A Longitudinal Study on the Impact of Indoor Temperature on Heat-Related Symptoms in Older Adults Living in Non–Air-Conditioned Households

BACKGROUND: Both chronic and acute heat result in a substantial health burden globally, causing particular concern for at-risk populations, such as older adults. Outdoor temperatures are often assessed as the exposure and are used for heat warning systems despite individuals spending most of their time indoors. Many studies use ecological designs, with death or hospitalizations rates. Individual-level outcomes that are directly related to heat-symptoms should also be considered to refine prevention efforts. OBJECTIVES: In this longitudinal study, we assessed the association between indoor temperature and proximal symptoms in individuals [Formula: see text] of age living in non–air-conditioned households in Montérégie, Quebec, during the 2017–2018 summer months. METHODS: We gathered continuously measured indoor temperature and humidity from HOBO sensors and repeated health-related questionnaires about health-related symptoms administered across three periods of increasing outdoor temperatures, where the reference measurement (T1) occurred during a cool period with a target temperature of 18–22°C and two measurements (T2 and T3) occurred during warmer periods with target temperatures of 28–30°C and 30–33°C, respectively. We used generalized estimating equations with Poisson regression models and estimated risk ratios (RRs) between temperature, humidity, and each heat-related symptom. RESULTS: Participants ([Formula: see text]) had an average age [Formula: see text] of [Formula: see text]. Higher indoor temperatures were associated with increased risk of dry mouth (T3 [Formula: see text]; 95% CI: 1.8, 3.5), fatigue ([Formula: see text]; 95% CI: 1.8, 3.0), thirst ([Formula: see text]; 95% CI: 2.5, 4.5), less frequent urination ([Formula: see text]; 95% CI: 1.8, 7.3), and trouble sleeping ([Formula: see text]; 95% CI: 1.6, 3.2) compared with T1. We identified a nonlinear relationship with indoor temperatures across most symptoms of interest. DISCUSSION: This study identified that increasing indoor temperatures were associated with various health symptoms. By considering the prevalence of these early stage outcomes and indoor temperature exposures, adaptation strategies may be improved to minimize the burden of heat among vulnerable communities. https://doi.org/10.1289/EHP10291