Sperm counts in men from America, Europe, Australia and New Zealand are on the decline, researchers have said.
In contrast, no significant decline has been reported in South America, Asia and Africa. The researchers, however, noted that far fewer studies have been conducted in these regions.
Sperm count is the average number of sperm present in one sample of semen and is assessed during routine semen analyses.
According to World Health Organisation (WHO) guidelines, experts consider a healthful sperm count to be 15 million per millilitre (ml), or at least 39 million per ejaculate.
A publication by Health News Today indicates that Doctors consider a sperm count under 15 million per ml to be low, and it may cause fertility issues
In Dr Shanna Swan's new book, “Count Down”, released earlier this year, the most likely culprit, she argues, is the proliferation of harmful chemicals such as bisphenol A (BPA)—which is most commonly found in household plastic goods.
According to Dr Swan, humans’ endocrine systems, which produce hormones including testosterone and oestrogen, can be adversely affected by these chemicals.
In some cases, they reduce fertility among both men and women.
Another study, conducted in Boston, looked at nearly 500 young men who hoped to donate sperm. It found that the share of applicants who were sufficiently fertile to donate had fallen from 69 per cent to 44 per cent in the ten years to 2013.
BPA chemicals may not be solely to blame. Another study, published by Environmental Pollution in 2018, collected the semen samples of 5,000 men living in northern Italy between 2010 and 2016. By geo-coding the men’s home addresses, it found that sperm counts deteriorated most when air pollution was highest.
Even if BPA chemicals are not the sole cause of the decline in sperm counts, regulators have been slow to catch on to the proven harm they cause.
In a similar study in 2017, Dr Swann and another Hagai Levine, who co-led the work at the Hebrew University-Hadassah Braun School of Public Health and Community Medicine in Jerusalem said a drop in sperm count was by over 50 per cent in less than 40 years.
The analysis, however, did not explore reasons for the decline, but researchers said falling sperm counts have previously been linked to various factors such as exposure to certain chemicals and pesticides, smoking, stress and obesity.
This suggests measures of sperm quality may reflect the impact of modern living on male health and act as a "canary in the coal mine" signalling broader health risks, they said.
Studies have reported declines in sperm count since the early 1990s, but many of those have been questioned because they did not account for potentially major confounding factors such as age, sexual activity and the types of men involved.
Working with a team of researchers in the United States, Brazil, Denmark, Israel and Spain, Levine screened and brought together the findings of 185 sperm count studies from 1973 to 2011 and then conducted a so-called meta-regression analysis.
Alarmingly, if the rich-world trend observed by Dr Swan in her 2017 study continued until 2045, it might render half the men of Europe and North America impotent.
Several studies have suggested that weight loss and exercise among people with overweight or obesity can lead to an improved or increased sperm count. However, the science linking a healthy body mass index (BMI) to a healthy sperm count is still weak.
Studies have indicated that regular exercise increased sperm count and motility in 45 men with obesity and sedentary lifestyles.
A 2016 meta-analysis that reviewed the results of over 20 studies with nearly 6,000 participants found that smoking consistently reduced sperm count.
The researchers found that people who smoked moderate or heavy amounts of tobacco had a lower sperm quality than people who smoked tobacco less heavily.
Some studies linked the worldwide use of drugs such as alcohol, marijuana, and cocaine to decreased sperm production. Some evidence is conflicting, so further research is necessary to confirm this link.