WASHINGTON • The quality of sperm from men in North America, Europe and Australia has declined dramatically over the past 40 years, with a 52.4 per cent drop in sperm concentration, according to a recent study.

The research, which takes the most comprehensive look at the topic, is the largest on it and involves data from 185 studies and 42,000 men around the world between 1973 and 2011.

The study, published on Tuesday, appears to confirm fears that male reproductive health may be declining, the Washington Post reported.

Lead researcher Hagai Levine, an epidemiologist, told the BBC that if the trend continued, humans would become extinct.

"If we will not change the ways that we are living and the environment and the chemicals that we are exposed to, I am very worried about what will happen in the future," he said.

"Eventually we may have a problem, and with reproduction in general, and it may be the extinction of the human species."

Professor Shanna Swan, one of the authors of the study published in the Human Reproduction Update, told the Washington Post the new meta-analysis is broad and comprehensive, and "shows the decline is strong and that the decline is continuing".

The analysis found drops only for men in Europe, North America and Australia and not for those in South America, Asia and Africa.

Prof Swan explained that this could mean there is something specific to certain cultures or regions that affects sperm, adding that it is also possible there just is not enough data yet to draw firm conclusions about the rest of the world.

There have been far fewer sperm studies conducted in non-Western countries. Some of the most recent ones have also noted a decline in aspects of sperm quality. A study published earlier this year about China's Hunan province found that 56 per cent of sperm donations met the criteria for health in 2001 versus only 18 per cent in 2015.

The most important data points in the study involved sperm concentrations for "unselected" men who have not yet proven they are fertile. These are men in the studies who are younger and are not yet fathers or do not have partners who are pregnant.

Researchers estimated that these men had an average sperm concentration of 99 million per ml in 1973 but this dropped to an average 47 million per ml in 2011.

According to World Health Organisation criteria, men with a sperm concentration of less than 40 million per ml are considered to have an impaired chance of conceiving and those with less than 15 million per ml are unlikely to be able to have children.

These numbers mean "surprisingly higher proportions of men are falling into the infertile and sub-fertile categories", Ms Swan said.

Many scientists say the most sensitive period may be during the first trimester, when the developing foetus' reproductive system may be affected by a mother smoking, stress she experienced or food she ate. Exposure to chemicals that can change hormone levels, known as endocrine disrupters, is among the issues being studied.

Over their lifespan, men are also exposed to a number of other things that could potentially influence sperm concentration: pesticides, lead, X-rays, stress and countless other factors.

The changes in the womb can also cause permanent damage, Prof Swan said, while the adult exposures are mostly reversible.

The issue of sperm is not just about having babies. Poor sperm health has been linked to cardiovascular issues, obesity, cancer and more generally to higher rates of hospitalisation and death.

Prof Swan said: "Having a low sperm count is a signal that there's something wrong in men's health overall."