Extreme heat affects men’s fertility and birth outcomes
Punishing hot weather not only affects a person’s health or work productivity, but also affects couples’ fertility and birth outcomes, a project by the National University of Singapore (NUS) has found.
Rising temperatures could further reduce Singapore’s resident total fertility rate, which dipped below 1 – a record low – in 2023.
Researchers from the NUS Yong Loo Lin School of Medicine studied sperm samples from 818 men that were already stored at the National University Hospital’s (NUH) andrology department.
The scientists then traced the men’s exposure to extreme heat – or when a day’s average temperature exceeds 29.8 deg C – by looking at weather records 90 days before they provided semen samples at NUH.
The team found that those who were exposed to extreme heat during the three months had a 46 per cent higher risk of low sperm count and a 40 per cent increased risk of low sperm concentration. The reproductive cells were also found to be less motile and more sluggish.
These findings were more pronounced for men between the ages of 25 and 35, who tend to be at the stage of entering fatherhood, said research fellow Samuel Gunther, one of the researchers in the team.
The fertility and heat research is part of Project HeatSafe, which brings together several studies and fieldwork by NUS researchers and partners over 3½ years to investigate how rising temperatures affect the health and productivity of people here and in the region – with a focus on outdoor workers.
“Conventionally, findings suggest that sperm quality decreases as one ages, but what we found in this study was that it was men in their (prime) reproductive time between 25 and 35 who were the most impacted by heat,” said Dr Gunther at a media briefing on March 18, where Project HeatSafe researchers gave a round-up of their projects.
“So just because you’re a young male, don’t think you’re invincible and don’t think you’re not also vulnerable to these impacts. Moving forward, the climate is going to get hotter. And that is also something that we need to bear in mind in family planning.”
Given the low fertility rate in Singapore, the researchers advised men who are planning to conceive in one to three months to avoid going outdoors on extremely hot days, and to sleep in cooler environments. They should also avoid saunas, hot baths and tight underwear during that period.
The 818 sperm samples from NUS came from men who had issues with conceiving, and a smaller portion of them were patients undergoing chemotherapy who wanted to preserve their sperm.
Heat is known to affect reproductive cells – it can lower sperm count and motility, and affect women’s ovulation cycle and egg quality.
However, the links between extreme heat and fertility have not been well-studied in tropical countries such as Singapore, added Dr Gunther.
The fertility study also scanned the birth records of more than 31,000 women, showing that pregnant women tend to take more protective measures, such as ramping up air conditioning. Avoiding extreme heat during the third trimester of pregnancy was therefore associated with a lower risk of premature births.
Associate Professor Chan Shiao-Yng from the medical school’s department of obstetrics and gynaecology, who was also involved in the study, said: “We know that women in their first pregnancy always behave more cautiously and impose more self-restrictions.
“We need to identify in future work the specific behavioural changes that actually make a difference to clinical outcomes. Then we can come up with guidelines that can be implemented across the board (for pregnant women).”
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