Claims that Scotland's childhood asthma pandemic has been due to the infestation of Scottish homes with house dust mites have been looked at by a BBC Scotland investigation.
The house dust mite is not quite visible to the naked eye
The house dust mite (HDM) at one third of a millimetre in size is not quite visible to the naked eye.
Millions of them exist in every home in the country attached to fibres in carpets, beds and sofas. They munch on moisture-bloated skin cells discarded by us with every movement we make.
The HDM produces an allergen in the form of a pellet that 80% of children with asthma are allergic to.
The World Health Organisation has suggested that there are limits for healthy living with the allergen.
A symptomatic allergic response is likely with 20ug of HDM allergen per gram of dust.
Charlie McSharry, clinical scientist in immunology at Glasgow University, tested Glasgow homes and found that half had higher levels than that.
He said: "Infestation in Glasgow seems to be fairly high in a global reference term, with these results likely to represent Scotland as a whole. "
Asthma can be triggered by viruses, but it is generally as the result of an allergic response.
Brian Lipworth, head of the Allergy and Asthma Research Group at Ninewells Hospital, said he believes that HDMs cannot be held responsible for the third of 13-14 year olds in Scotland having asthmatic symptoms.
His view is that Scottish doctors have been very efficient at spotting asthma, pushing up the statistics.
But Stirling Howieson. of Strathclyde University's Centre for Environmental Design and Research, said he is convinced that extreme levels of house dust mite are to blame for the pandemic.
Dr Howieson argued that it is the "only hypothesis that bears water".
It is his role as expert witness that is driving forward legal arguments in cases where tenants believe their damp accommodation has caused their child's asthma. HDMs thrive in damp environments.
Mike Dailly, of Govan Law Centre, said he believes convincing a sheriff that a damp house inflicted asthma upon a child would revolutionise landlord responsibilities.
He cites the example of the United States.
He said: "It's interesting in America when it comes to damp, mouldy housing that insurance companies, as I understand it, will exclude such claims because the damages are so high in America, and that is a real incentive for landlords to take action."
Mr Dailly hopes to replicate the asbestos campaign, but says it is a challenge as there is no proof that HDMs actually cause asthma.
'No specific risk'
Dr Howieson has his gaze focused further than simple maintenance.
He believes the drive for well insulated houses has produced decades of draft free homes.
He added: "We have built in the last 30 years buildings that are much warmer, much tighter and for that reason the indoor air quality is poorer and the relative humidity- the amount of moisture in the dwellings, is higher."
The Scots love timber-framed homes. We build a higher proportion than the rest of the UK.
Dr Howieson said the means of insulating timber-framed houses promotes a highly moist domestic environment "so people living in modern timber framed dwellings are living inside plastic bags".
The UK Timber-Frame Association rejected this suggestion. They stated that "there is no specific risk of poor air quality… because no room or part of the home is 'sealed'".
The idea that modifying our home environment to reduce HDMs can prevent or control asthmatic symptoms is still controversial.
Is our housing stock contributing to the pandemic?
Stirling Howieson said: "I personally don't need any more evidence", but the remainder of the jury is still out.
You can hear more on the controversial claims surrounding the cause of childhood asthma on BBC Radio Scotland's Morning Extra programme from 0900 BST on Monday, 7 July.