The Effects of Ambient Temperature on Sperm Quality in the Negev

Background: There is a growing body of literature indicating a decline in sperm quality. As the climate warms, temperature becomes a more significant factor that can potentially affect sperm quality. Assuming that the human body can adapt to new and/or extreme climates, the effect on sperm is expected to differ among native populations and immigrants. This study is based in the Negev region of Israel, which is unique for its high temperatures and a high prevalence of immigrants.

Primary Aim: The primary aim of this study is to examine the effects of ambient temperature, and potential adaptation to warming temperatures, on sperm quality in the Negev population.

Secondary Aims:

1. To analyze the impact of immigration status years since immigration on potential adaptation to increased temperatures.
2. To study non-environmental factors such as age, ethnic origin, medications, and comorbidity on sperm quality in the Negev.
3. To study the trend of sperm quality over time as it relates to ambient temperature.

Hypothesis: Our hypothesis is that higher temperatures in the three months preceding semen testing negatively affect sperm quality across all parameters. This effect is seen more prominently among naïve populations than Israeli-born individuals.

Methods: This project is a retrospective cross-sectional study. We analyzed sperm quality data from the SUMC fertility clinic collected during 2009-2022 against ambient temperature, comparing Israeli-born and naïve populations. We obtained data on the minimum and maximum daily temperature from the closest Israeli Meteorological and Ministry of Environmental Protection stations to the patients’ residence for up to 90 days prior to sample collection. We explored trends in sperm quality over the 20 years of data collection using time series methodology. The effect of heat on sperm quality was investigated within layers of immigration status, indicating the level of adaptation of men to heat. Sperm concentration, sperm volume, percent motile (mobile) sperm, and progressive motility were the main parameters of sperm quality analysis.

Statistical Analysis: In addition to t-tests, Chi2, and correlation tests, we used Directed Acyclic Graphs (DAGs) to select potential confounding variables to include in regression models. We employed distributed lag models (DLMs) and time series analyses to analyze the delayed response to heat exposure.

Results: To investigate the relationship between temperature and motility, we constructed crossbasis matrices representing the lagged effects of temperature on motility. We created a second crossbasis to predict the behavior of relative humidity. Each matrix was built for lag lengths preceding the sperm analysis: 90, 30, 14, and 7 days lag. A linear function was used to predict environmental factors’ patterns, and a fourth-degree polynomial function was used to model lag effects.

We formulated a linear model to predict motility based on the temperature and relative humidity matrices. The socioeconomic level was included in the model to account for potential confounding. To accommodate clustering of observations within individual participants (PID), we specified PID as a random intercept term. We assumed a Gaussian distribution for the response variable, consistent with our continuous outcome variable (motility).

A Generalized Additive Mixed Model (GAMM) was used to analyze the 7-day lag period, including smoothed socioeconomic and relative humidity variables as covariates and PID as the random cluster intercept. The coefficient effect for the mean maximum daily temperature for the 7 days preceding testing in summer months was -0.1766 with borderline significance (p-value 0.121). This effect was stronger amongst nonimmigrants (-0.2330, p-value= 0.0861). Interestingly, the effect was reversed in the winter months (0.35198, p-value < 0.001), indicating that an increase in temperature increased the percentage of motile sperm.

Conclusions: These results suggest that the rise in temperature interacts with sperm quality differently throughout the seasons. The effects of increased temperature on percent motile sperm, as modeled in both the DLM and GAMM, are more pronounced in the summer months. In winter, higher temperatures are associated with improved sperm motility. These effects did not significantly differ between immigrants and nonimmigrants. Socioeconomic level was strongly correlated with low motility.

Discussion: Based on these results, it appears that both high and low temperatures can decrease percent motile sperm. The next step is to examine the effects of heatwaves and cold spells on sperm motility and compare these effects between socioeconomic and ethnic subgroups.

Significance of Study: This study serves as a case study for the effects of climate change on sperm quality in arid climates.

Study Limitations: The major limitation in this study is the difficulty of assessing individual exposure to environmental factors to an exact degree. We will address this difficulty by controlling for socioeconomic status, approximated by membership to Clalit HMO clinic.

Keywords: Adaptation, sperm quality, spermatogenesis, ambient temperature, climate change, lag-distributed model.