Can a boiler release carbon monoxide when off? No. 

If your boiler is switched off at the mains, your boiler will not be burning fuel and therefore no waste carbon monoxide gas will be produced.

How to know of your boiler could be leaking:

1. black, sooty marks on the front covers of gas fires.
2. sooty or yellow/brown stains on or around boilers, stoves or fires.
3. smoke building up in rooms because of a faulty flue.
4. yellow instead of blue flames coming from gas appliances.
5. pilot lights frequently blowing out.

Carbon monoxide is a poisonous gas that has no smell or taste. Breathing it in can make you unwell, and it can kill if you’re exposed to high levels.

Every year there are around 60 deaths from accidental carbon monoxide poisoning in England and Wales.

After carbon monoxide is breathed in, it enters your bloodstream and mixes with haemoglobin (the part of red blood cells that carry oxygen around your body) to form carboxyhaemoglobin.

When this happens, the blood is no longer able to carry oxygen, and this lack of oxygen causes the body’s cells and tissue to fail and die.

Symptoms of carbon monoxide poisoning

The symptoms of carbon monoxide poisoning are not always obvious, particularly during low-level exposure.

A tension-type headache is the most common symptom of mild carbon monoxide poisoning. 

Other symptoms include:

• dizziness
• feeling and being sick 
• tiredness and confusion 
• stomach pain 
• shortness of breath and difficulty breathing 

The symptoms of exposure to low levels of carbon monoxide can be similar to those of food poisoning and flu. 

But unlike flu, carbon monoxide poisoning does not cause a high temperature.

The symptoms can gradually get worse with prolonged exposure to carbon monoxide, leading to a delay in diagnosis.

Your symptoms may be less severe when you’re away from the source of the carbon monoxide. 

If this is the case, you should investigate the possibility of a carbon monoxide leak and ask a suitably qualified professional to check any appliances you think may be faulty and leaking gas. 

The longer you inhale the gas, the worse your symptoms will be.

You may lose balance, vision and memory and, eventually, you may lose consciousness. 

This can happen within 2 hours if there’s a lot of carbon monoxide in the air.

Long-term exposure to low levels of carbon monoxide can also lead to neurological symptoms, such as:

• difficulty thinking or concentrating
• frequent emotional changes – for example, becoming easily irritated, depressed, or making impulsive or irrational decisions

Breathing in high levels of carbon monoxide gas can cause more severe symptoms. 

These may include:

• impaired mental state and personality changes (intoxication)
• the feeling that you or the environment around you is spinning (vertigo)
• loss of physical co-ordination caused by underlying damage to the brain and nervous system (ataxia)
• breathlessness and a heart rate of more than 100 beats per minute (tachycardia) 
• chest pain caused by angina or a heart attack
• an uncontrollable burst of electrical activity in the brain that causes muscle spasms (seizures) 
• loss of consciousness – in cases where there are very high levels of carbon monoxide, death may occur within minutes

What causes carbon monoxide to leak?

Carbon monoxide is produced when fuels such as gas, oil, coal and wood do not burn fully.

Burning charcoal, running cars and the smoke from cigarettes also produce carbon monoxide gas.

Gas, oil, coal and wood are sources of fuel used in many household appliances, including:

• boilers 
• gas fires 
• central heating systems 
• water heaters 
• cookers 

Incorrectly installed, poorly maintained or poorly ventilated household appliances, such as cookers, heaters and central heating boilers, are the most common causes of accidental exposure to carbon monoxide.

The risk of exposure to carbon monoxide from portable devices may also be higher in caravans, boats and mobile homes.

Other possible causes of carbon monoxide poisoning include:

• blocked flues and chimneys – this can stop carbon monoxide escaping, allowing it to reach dangerous levels 
• burning fuel in an enclosed or unventilated space – for example, barbecue inside a garage, poorly installed heating systems, old boilers or a faulty boiler in an enclosed kitchen 

Source: www.nhs.co.uk