A new report reveals that sperm counts among men in Western countries, including men in North America, Australia, New Zealand and Europe, have dropped substantially over the years. According to study authors, in less than 40 years, collective sperm count among this group of men has declined more than 50%. Sperm count is currently considered the best measure of male fertility.

The new study, published Tuesday in the journal Human Reproduction Update, not only shows that men’s sperm counts are dropping, but that the continued decline does not appear to be leveling off.

“The results are indeed very profound, and even shocking,” says study author Dr. Hagai Levine, head of the environmental health track at the Hebrew University-Hadassah Braun School of Public Health and Community Medicine in Jerusalem.