Changes to temperatures in children's bedrooms can affect their lung function for at least two weeks, according to a new study from the University of Otago.
Researchers from the Housing and Health Research Programme/He Kainga Oranga looked at more than a million temperature measurements in 405 homes.
They found that changes in children's bedroom temperatures were much more important than those in living room temperatures.
Researcher Nevil Pierse said the effects were greatest when children were exposed to temperatures of less than 12C, and a decrease in lung function was still detectable two weeks after exposure.
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"Kiwi homes are much colder than those overseas. The World Health Organisation recommends children sleep in rooms no less than 20C, and the harm of changes below 12C is more than 10 times that of changes at 18C.
"With New Zealand having one of the highest rates of asthma in the world, the research further confirms the importance of keeping our children warm at night."
Dr Pierse said the Warm Up New Zealand campaign had insulated about 230,000 houses since it began in 2009, but the benefits were not reaching the group that needed them most - tenants.
He believes the programme should continue, supplemented by support for initiatives such as the He Kainga Oranga and New Zealand Green Building Council plan for a Warrant of Fitness-type assessment for homes.
"As a new father myself, I know how expensive it is to keep New Zealand homes warm," Dr Pierse said.
"We must do everything we can to ensure warmer homes. It's critical to our children's growth and development, and reducing our extremely high asthma rates."
The research has been published by the Journal of Epidemiology & Community Health.