Quantifying the exposure-response relationship between temperature exposure and semen quality.

Plain-language summary

This study measured how outdoor temperature tracks with semen quality, using records from 5,114 men attending a reproductive medicine centre in Wenzhou, on China's east coast. The measure it used is apparent temperature, the "feels like" figure that combines air temperature with humidity, rather than the thermometer reading alone. Each 1 degree Celsius rise in apparent temperature over the 90 days before a sample corresponded to a fall of about 2.1 percentage points in progressive motility, meaning the share of sperm swimming forward usefully, and about 4.9 percentage points in total motility, the share moving at all. The authors report the effect showing up at lags of 15 to 69 days and 70 to 90 days before the sample, which covers most of the roughly three months it takes to make sperm. Their conclusion also advise avoiding heat in the two weeks immediately before ejaculation. This is an observational study, so it shows temperature and semen quality moving together rather than showing that one caused the other.

Key findings

• Each 1 degree Celsius rise in apparent temperature was associated with a fall of about 2.1 percentage points in progressive motility.
• The same rise was associated with a fall of about 4.9 percentage points in total motility.
• The association appeared at lag periods of 15 to 69 and 70 to 90 days before the sample.
• The analysis covered 5,114 men attending one reproductive medicine centre in eastern China.