Why winter is a fertility minefield for men

sperm count
sperm count

Brass monkeys beware. When the mercury drops, some of the most common comforts we turn to in order to stay snug may be having a detrimental effect on male fertility.

Winter warmers such as heated car seats, electric blankets, saunas and even thermal underwear could be playing havoc with the crown jewels, warn experts.

That winter could be bad for male fertility flies in the face of some research. While humans are not seasonal breeders, studies have found differences in sperm counts, with an average 30 per cent reduction in sperm counts in summer compared with winter. The science is not conclusive, however, and persistent cold could also be a problem – another study found that long-term exposure to cold could induce sperm DNA injury, although it does not affect sperm quality.

Problems arise when heat is applied to the testes indirectly through a range of winter devices. The testes work best when they are two to four degrees below body temperature. Any significant increase in testicular temperature for a prolonged period can damage sperm and reduce sperm count, meaning many of our favourite winter warming techniques could be out of bounds for men concerned with fertility.

Dr Jonathan Ramsay is a consultant urologist who deals exclusively with male fertility. He explains: “The testicles function optimally at temperatures of 32.5 degrees, because we were designed to run around naked. So, if you are sitting on a heated car seat wearing tight thermal underwear it’s going to be like sitting in a hot bath, which is not good. As a rule, uncommonly hot and uncommonly cold are not ideal. Winter is not a good season for sperm.”

Heated car seats

Warming up your back and posterior with a heated seat on the way to work on a frosty morning is one of life’s luxuries. But stewing your privates in your Peugeot for any prolonged period may not be doing your sperm count any favours.

Dr Channa Jayasena, clinical senior lecturer at Imperial College London and consultant in reproductive endocrinology and andrology at Hammersmith and St Mary’s hospitals, explains: “Anything that heats up your testes for a sustained period is going to be bad for them. If you are driving very long distances using a heated car seat that makes your groin very hot for several hours, it will undoubtedly have an effect on your fertility.

“While we don’t have many direct studies on car seats, we do know how the testes work and we know that for brief periods there shouldn’t be a problem. Most men don’t use heated car seats in that way, so they are fine. But if you use one for hours and if you have contributing factors – if you are overweight, if you smoke, if you drink alcohol excessively, these things pile up.”

Studies show that long exposure to heated car seats can reduce sperm function
Studies show that long exposure to heated car seats can reduce sperm function - iStockPhoto

One study suggests that 90 minutes is enough to start to heat up the testes to problematic levels. In a University of Giessen study, 30 healthy volunteers sat on heated car seats for 90 minutes while fitted with temperature sensors. An hour into the trial, the average scrotal temperature of the men sitting on the heated car seats was 37.3°C, with one man reaching a high of 39.7°C. In contrast, the average scrotal temperature of volunteers sitting on non-heated car seats was 36.7°C.This was enough of an increase to damage sperm production, according to Andreas Jung, leader of the study.

Dr Shabana Bora, Consultant Gynaecologist and Fertility Specialist at the Lister Fertility Clinic agrees. “While there is no evidence, you can assume that if you are sitting in a heated car seat for hours and it’s really hot, it is probably not going to be very good for your sperm function because your testicles are designed to stay cool,” she says.

Thermal underwear

Studies have shown that wearing tight underwear can increase testicular temperature, but whether this heating causes infertility has yet to be definitely proved.

Nevertheless, fertility clinics routinely advise men suffering from fertility issues not to wear tight-fitting underwear.

Dr Channa says: “If you are a man who wants to maximise fertility or has had problems with fertility, we advise about underwear. Thermal underwear will have the same effect as tight fitting underwear. Anything that in effect makes the testes the same temperature as the body could be detrimental.”

In the same vein, tight fitting Lycra is also advised against, particularly if worn on a long winter bike ride, as studies have shown that cycling can also be bad news for sperm.

Electric blankets

While regular use of an electric blanket should not be a problem, overuse could start to affect fertility, as Dr Channa explains.

“They are useful for many people, and it is very unlikely there will be a problem unless you are using them more than you should, or getting overheated to the point where you are sweating,” he says.

Overusing an electric blanket could start to affect fertility - Getty Images

With any device that warms the nether regions, he advises that sweating is a good benchmark to use when determining if you might be causing yourself a problem.

Bubble baths and hot tubs

It sounds obvious, but science proves that regularly boiling your balls in a hot bath is not a good idea. One study involving men who took hot baths for at least 30 minutes a week found that most saw an average increase in total motile sperm counts of 491 per cent (motility relates to sperm movement) after they stopped bathing in hot water.

Dr Shobana says: “Frequent hot baths are not recommended if you are concerned about fertility. If you take them every day for an hour at a time, that’s probably not good. Moderation is key.”

Harding: 'It sounds obvious...but regularly boiling your balls in a hot bath is not a good idea'
Harding: 'It sounds obvious...but regularly boiling your balls in a hot bath is not a good idea' - Getty Images

Dr Channa adds: “Studies have shown that taking a long hot bath for 20 minutes or more will negatively affect sperm production in the testes.”


One study found that exposure to a sauna for an average of two hours and 24 minutes every two weeks decreased sperm counts by a maximum of 50 per cent.

And while ice baths and plunges are sometimes claimed to increase testosterone levels, there is no evidence to support this. In fact, prolonged exposure to cold could damage sperm production.

Dr Ramsay says: “Cool is good, but cold could be bad and if you are the type of man who gets competitive in the plunge pool or ice bath and stays in for prolonged periods you might be reducing your sperm levels. Ideally you want to be on the uncomfortable side of cool, but not so cold it hurts.”

Winter chills

Unavoidable as they are, winter chills and fevers have a detrimental effect on fertility and Covid appears to have an effect on sperm production.

Harding: winter chills and fevers have a detrimental effect on fertility
Harding: winter chills and fevers have a detrimental effect on fertility - Getty Images

Dr Channa explains: “Men who get ill tend to have reduced libido. The hormone signals are affected. Heat from a fever doesn’t necessarily damage sperm but being ill will temporarily reduce fertility because the sex drive is reduced.”

And there also appears to be a link between Covid and reduced male fertility. “There have been cases where people have had Covid, and it has severely affected their sperm function,” Dr Shobana explains. “It can take months to recover.”


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