Sperm count is not only an indicator of human fertility but also that of men's health, with low levels being associated with increased risk of chronic disease, testicular cancer and a decreased lifespan, the researchers said.

The decline reflects a global crisis related to modern environment and lifestyle, with broad implications for the survival of the human species, they said.

The study, published on Tuesday in the journal Human Reproduction Update, used data from 53 countries. It includes an additional seven years of data collection (2011-2018) and focuses on sperm count trends among men in regions not reviewed previously, specifically South America, Asia and Africa.

The data shows, for the first time, that men in those regions share the significant decline in total sperm counts (TSC) and sperm concentration (SC) seen previously in North America, Europe and Australia.

The study shows an accelerated post-2000 decline in TSC and SC globally.

"India is part of this larger trend. In India, due to availability of good data (including 23 estimates in our study, one of the countries with the richest data), we have more certainty that there is a strong and sustainable decline, but it's similar globally," Professor Hagai Levine of Hebrew University of Jerusalem in Israel, told PTI. "Overall, we're seeing a significant worldwide decline in sperm counts of over 50 per cent in the past 46 years, a decline that has accelerated in recent years," Levine said.

While the current study did not examine the causes of sperm count declines, Levine pointed to recent research indicating that disturbances in the development of the reproductive tract during fetal life are linked to lifetime impairment of fertility and other markers of reproductive dysfunction.

"Additionally, lifestyle choices and chemicals in the environment are adversely affecting this fetal development," Levine explained.

"Our findings serve as a canary in a coal mine. We have a serious problem on our hands that, if not mitigated, could threaten mankind's survival.

"We urgently call for global action to promote healthier environments for all species and reduce exposures and behaviors that threaten our reproductive health," Levine said.

The scientist noted that a separate study in India should be done, better by following the same population over time.

"However, there is no reason to think the trend in India is different," he added. Professor Shanna Swan at the Icahn School of Medicine, Mount Sinai, US, stressed that low sperm counts do not only affect men's fertility, but have serious ramifications for men's health more generally, and are linked with other adverse trends, termed together as testicular dysgenesis syndrome.

"The troubling declines in men's sperm concentration and total sperm counts at over 1 per cent each year as reported in our paper are consistent with adverse trends in other men's health outcomes, such as testicular cancer, hormonal disruption, and genital birth defects, as well as declines in female reproductive health," Swan added