A toxic gas could improve the lives of patients with serious lung conditions, scientists suggest.
Carbon monoxide - found in car exhausts and produced by faulty heaters - causes more than 50 deaths a year in the UK.
Dutch researchers, reports New Scientist magazine, believe at very low doses, it could relieve asthma and chronic obstructive pulmonary disease.
However, a second specialist says he has serious doubts about its safety as a treatment.
"The difficulty with carbon monoxide is that there is a fine balance between levels that influence COPD, and toxic levels, and it would be very hard to gauge a safe dose for patients"
Carbon monoxide, an odourless and invisible gas, is dangerous in high doses because it is taken up and stored by red blood cells instead of oxygen, with the result that insufficient oxygen is transported around the body.
The gas is produced naturally in the body as a by-product of some chemical reactions, but the researchers from the University Medical Center at Groningen said it has some surprising - and useful - properties.
Serious diseases such as COPD, often a combination of chronic bronchitis and emphysema caused by years of smoking, involve serious inflammation of lung tissues, and carbon monoxide appears to be able to ease this reaction, potentially improving symptoms.
The Dutch team gave 18 people a low dose of carbon monoxide for two hours on four consecutive days.
They then measured the level of a certain type of immune cell linked to inflammation in the lung mucus of the patients, and found that, on average, it had fallen by a third.
The lungs themselves also became more resistant to the effects of an irritating chemical.
Toxic warning
Bigger trials are planned, and drug firms are looking for ways to make drugs which slowly release carbon monoxide, so it does not have to be breathed.
However, Dr Paolo Paredi from the National Heart and Lung Institute in London, who has carried out his own research into the presence of carbon monoxide in the breath of COPD patients, insisted that the gas was not a good candidate treatment.
He said: "Personally, I don't see any future in this idea, and I am far more excited about other drugs for COPD that are being researched.
"The difficulty with carbon monoxide is that there is a fine balance between levels that influence COPD, and toxic levels, and it would be very hard to gauge a safe dose for patients. This makes it potentially quite dangerous."
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