Damp and mould-infested houses could be the cause of permanent asthma in children, researchers have said.
Asthma is thought to have many potential triggers
Poor housing conditions are already linked to the illness but there is debate whether they cause asthma, or simply trigger attacks.
Finnish researchers writing in the European Respiratory Journal say they have proved this after surveying the homes of more than 300 children.
However, UK asthma experts are still not convinced mould can cause asthma.
Asthma is now the most common chronic disease of school-age children, and rates have risen steadily in recent years in industrialised countries.
Dr Juha Pekkanen, from the National Public Health Institute in Kuopio, suggests that as many as one in five cases of child asthma may be caused by moisture and mould in the home.
His team found that the severity of asthma increased alongside the severity of the damp in living areas.
In all, the homes of 121 asthmatic children were compared to those of 241 non-asthmatic children.
Leaks and stains
As well as a detailed interview and allergy test to rule out specific asthma triggers in the environment, experienced civil engineers ranked all the houses in terms of the level of damp and presence of mould.
Everything from leaks, condensation and damp stains to peeling on the surface of walls was recorded.
Evidence of serious damp or visible mould was seen two to three times more often in homes inhabited by asthmatic children. Mould and damp in 'non-family' parts of the house, however, was not linked to the illness.
The researchers said this was clear evidence that mould and damp caused asthma in children, as opposed to worsening or triggering attacks in children whose asthma had another underlying cause.
Dr Pekkanen said: "Damp in the home does more than merely exacerbate existing asthma - it can contribute to the onset of persistent asthma."
He called for doctors to be aware of the link when treating child asthmatics.
However, not all experts are convinced that the evidence points to a cause for asthma.
Dr Michael Burr, a researcher at Cardiff University whose work on mould and asthma is funded by the charity Asthma UK, said: "This study suggests that Finnish children with newly-diagnosed asthma are more likely than other children to have moisture damage and mould in their homes.
"Together with existing evidence, this suggests that mould probably triggers respiratory symptoms and may contribute to causing asthma.
"However, it is not possible to distinguish conclusively between the role of moisture damage and mould as a trigger factors and any causal link with childhood asthma based on the current evidence."