Cathy Killick reports from Bradford

The foundation has just completed two years of clinical trials on the device which works in a similar way to an extractor fan - it removes dirty air from a room, replacing it with clean fresh air. But the humidity of the incoming air is reduced and scientists say this will kill house dust mites, a major trigger for asthma.

Dr John Baruch from Bradford University says the air exchange system is very effective.

"The house dust mite only lives when the humidity is above 50%. People are quite happy in humidity down to 30%. Below that it gets a bit dry, but at 40% people are very comfortable and the dust mite is killed," Dr Baruch said.

The extractor costs around £300 to install and Mike Hodder, who has an asthmatic child, has fitted them to all the rooms in his house. He says it has made a big difference for his son Jake: "When he had the attacks it was pitiful to watch; he really suffered with it.

"I can only say what's happened is remarkable. I don't think it's cured him, but the improvement is due to the system that we have in the house which circulates the air efficiently and cleans it," he said. Jake has been able to reduce his medication.

The findings of the clinical trials into the system are due to be published shortly, but initial results have already excited the British Allergy Foundation. They hope the extractor will help many more children.

For inventor Chris Palmer, the alternative use for his smoke extractor was unexpected.