What is carbon monoxide and carbon monoxide poisoning?

Last updated: 9 months ago

The deadly gas is a risk in nearly every home. But what is carbon monoxide?

You’ve probably heard that carbon monoxide (CO) is a colourless, odourless gas that is deadly to humans and animals. In the UK alone, it kills up to 50 people per year and sends another 4,000 to the hospital. But what is carbon monoxide, exactly, and where does it come from? We’ll cover these questions – and most importantly, how to protect yourself and your family – in this article.

What is carbon monoxide?

If you can, think back to the big periodic table of elements diagram that hung on the wall of your science classroom: hydrogen, helium, lithium, beryllium...

Periodic Table of Elements

Atoms bind together to form molecules, the building blocks of everything we encounter in the physical world. For example, hydrogen dioxide, or H20, is two hydrogen atoms (H2) and one oxygen atom (O) bound together.  Of course, we know this simply as “water”.

Similarly, carbon monoxide, or CO, is a single carbon molecule (C) bound to a single oxygen molecule (O).

Carbon Monoxide Molecule

How is carbon monoxide (CO) formed?

Carbon monoxide is created when a carbon-based fuel is burned. Most of the fuel sources we use to heat our homes and power our vehicles (natural gas, oil, wood, petrol, etc) are carbon-based. The leftover carbon molecules bind with leftover oxygen molecules, creating carbon monoxide.

What is carbon monoxide poisoning?

Carbon monoxide poisoning occurs when there is too much carbon monoxide in your bloodstream and not enough oxygen. This lack of oxygen to your vital organs causes tissue damage and eventually death. Because carbon monoxide is colourless and odourless, and the symptoms can be gradual, it is often hard to know you’ve been poisoned until it’s too late.

A large carbon monoxide leak can easily kill you in a matter of hours. Prolonged exposure to a milder leak can cause vague symptoms as your tissues are progressively damaged. If you survive carbon monoxide poisoning, you can still be left with permanent brain and tissue damage.

Symptoms of carbon monoxide poisoning

The symptoms of CO poisoning can be subtle, gradual, and easily attributed to other causes, which can make it hard to diagnose. A headache or feeling sleepy doesn’t usually set off any alarm bells, which is why it’s important to have a carbon monoxide detector that will alert you to a leak before it can harm you. Unfortunately, by the time you notice symptoms, the damage has often already occurred.

Symptoms can include:

  • Headache
  • Confusion
  • Tiredness
  • Memory loss
  • Vison loss
  • Dizziness or vertigo
  • Loss of balance
  • Hallucinations
  • Nausea or vomiting
  • Stomach pain
  • Difficulty breathing or shortness of breath
  • Seizures

There can be other signs that these symptoms are related to CO poisoning, for instance:

  • Multiple people in the household exhibit similar symptoms
  • Symptoms improve when away from home (e.g. on holiday)
  • Symptoms are seasonal (especially during heating season)
  • An unexplained death of a household pet

It has been theorized that many reported “hauntings” from the past could be attributed to mild carbon monoxide poisoning from using gas lamps indoors. This theory is strengthened by the fact that reports of ghosts and hauntings sharply decreased as electricity became commonplace in homes.

What causes carbon monoxide leaks?

Boilers, woodstoves, space heaters, and fireplaces are common sources of carbon monoxide leaks in the home.

Normally, any carbon monoxide produced while heating your home is vented away safely by the chimney, dissipating as soon as it mixes with the outside air. However, if the chimney is blocked or your heater malfunctions, it can lead to a rapid build-up of the dangerous gas indoors.

Carbon monoxide build-up can also happen for other reasons, like running a car in a closed garage, or operating a fuel-burning space heater in a tent. No matter how tempting it might be, don’t bring your barbecue indoors.

Preventing carbon monoxide poisoning

Preventative measures go a long way in protecting against carbon monoxide leaks and poisoning, like properly maintaining your boiler or heater, having the chimney cleaned and inspected regularly, and generally not burning fuel in enclosed spaces.

However, the consequences of carbon monoxide poisoning are so dire you need to have a working carbon monoxide detector, even if you’re taking all other precautions.

Carbon monoxide alarms

Like a smoke alarm, a carbon monoxide alarm (also called a carbon monoxide detector) can detect the presence of carbon monoxide in the air with a variety of methods. Unlike smoke and fire, carbon monoxide is impossible to detect without a sensor. While nearly everyone understands the importance of a smoke alarm, not as many people are aware of the necessity of a carbon monoxide alarm.

Where should carbon monoxide alarms be placed?

Your home should have at least one carbon monoxide alarm on every level, and one outside of any bedroom or sleeping area.

Carbon monoxide alarms come in mains-powered and battery-powered versions. Before you purchase a mains-powered detector, make sure you have hardwired connection points at every location an alarm is required.

Our smart carbon monoxide detectors


This popular detector covers all the bases. Combining both a smoke and carbon monoxide detector, it protects your household in one smart package. The Nest Protect checks its own batteries and sensors over 400 times a day. It also performs monthly speaker and siren tests once per month at your preferred time (so you’re not jolted out of bed!) The smoke detector can distinguish between fast burning and slow smouldering fires, in addition to knowing the difference between smoke and steam. A built-in light illuminates your path if the alarm sounds at night. If you also have a Nest thermostat, you can configure it to automatically shut off your boiler if carbon monoxide is detected. It's available in both battery-powered and mains-powered versions.


This carbon monoxide detector works with the Honeywell Evohome Security System.  The Evohome system stays connected even without WiFi, ensuring you receive notifications even during a power outage. Its internal memory retains information for 24 hours, so you’ll know about events even if you’re not available for the notification. Different alarms sound depending on the amount of carbon monoxide detected.

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