NUS Study Shows That Hot Weather Affects Men’s Fertility & Sperm Count

No one is surprised by Singapore’s historically low fertility rate of 1.10 per woman now, considering the rising financial costs of child planning, difficulty balancing work and family commitments, emotional labour of raising a child and…rising temperatures in Singapore?

Yes, you’re not seeing things. I am also talking about the heat.

According to Climategov, the year 2023 was the warmest year since global records began in 1850 at 1.18°C above the 20th-century average of 13.9°C. 

This has raised concerns for the health risks associated with rising temperatures. Majority of people experiencing heat-related stresses may present with fatigue, or vague symptoms like lethargy or exhaustion. 

Recently, NUS researchers have drawn a link between Singapore’s decreasing fertility rate and its increasing temperatures, backed up by credible evidence and thorough research. This can only suggest one thing: is Singapore’s fertility rate going to continue plummeting?

What Even is This Study?

Project HeatSafe is a large-scale study by National University of Singapore (NUS) researchers aimed at understanding the impacts of rising temperatures on the productivity, well-being and health of occupational workers living in Singapore. 

The study was led by NUS Yong Loo Lin School of Medicine (NUS Medicine), Singapore’s first and largest medical school, and took place between the course of October 2020 and March 2024. Its research findings were released on 18 March.

The research project comprised nine study areas — including environmental monitoring at workplaces, a fertility and pregnancy study, and an economic analysis — each led by different researchers and spanning different study durations. 

How Was It Done?

According to The Straits Times, to gather the data, NUS researchers took 818 sperm samples which had already been stored in andrology in National University of Singapore (NUH). These sperm samples were from men who had trouble conceiving and those who were undergoing chemotherapy and wanted to preserve their sperm.

The researchers then traced each man’s exposure to extreme heat (in this case, when days in Singapore rose to 29.8 degrees celsius or higher). They did this by examining the weather records in Singapore 90 days before they were provided with the sperm samples by NUH.

What Does This Mean?

The researchers found out that men who had been exposed to extreme heat during the 90 days were 46% more likely to have a lower sperm count and a 40% more likely to have a low sperm count; the study showed that their reproductive cells were more sluggish and less motile, which could be linked to Singapore’s declining fertility rate. 

The research findings stood out more for men who were between the ages of 25 and 35, as men between these ages are typically those entering the stages of fatherhood. 

“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 according to the Straits Times.

According to the National Institute of Health (NIH), heat stress can have large effects on most aspects of reproductive function in mammals.

These include disruptions in spermatogenesis and oocyte development, oocyte maturation, early embryonic development, foetal and placental growth and lactation.

However, while some may say the 818 men came from a biased group since they were from those who reported trouble conceiving or undergoing chemotherapy, the study was about linking sperm quality with heat exposure, said Associate Professor Chan Shiao-Yng from the medical school’s Department of Obstetrics and Gynaecology, who was also involved in the study. 

Given the already  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. For example, avoiding saunas, hot baths and tight underwear during that period.

It’s only going to get hotter from here onwards.