A scoping review on the impact of ambient temperature on human infertility.

Ambient temperature is a well-known environmental factor affecting gamete production in mammals. Despite a growing interest in biological mechanisms and epidemiologic studies for ambient temperature and human reproductive outcomes, systematic evidence synthesis is limited. Mapping human infertility outcomes in relation to ambient temperature is important to inform public health and identify research gaps. This scoping review systematically searched (n = 21,332) and included eligible epidemiologic studies (n = 135) on temperature and fertility in human populations to identify the types of temperature metrics and outcomes examined. Included studies used a variety of temperature exposure metrics: ambient temperature measurements, thermal indices, extreme temperatures (e.g., heatwaves), season, occupational heat exposure, climate region, and outdoor activities. Seven types of outcomes were identified across various geographical regions: sperm parameters, outcomes of assisted reproductive technologies, pregnancy loss, population-level reproductive outcomes (e.g., birth rate), infertility diagnosis (yes/no), testicular torsion, and ovarian function. The largest number of studies was for sperm parameters, followed by outcomes of assisted reproductive technologies. Results consistently indicated that higher temperature exposure was associated with reduced sperm parameters (e.g., motility) and increased odds of infertility, while other outcome types showed heterogeneous exposure-response associations. While the methodologies vary by the type of exposures and outcomes, most studies lacked longitudinal or prospective study designs, detailed description of exposure assessments, and consideration for non-monotonic exposure-response associations. This review maps research on temperature and human infertility, highlighting methodological limitations and knowledge gaps to guide future epidemiologic studies and systematic evidence synthesis for evidence-based public health and clinical decision making.