuinely solves most of the problem — provided it is present, powered, and not expired.

Four things people get wrong

CDC and manufacturer guidance.

1. NEAR EVERY SLEEPING AREAThe alarm has to wake you. One in a hallway two floors down is decoration. Every level, including the basement.

2. THEY EXPIRE — USUALLY 5 TO 7 YEARSThe sensor degrades whether or not it has ever alarmed. Check the date printed on the unit; replace the whole alarm, not the battery.

3. A SMOKE ALARM IS NOT A CO ALARMDifferent sensor, different hazard. Combination units exist and are clearly labelled for both — if yours only says smoke, you are not covered.

4. GET A DIGITAL DISPLAYCDC suggests a readout model. It shows peak concentration recorded, which tells a technician — or a hospital — something a beep cannot.

Source: U.S. Centers for Disease Control and Prevention, CO detector placement and maintenance guidance.

If the alarm sounds

There is no version of this where you investigate first.

Source: CDC emergency guidance for carbon monoxide alarms.

What to own (paid link)

Short list, because the answer here really is one object per floor.

CO ALARM WITH DIGITAL READOUTLook for UL 2034 certification and a display showing peak reading. One near each sleeping area.
Browse on Amazon →

COMBINATION SMOKE + CO ALARMSensible where you would otherwise mount two units. Check it explicitly states both hazards.
Browse on Amazon →

We are not linking “CO detector cards” — the small colour-change patches sold as a cheap alternative. They require someone to be looking at them, which excludes everyone asleep, and that is the group this hazard kills. This is a category where the audible alarm is the product and there is no budget version of it.

Questions people ask

High or low on the wall?

Carbon monoxide has almost the same density as air, so it mixes rather than sinking or rising. Height matters far less than people assume — follow the manufacturer’s instructions for that specific unit. What matters much more is being near where people sleep, and not tucked behind furniture or right beside a fuel-burning appliance where normal brief spikes cause nuisance alarms.

I have an all-electric home. Do I need one?

Lower risk, not zero. An attached garage, a fireplace, a shared wall in an apartment building, or a neighbour’s generator during an outage all put CO within reach. In a flat with no attached garage and no combustion anywhere in the building, the argument is genuinely weak — but at twenty dollars, most people find the maths easy.

What happens at the hospital?

A blood test measures carboxyhaemoglobin — how much of your haemoglobin is carrying CO instead of oxygen. Treatment is high-flow oxygen, which displaces it far faster than fresh air alone; severe cases may involve hyperbaric oxygen. Tell them you suspect CO, because a standard pulse oximeter on the finger reads falsely normal in CO poisoning and can be reassuring when it should not be.

My alarm went off once and then stopped. Ignore it?

No. A brief alarm means CO reached a level worth alarming about, and something produced it. Get it checked — a heating engineer can test the appliances, and an alarm with a peak-reading display will tell them the number it saw. Intermittent alarms are frequently the first sign of a cracked heat exchanger.

The short version

• Over 400 Americans a year die from unintentional CO poisoning unrelated to fires, with 100,000+ emergency visits.
• Symptoms mimic flu — but CO causes no fever, and it makes everyone in the building ill at once, including pets.
• If symptoms improve when you leave the house and return when you come home, that is the pattern.
• People asleep or intoxicated can die before any symptoms. The alarm has to do the noticing.
• Biggest killers: portable generators and cars in garages — not faulty boilers.
• Alarms go near every sleeping area, on every level, and they expire in about 5–7 years.
• A smoke alarm is not a CO alarm. Check the label.
• If it sounds: everyone outside, call from outside, do not go back in.

Suspected carbon monoxide poisoning is a medical emergency. Get everyone into fresh air and call emergency services from outside the building. Tell medical staff you suspect CO — it changes both the test and the treatment.

On the links above: some are affiliate links, marked (paid link). If you buy through one we may earn a commission at no additional cost to you. As an Amazon Associate I earn from qualifying purchases. We link to product searches rather than specific items so recommendations do not break as models change, and we say plainly when we are choosing not to link something. Full policy: Affiliate Disclosure.

Sources

• U.S. Centers for Disease Control and Prevention. “Carbon Monoxide Poisoning Basics” and CO detector guidance.
• U.S. Consumer Product Safety Commission. Portable generator carbon monoxide hazard reporting and winter safety guidance.
• National Safety Council. Carbon monoxide safety topic.