Background While the association between indoor temperature and health has been indirectly investigated using variables including self-reported cold homes and the presence of insulation as proxies for temperature, research using indoor temperature directly has been lacking. We hypothesised that lower indoor temperatures would be associated with increased odds of poor general health, which would be partially explained by socioeconomic and housing variables.

Methods We used data from the Health Survey for England collected between 2003 and 2014. The analytical sample consisted of 74 735 participants aged over 16. Indoor temperature was approximated using a one-off reading of indoor air temperature in participants’ homes. Self-reported general health was used for the main outcome, dichotomised into very good/good/fair vs. bad/very bad.

Using Stata, logistic regression models were built for the data, using maximum likelihood estimation. The complex survey design and weights were taken into account in the analysis. The influence of socioeconomic and housing factors was assessed by adjusting for these variables in the models.

Results The mean indoor temperature was 20.6°C (SE 0.09), which varied seasonally and regionally. Warmer indoor temperatures were associated with greater odds of poor general health. The age and sex-adjusted odds ratio (OR) of poor health for each degree increase in indoor temperature was 1.01 (95% CI 1.01–1.02 P<0.01). Lower social class and lower education level were associated with warmer temperatures and poorer health. The OR decreased after adjusting for these confounding variables. Higher income was associated with warmer temperatures and fairer health. After adjusting for both the suppressor and confounding variables, the OR for poor health for each degree increase in temperature has a statistically significant increase by 19% to 1.02 (95% CI 1.01–1.03 P<0.01).

Conclusion Our analysis unexpectedly showed an association between high indoor temperature and poor health. The relationship between socioeconomic factors and indoor temperature was complicated, with different socioeconomic variables having different directions of association with indoor temperature. Further research is needed to understand the temporality and direction of the association. The research is timely given the potential for data from increasingly prevalent smart thermostats.