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The first domino piece was a series of studies during Gustavo Luis Verón’s PhD when he was analyzing semen quality based on different factors in Argentina. Obesity, age and climatic factors were some of the variables he investigated. At the end of 2021, the results showed that semen samples generated in winter were noticeably of better quality than those in summer. These data – far from going unnoticed – made him obsessed with another question: Can increasing heat waves then put male fertility in jeopardy? The latest findings by Verón and his colleagues seem to indicate that they can. According to the report published this yearthe semen samples obtained during these events presented 10% fewer motile spermatozoa – the only ones capable of fertilization – than the samples that were never exposed. These high temperatures also affected their morphology.
For Dr. Monica Vazquez-Levin, principal investigator at the National Scientific and Technical Research Council (Conicet) and one of the authors of the report, given the frequency and intensity of these events, “the climate will eventually be a variable for planning parenthood”: “The idea that men always have sperm and that they are always good is a myth. They will have to start taking measures and will end up preserving their sperm at an early age, just as women do with their eggs today.”
While there are previous studies that have reported a negative association between temperature and semen quality, the focus has mainly been on Asian and European populations and not necessarily on the impact of heat waves. One of papersby a group of Chinese academics, revealed the impact on bakers and welders. The research by Dr. Verón, Genetics graduate Ania Manjon, and Dr. Vazquez-Levin, is the first report to focus on the Latin American population and, specifically, the Argentine population. The team identified an average of 6.5 heat waves per year, between 2005 and 2023, the period studied, and an increase of half a degree per decade. Although they recorded 287 days with high temperatures, they focused on episodes of at least three consecutive days between 22 and 32.3°, based on how the scientific community defines heat waves.
The study also estimated an increase of six days of heat waves per decade. This figure doubles the warming trend that Spain is experiencing, for example, according to the Report on the State of the Climate in Spain 2023, prepared by the State Meteorological Agency (Aemet), which points to an average of three days per decade.
Testicular temperature in mammals is vital for efficient spermatogenesis [el momento en que los testículos generan los espermatozoides]. The evolutionary transition of the human being required the externalization of the testicles precisely as a thermal regulator in practically all mammals, except for some such as elephants. And, consequently, the conditions that induce thermal stress are associated with a decrease in semen quality. Currently, the WHO has issued guidelines for semen evaluation which take into account sperm count, vitality, motility (whether and how they move) and morphology; as well as external factors. The latest publication, from 2021, did not yet include climatic factors.
In addition to the effect on motile sperm, the scientists noted that the moment in which the heat wave occurs is also very relevant. Men exposed to increases in temperature during spermatogenesis exhibited a lower number of sperm and a worse morphology than those who suffered the heat wave at the time of ejaculation. In an increasingly less distant future, this information could serve to shape a useful medical tool. “One cannot control by oneself when there is a heat wave or not,” explains Vazquez-Levin, founder and coordinator of AndroLATAM, a network of professionals in Male Reproductive Health in Latin America. “But it can recommend when it is best to try to conceive based on the days that have passed since the hot moments.”
Another variable taken into account in this study carried out with 54,926 men aged between 18 and 60 was age. The results show that men under 40 years of age exposed to heat had a lower volume of semen, as well as sperm, although the morphology was normal. Men over 40 also had a lower proportion of live sperm and their morphology was affected. Additional analysis revealed a significantly negative association between semen quality and the length of the heat wave. That is, prolonged exposure is more harmful than acute exposure.
![Sitting on hot concrete, a homeless man asks for support from a woman wiping her sweat during a heat wave in Mexicali, Mexico, on June 14.](https://imagenes.elpais.com/resizer/v2/AZ4JG6MGVZE6JPUFV7AGZBTFUI.jpg?auth=de5f6c465be789a524bc97385ee4fe412a7eeccda1cb6b6301130320db0e6fb9&width=414)
All these findings are enough for Ms. Manjon to encourage men to break the taboo of having regular check-ups. “Beyond the desire to conceive or not, we are talking about men’s health,” she says via video call. The doctor, who accompanies her in the interview, nods and adds: “We know much more about women’s infertility than about men’s. But governments have to make decisions to reverse these climatic phenomena, now that we know that they affect them. Because we are talking about the permanence of humans on earth. There is already literature about the extinction of species due to heat. And we are just another species.”
The gap in male infertility research was another reason why they decided to tackle it. Another reason was to want to “take a weight off women’s shoulders”: “Historically, there has been a huge burden on our shoulders. The inability to conceive has always been attributed to women,” says Vazquez-Levin. Science — once again — shows how wrong we were. Nearly 17.5% of the world’s adults (one in six) will be infertile, according to the WHO. Half of these cases will be due to male infertility. “One of the causes we already know is global warming. Heat waves are going to have an impact on the world’s demographics if we do nothing,” she concludes.
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