Semen quality in Western countries has been declining over the past several decades, and we are beginning to understand why.

Heavy television watching has been associated with many health-related consequences because of decreased physical activity. But it can also have effects on scrotal temperature, which may be a cause of poor sperm quality.

A new study in the British Journal of Sports Medicine (published online ahead of print) examined 189 men ages 18-22, evaluating their levels of physical activity, hours of TV watching, and semen quality.

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TV watching had an inverse relationship with sperm concentration and total sperm count. Men in the highest quartile of TV watching (more than 20 hours per week) had a 44% lower sperm concentration than men who watch no TV at all.

Not surprisingly, men in the highest quartile of moderate-to-vigorous activity (more than 15 hours per week) had a 73% higher sperm concentration than men in the lowest quartile (less than 5 hours per week).