Men who visit saunas may be damaging their sperm, Italian researchers have warned.
They found that healthy men who regularly used them had a lower sperm count - and the damage could last several months.
In the study, Finnish men in their 30s who had normal sperm counts were monitored.
They all spent 15 minutes in a sauna twice a week for three months, after which their sperm counts were tested.
Overheating: Men who regularly used had lower sperm counts, with the damage lasting several months
The researchers discovered that the men's sperm counts were lower at the end of the study than at the beginning - and only six months later did they return to normal.
Dr Andrew Kramer, a urologist at the University of Maryland Medical Centre told the website LiveScience that heat was to blame, as it affects sperm production.
The researchers found that during the sauna sessions, the temperature of the men's scrotums increased by three degrees Celsius (5.4 F).
'The testicles hang down from the body in men to cool them,' added Dr Kramer. He added this may be why 'men with undescended testicles can have impaired sperm product and fertility'.
Men who used the saunas also had differences in their sperm's DNA, the researchers noted.
Previous research has shown that heat from laptops can damage sperm, affecting fertility as it struggles to swim to the egg.
Reporting in the journal Human Reproduction, the researchers in the sauna study said: 'The large use of Finnish sauna in Nordic countries and its growing use in other parts of the world make it important to consider the impact of this lifestyle choice on men's fertility.'
Men who used the saunas also had differences in their sperm's DNA, the researchers noted
They added that in countries were saunas are widely used, doctors should flag up the problems they may cause to couples struggling to conceive.
Sperm counts are falling at an alarming rate - up to 38 per cent in a decade - with diet and lifestyle largely to blame.
A recent Spanish study found that even in young men, sperm concentration fell by an average of two per cent a year - and could soon hit levels where fertility is compromised.
A ten year-study of more than 200 men found the average concentration went from 72 million spermatozoids per millilitre in 2001 to 52 million/ml in 2011.
Meanwhile a British survey published last month found that nearly 20 per cent of couples spend more than a year trying for a baby.
And the effect of not being able to conceive is so bad for some couples that it forces them apart - or places their relationship under great pressure.
Experts have blamed women delaying motherhood and the obesity epidemic as the two most likely reasons for rising infertility.
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