Diurnal scrotal skin temperature and semen quality. The Danish First Pregnancy Planner Study Team.

Plain-language summary

This study asked whether the ordinary, everyday variation in scrotal temperature is enough to matter for sperm. Sixty men from a group of couples planning their first pregnancy wore a portable recorder that logged scrotal skin temperature for three days, kept a daily diary of working hours and posture, and gave a semen sample. The median reading was 33.3 degrees Celsius (91.9 degrees Fahrenheit) in the daytime and 34.8 degrees Celsius (94.6 degrees Fahrenheit) at night, and stretches of sedentary work ran about 0.7 degrees Celsius (1.3 degrees Fahrenheit) warmer than the rest of the day. Men whose daytime readings sat above 35 degrees Celsius (95 degrees Fahrenheit) more than three quarters of the time had a median sperm concentration of 33.4 million per milliliter, against 91.8 million for men who were that warm less than half the time. This is one of four papers from the same Danish research program. Its larger sibling on temperature and semen quality, already in this library, reported that sedentary work by itself did not track with semen quality, so the program is not of one mind about how much the sitting itself matters.

Key findings

• Scrotal skin temperature ran at a median of 33.3 degrees Celsius (91.9 degrees Fahrenheit) by day and 34.8 degrees Celsius (94.6 degrees Fahrenheit) at night in these 60 men.
• Sedentary work raised scrotal temperature by about 0.7 degrees Celsius (1.3 degrees Fahrenheit).
• Men who were above 35 degrees Celsius (95 degrees Fahrenheit) for more than three quarters of their daytime readings had a median sperm concentration of 33.4 million per milliliter, against 91.8 million for men who were that warm less than half the time.
• Temperature was higher in the daytime, in summer, and during leisure time than during working hours.