What is carbon monoxide?
A gas that has no colour or smell, so it's very hard to tell if there is a leak.
What causes it?
It's let off when the things we use in boilers - gas, oil, coal and wood - don't burn properly.
Why doesn't it burn properly?
Often because the boiler is faulty. Sometimes it hasn't been properly put in, or it's broken down and no-one has noticed.
How can you stop this?
It's important to get trained people to carry out regular checks to make sure boilers are working correctly.
Most people in the UK make sure their boilers are working properly and get the checks carried out, so carbon monoxide poisoning is quite rare.
Alarms are also available, which work just like smoke alarms, beeping and flashing when they sense carbon monoxide.
Why is carbon monoxide so harmful?
Once it gets into people's lungs, it starts removing oxygen from the blood. People need oxygen to make sure their heart and brain are working properly.
Once carbon monoxide gets into the blood, it puts the heart and brain at risk.
People affected by carbon monoxide poisoning get headaches. If they take on more gas, they can feel sick and dizzy.
If too much gas is breathed in, it can kill people.
Children and people with breathing problems are particularly at risk, often feeling ill before healthy adults start getting the symptoms.