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.

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).