The association between extreme heat and sperm quality in a chronically heat-exposed population

Background: Extreme heat has detrimental effects on male fertility. Previous studies conducted mainly in temperate countries have reported an association between higher ambient temperatures and decreased sperm quality. These findings might not be applicable to populations in tropical regions, who are chronically exposed to high ambient heat. Objectives: To address this knowledge gap, we measured the association between extreme heat during the spermatogenic life cycle and sperm quality in a population of 914 males who are 21–43 years old residing in a tropical country, Singapore. Methods: Medical records from 2011 to 2022 were analyzed alongside daily weather station records. Linear regression was used to estimate the association between the occurrence of days with maximum temperature >34.7°C during the 90-day spermatogenic life cycle and indicators of sperm quality. Sensitivity analyses were conducted for participants exposed to extreme heat multiple times during spermatogenesis. Results: Adjusting for age, race, body mass index, and days of abstinence before sample collection, exposure to extreme heat during the late stage of spermatogenesis (0–33 days before collection) was associated with lower sperm concentration (β = −9.3 million/mL, 95% confidence interval: –17, –1.8, P = 0.015). This amounted to a 20% reduction from the mean sperm concentration in the sample. Multiple (≥2) days of exposure to extreme heat in the late stage of spermatogenesis were associated with a larger reduction in sperm concentration (β = −12 million/mL, 95% confidence interval: −22, −2, P = 0.019). Conclusion: Even in tropical regions, with high ambient temperatures year-round, the occurrence of extremely hot days during the late stages of spermatogenesis is associated with lower sperm concentration. The degree of risk increases with more frequent exposures to extreme heat.