ITV News was filming a routine gas inspection this morning when the engineer detected potentially lethal levels coming from a gas fire in a house in Morden.
Badly fitted or poorly maintained appliances can mean a killer is in your home without you knowing - perhaps until it’s too late.
Around 50 people are killed by carbon monoxide each year.
New Freedom of Information disclosures from UK Fire Services show incidents of poisoning rose from 2450 incidents in 2014 to 3249 in the last 12 months - an increase of 32%.
Campaigners say 40 million people are at risk because they have no alarm.It is so dangerous that even small amounts can poison you slowly - and at high levels it can kill in minutes.
For years there have been warnings, yet the government has missed the opportunity to substantially reduce the danger.
Currently the law in England, Wales and Northern Ireland only requires landlords to fit carbon monoxide alarms if the residence has a solid fuel burning appliance. Some MPs, campaigners and even many landlords believe all homes with gas should have mandatory alarms.
You can’t see, smell or taste carbon monoxide - which is why it is so often called "the silent killer".
Some will ask why government remains silent on this important safety reforms that seem long overdue.
What are the symptoms of carbon monoxide poisoning and who is most at risk?
Symptoms may be hard to distinguish from other commons ailments.
Dizziness, tiredness, headaches, nausea and generally feeling unwell are common.
Carbon monoxide poisoning is commonly mistaken for other illnesses, leaving the elderly and very young most vulnerable.
The gas is known as a silent killer; due to the fact it does not have a colour, smell or taste.
How can I stay safe from being poisoned by carbon monoxide?
The only effective protection from carbon monoxide is to have an alarm installed.
An estimated two thirds of homes are unprotected, leaving around 40 million people at risk.
Alarms should be installed between one and three metres away from any fuel-burning appliance, as well as in areas like bedrooms and living rooms.
Gas appliances should also be installed and serviced regularly by a Gas Safe registered engineer.
What should I do if my carbon monoxide alarm goes off?
If your alarm starts making a noise, you should open external doors and windows to ventilate the space. Make sure you turn off gas appliances before leaving the building.
It's important to not turn on any electrical appliances or smoke, a spark could ignite a gas explosion.
If you are feeling unwell, seek medical advice by calling 999.
The Gas Emergency Service line should also be contacted on 0800 111 999, before you return to the property.
It's advised to then have your appliances serviced by a qualified and registered gas engineer.
When should I get my appliances tested and whose responsibility is it to pay for it?
In a rented property, it is the landlord's responsibility to ensure that all equipment is properly maintained. This applies to any heat-generating appliance, regardless of the fuel it uses.
Landlords should provide a gas safety certificate to say an inspection has been done.
If you have flues which extract fumes then these also need to be checked.
If you own your home, you'll need to pay for servicing and maintenance yourself.
Some energy suppliers will do free gas safety checks for people on means-tested benefits, it's worth contacting your provider to see if this applies to you.