Cooler bedroom temperatures are harming children's lungs, a University of Otago study has found.
New findings from the housing and health research programme at the university have shown that temperatures under 12degC can have a large effect on a child's lung function and can increase asthma symptoms.
Researcher Dr Nevil Pierse recorded a 20% reduction in lung function for some children living in colder environments through the winters of 2005 and 2006.
As little as a 1degC reduction over an hour could reduce a child's lung function by 10ml per second.
The study looked at children between 4 and 13 years old in 405 homes in five separate centres - Bluff, Dunedin, Christchurch, Porirua and the Hutt Valley, near Wellington.
Dr Pierse said the effects were greatest when children were exposed to temperatures less than 12degC and that a reduced lung function was still detectable two weeks later.
He could not be sure whether the cold temperatures had a long-term effect on lung function, but said asthmatic symptoms did start to appear in children in colder homes.
The World Health Organisation recommends children sleep in rooms no cooler than 20degC. It added the harm caused by changes below 12degC was more than 10 times that at 18degC.
Dr Pierse said New Zealand had one of the highest rates of asthma in the world and his research further confirmed the importance of keeping children warm at night.
He said raising the bedroom temperature above 12degC was as effective at asthma prevention as an asthma inhaler or other asthma preventers.
Despite the Government's Warm Up New Zealand scheme, under which about 230,000 houses had been insulated since 2009, many of those most in need were missing out, he said.
Non-insulated and older homes needed attention from the Government, he said.
Dr Pierse believed the programme should be continued. Establishing a minimum requirement or ''warrant of fitness'' was a possible solution.
Dr Pierse said parents need not worry about overheating a room, as the perfect room temperature lay between 21degC and 30degC.