Image copyright Roland Wessling Image caption Hazel and Roland together just before the accident in 2011

When Roland Wessling looked over to his partner Hazel Woodhams he knew instantly that she was dead. What he didn't yet know was that her killer was still in their tent, and that his life was also in danger.

Carbon monoxide, a colourless, odourless gas, had leaked from their extinguished barbecue. Hazel and Roland had been poisoned as they slept.

Half of the world's population use basic carbon-based fuels such as coal, wood and dung to cook and heat their homes, often in poorly ventilated spaces.

The dangers of being too close to an open fire are clear to most of us, but what many people don't know is the potentially fatal threat of a carbon fuel fire that is no longer burning.

Silent killer

In 2011 Hazel and Roland took up camping and bought a new tent. Hazel turned 30 and they decided to take a short trip to the Norfolk Broads for a weekend of camping and canoeing.

Barbeque safety

Never take a smouldering or lit BBQ into a tent, caravan or cabin. Even if you have finished cooking the BBQ should remain outside as it will still give off fumes for some hours after use

Never use a BBQ inside to keep you warm

Never leave a lit BBQ unattended or while sleeping

Place your cooking area well away from your tent. Always ensure there is an adequate supply of fresh air in the area where the BBQ is being used

Only use your BBQ in accordance with the operating instructions

Remember the signs and symptoms of carbon monoxide poisoning - headaches, dizziness, breathlessness, nausea, collapse and loss of consciousness

Source - Gas Safety Register

She had seen the deadly effects of carbon monoxide in her job as a scenes-of-crime officer, and the couple had a carbon monoxide alarm in their home, but they had not thought to bring it with them on their trip.

"We were doing all the cooking, nice and safe, outside the tent," Roland explained. But several hours later they decided to bring the barbecue inside the tent to protect it from rain and passers-by.

"The barbecue was cold to the touch. There was no smoke coming off it, no glowing, it seemed to be completely inactive."

Media playback is unsupported on your device Media caption Claudia Hammond finds out how carbon monoxide poisoning can be treated

"We put it into the porch area of our tent and simply went to sleep."

But the barbeque wasn't inactive at all.

When fuels are burning, the carbon monoxide being emitted is converted into relatively harmless carbon dioxide. But once the flame has gone out carbon monoxide continues to be produced and is no longer burnt off.

In a well-ventilated area this is no problem, but inside Roland and Hazel's air-tight tent the carbon monoxide accumulated to toxic levels and overwhelmed them.

"I remember waking up in the morning feeling extremely sick, extremely disorientated. I can honestly say I never felt worse in my life," recalls Roland.

"I started screaming for help but we had picked a particular part of the campsite that was very secluded. Nobody heard me."

Eventually Roland did summon help and was rushed to hospital. It was only then that doctors realised that the levels of carbon monoxide in his blood were sky-high.

As carbon monoxide is inhaled, it enters the bloodstream and binds to the haemoglobin in red blood cells, replacing and blocking the oxygen molecules which are normally attached.

Carbon monoxide advice

Warning signs can include symptoms that disappear if you are away from your house, or which are worse in winter when the central heating is on more

Other people in the house, or pets, fall ill with similar symptoms

Clues to a leak can include black, sooty marks around gas fires, boilers or stoves

Smoke building up in rooms due to faulty flues

Do not sleep in a room that has an unflued gas fire or a paraffin heater

Make sure your kitchen has an extractor fan

Yellow instead of blue flames from gas appliances

To be safe, you should never use ovens or gas ranges to heat your home

Never use oversized pots on your gas stove, or place foil around the burners

Make sure rooms are well-ventilated and do not block air vents

Sources: NHS Choices

At extremely high levels, like in Roland and Hazel's cases, carbon monoxide can quickly replace nearly all of the oxygen. Victims are effectively suffocated and organs are quickly starved of the oxygen they need.

The brain is particularly vulnerable and can be suffocated in as little as four minutes. Those who do survive may see the effects worsening over the coming days and weeks.

They can develop symptoms including difficulty with concentration, hearing and vision, and mood effects including anxiety and depression.

High-pressure therapy

Administering pure oxygen displaces carbon monoxide from the haemoglobin and is necessary in the early minutes and hours after poisoning. This is normally done using a basic oxygen mask.

But Roland was offered hyperbaric oxygen therapy - where oxygen is delivered at high pressure - which experts believe helps to reduce inflammation and prevent long-term brain damage.

He was lucky to be close to a treatment centre and his neurological recovery has been good.

During therapy, patients sit in a pressurised oxygen chamber, the kind originally used for scuba divers with decompression sickness.

"It is a bit like being in a plane and going on a journey with yourself and your companions," explains Dr Pieter Bothma, medical director of the London Hyperbaric Unit at Whipps Cross Hospital.

"We give them oxygen under pressure for about 30 minutes, then they get an air break, and then they have a further session of oxygen."

Image caption Patients in hyperbaric chamber

But the research on the use of hyperbaric oxygen in the treatment of carbon monoxide poisoning is conflicting. A 2011 review of the best studies found insufficient evidence to recommend it as a treatment. Experts agree further work is needed.

But even hyperbaric treatment is not perfect. The best way to deal with carbon monoxide is not to be poisoned by it in the first place.

"It's like any other neurological injury", says Dr Bothma. We understand the brain, and how to lessen damage to it, very poorly. Prevention is absolutely the key."