Introduction Many epidemiological studies have shown that low outdoor temperatures lead to increased mortality and hospitalisation, generally finding the strongest association to be with temperature below a given threshold measured at a lag of 7–14 days. In contrast, very little work has been done on the health effects of exposure to low indoor temperatures, and none on the best metric of this exposure, despite the fact that people have greater exposure to the indoor environment. The scarcity of studies on the association between indoor temperature and health is due to the difficulty in measuring indoor temperatures and health outcomes regularly and simultaneously over an extended time period.

Methods The Heating Housing and Health Study is an RCT which investigates the effect of installing heaters in asthmatic children's homes. It has detailed measurements of lung function (daily) and indoor temperature (hourly).

Lung function and indoor temperature were measured for 309 children over 12049 child-days. For four measures of lung function (PEFR morning, PEFR evening, FEV1 morning and FEV1 evening) we attempted to find the strongest association between exposure to low temperatures below particular thresholds averaged over various periods.

Results Indoor temperatures were found to have a small, but significant, association with short-term variations in the lung function in children with asthma. This association was greatest for temperatures below 11°C in the child's bedroom averaged over the preceding 11 days.

Conclusion These findings provide valuable information for future studies looking at the effects of low indoor temperatures on respiratory health.