US researchers have said that men who take hot baths may reduce their fertility. The conclusion came after a study which found that 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. The findings from the three year study support current advice that men should avoid 'overheating' their sperm.

The researchers from the University of California, San Francisco, carried our their study because although 'wet heat' was believed to damage fertility, there had been very little actual research into it. For the study, 11 men were recruited who attended a fertility clinic and who were exposed to more than 30 minutes per week of 'wet heat' through hot baths, Jacuzzis or hot tubs.

After three to six months of staying out of the bath, five of the 11 men showed dramatic five-fold improvement in sperm count, with sperm motility increasing from 12% to 34% in the men who responded to cutting out baths. Of the six men who showed no improvement five were chronic smokers, which the researchers said could have influenced the lack of response.

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."

A previous study in 1965 had looked at the link between hot baths and fertility and found a temporary decline in sperm production in men who were exposed to 'wet heat' for 30 minutes on alternating days. However, the study did not look at sperm quality before and after the study, which is where the more recent study succeeded.

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."

Sperm develop best in cool surroundings which is why the testicles are situated outside the man's body within the scrotum.