Soaking in the tub may reduce men's fertility, say US researchers.
Heat can affect sperm production
Findings from a three-year study support current advice that men should avoid 'overheating' their sperm.
Sperm counts in five of 11 men with fertility problems soared by 491% after they stopped having baths or using the hot tub for a few months.
Other research has shown heat from laptop use and wearing tight underwear can reduce fertility the Journal of the Brazilian Society of Urology reports.
The researchers from the University of California, San Francisco, said although it had been believed for decades that 'wet heat' could damage fertility, there had been very little research.
Men attending a fertility clinic who were exposed to more than 30 minutes per week of 'wet heat' through hot baths, Jacuzzis or hot tubs, were recruited to the study.
After three to six months of staying out of the bath, just under half the men showed dramatic five-fold improvement in sperm count.
Sperm motility increased from 12% to 34% in the men who responded to cutting out baths.
Five of the six men who showed no improvement were chronic smokers, which the researchers said could have influenced the lack of response.
Sperm are known to develop best in cool surroundings which is why the testicles are situated outside the man's body within the scrotum.
Study leader, Dr Paul Turek, director of the UCSF Male Reproductive Health Center said: "These activities can be comfortably added to that list of lifestyle recommendations and 'things to avoid' as men attempt to conceive."
He added that if men could improve their fertility through avoiding hot baths, couples may be able to avoid IVF or choose less invasive treatment.
"Couples really prefer having kids at home and not with technology. This is a way to help them do that."
According to Dr Turek, the only other published study looking at the link between hot baths and fertility was done in 1965.
After exposing men to 'wet heat' for 30 minutes on alternating days, researchers found a temporary decline in sperm production but did not look at sperm quality before and after the study.
Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield said it seemed intuitive that hot baths could contribute to reduced numbers of sperm but it was unclear whether it actually contributed to fertility.
"Ideally, this study needs to be repeated with a much larger number of patients, and with a clearly defined control group, before we can be certain that hot baths are a genuine risk factor for male sub-fertility
"Changes in sperm quality are one thing, but it is pregnancies that matter.
"However, it would do no harm for men who are concerned about their fertility to take a shower instead of a bath."