A newly published study shows sperm counts in men have declined significantly in recent years, raising concerns about human extinction.
The study, which assesses the results of nearly 200 samples, was carried out by a multinational team and published on the Human Reproduction Update journal of Oxford University Press.
It demonstrates a significant decline in sperm counts between 1973 and 2011, a 50-60 percent decline among men unselected by fertility from North America, Europe, Australia and New Zealand.
Specifically, the researchers found a 52.4-percent decline in sperm concentration, and a 59.3-percent decline in total sperm counts in those areas. The study also indicates that the rate of decline is continuing and possibly even increasing.
Hagai Levine, lead researcher of the study, said in an interview with BBC that he was "very worried" about what might happen in the future.
"Eventually we may have a problem, and with reproduction in general, it may be the extinction of the human species," Levine said.
There is no clear reason for the apparent decrease. However, possible causes include exposure to chemicals used in pesticides and plastics, obesity, smoking, stress, diet and even watching too much TV.
Because of the significant public health implications of the results, the study calls for more research on the causes of the decline.
"We must take action, for example, better regulation of man-made chemicals, and we must continue our efforts on tackling smoking and obesity," Levine said about ways of reversing the descending trend.