Scientists searching for the causes of falling sperm counts are getting a clearer picture of the role played by chemical pollutants - and it’s not a pretty one.
A study of urine samples from nearly 100 male volunteers has uncovered "alarming" levels of endocrine disruptors known to reduce human fertility.
Cocktails of chemicals such as bisphenols and dioxins, which are believed to interfere with hormones and affect sperm quality, were present at levels up to 100 times those considered safe.
The median exposure to these chemicals was 17 times the levels deemed acceptable.
"Our mixture risk assessment of chemicals which affect male reproductive health reveals alarming exceedances of acceptable combined exposures," wrote the authors of the study, published on Thursday in the journal Environment International.
The study measured nine chemicals, including bisphenol, phthalates, and paracetamol, in urine samples from 98 Danish men aged 18 to 30.
Additionally, it used existing data, mostly from the European Food Standards Agency (EFSA), to estimate their likely exposure to 20 other chemicals.
The team then compared the results with acceptable levels of exposure derived from the scientific literature.
This resulted in a measure of the potential impact of each chemical, which the scientists combined to produce an overall risk measure - or "hazard index" - for the cocktail of compounds.
The study authors, led by Professor Andreas Kortenkamp of Brunel University London, said they were "astonished" by the magnitude of this hazard index in the volunteers studied.
They were also surprised to find that bisphenol A (BPA) was the dominant risk factor, given that recent research had focused on phthalates, which are used in plastics.
BPA was followed by dioxins, paracetamol, and phthalates. Removing BPA from the mix did not bring down the combined exposure to acceptable levels, and paracetamol was described as "a driver of mixture risks among subjects using the drug".
The researchers acknowledged some limitations to their research.
For example, the data used dated from 2009 to 2010, and exposure to BPA may have decreased since, while exposure to other chemicals may have increased.
Another uncertainty is whether women of reproductive age have the same levels of chemical exposure as the men in the study.
But the researchers stressed their research may actually underestimate the risk posed by exposure to these chemical cocktails, given "the multitude of chemicals humans are exposed to" - which were not all measured in this study.
Sperm count collapse
Sperm quantity and quality have dramatically declined across Western countries in recent decades, with research suggesting sperm counts have been more than halved in the space of 40 years.
Meanwhile, other reproductive health disorders have been on the rise, such as non-descending testes and testicular cancer.
Scientists around the world have considered a range of other possible causes behind falling sperm counts, including lifestyle factors, tobacco consumption, and air pollution.
But recent studies have increasingly zeroed in on the role played by chemicals.
"Our analysis has the character of a prediction which could be verified in suitably designed epidemiological studies of semen quality," Kortenkamp and his colleagues wrote.
Pending further studies in populations, the researchers called for urgent regulatory action such as a ban of BPA from food contact materials as a precautionary measure.
They added that animal studies looking at the impact of different doses of paracetamol on semen quality were "missing altogether and are urgently needed".