Associations of chronic and acute ambient heat exposure with sex hormones: A longitudinal study in women receiving assisted reproductive technology

BACKGROUND: Acute and chronic heat stress disturb sex hormone levels. However, epidemiological evidence on their associations remains lacking. We aimed to explore the relationship between chronic and acute heat exposure and female sex hormones. METHODS: We conducted a longitudinal study including 19790 women receiving assisted reproductive technology from 2014 to 2020. We extracted participants' daily mean temperatures from a validated grid dataset. Chronic exposures were represented using the proportions of medium, high, and extreme heat days during the year before hormone measurement. Acute exposures were six different defined heatwaves during the week before measurement. To quantify their associations, we used linear mixed-effect models with subject-specific intercepts. Besides, we introduced the heat index (HI) to investigate the combined effects of heat exposure and humidity on sex hormones, and also explored potential modifying effects of greenness. RESULTS: For every 10% increase in the proportion of extreme heat days, E2 decreased by -15.1% (-19.2%, -10.7%), P decreased by -21.1% (-26.8%, -14.9%), T decreased by -9.9% (-13.8%, -5.8%), and LH increased by 28.6% (11.9%, 47.8%). Acute heatwave exposure across six definitions was all significantly linked to reduced E2 and elevated LH. Differing from heat exposure alone, chronic HI was associated with elevated E2 [28.1% (18.6%, 38.4%)] and P [20.4% (9.6%, 32.3%)], and acute HI was associated with elevated P [2.4% (1.3%, 3.5%)]. Besides, high greenness modified the negative impact of chronic heat exposure on P. CONCLUSION: Heat exposure may adversely affect female sex hormone homeostasis, with humidity and greenness playing a moderating role.