Relationships between GPS-derived outdoor activity space ambient heat exposure, mental health, and salivary cortisol in a longitudinal, repeated measures sample of adult detroiters

Exposure to heat in urban settings is a significant and growing public health concern. Yet, few studies have longitudinally measured everyday heat exposure using GPS-derived activity space data, paired with fine resolution satellite-derived heat measures, and no studies have measured effects on salivary cortisol in a community-dwelling sample. We address these gaps using data collected 2019-2023 across 11 neighborhoods in Detroit. Participants wore a GPS device and accelerometer over a one-week period. They completed a survey on demographics and mental health (anxiety and depression symptoms) and provided salivary cortisol samples (used to calculate diurnal slope) each year. We used a novel land surface temperature fusion product from ConstellR at daily cadence and 30-m spatial resolution to calculate heat exposure. We linked the seasonal time series of 2-band Enhanced Vegetation Index values calculated from the Harmonized Landsat-Sentinel-2 product with accumulated growing degree-days calculated from the ConstellR fusion product to model the land surface phenology at each pixel and used these modeled EVI2 values to estimate the exposure to greenness. We then tested whether higher heat exposure during routine daily outdoor activities was associated with poorer mental health or salivary cortisol data and whether these effects were attenuated by exposure to greenness. Among participants with ≥ 60 minutes of outdoor non-vehicle time, we observed a positive association between heat and anxiety (coef = 0.007, p-value = 0.062) and depression (coef = 0.007, p = 0.036) symptoms, after full confounder adjustment. Greenness was not independently, significantly associated with mental health measures and did not alter the effect size of heat exposure. We also observed a significant, positive association between heat exposure and cortisol slope (coef < 0.001, p-value = 0.029), indicative of higher stress. The consistency in our findings strengthens the evidence of the mental health impacts of everyday heat exposure, beyond extreme heat events. These findings have urban planning implications for increasing tree canopy, lowering impervious surfaces, providing heat shelters, and other urban cooling strategies. City-level or neighborhood-level heat action plans may help reduce the broader health impacts of rising temperatures in a warming climate.