Researchers at the University of Copenhagen
found that filters commonly used in sunscreens to absorb ultraviolet light could affect male fertility by stopping sperm from functioning properly.
"Globally, we see that semen quality is generally very poor," says Niels Skakkebaek
, an endocrinologist at the university's Rigshospitalet
hospital unit who led the study
, presented at the Endocrine Society's
annual meeting in Boston earlier this month. The findings are yet to be published in a peer-reviewed journal.
These filters are one of a range of chemicals used in consumer products, known as endocrine disruptors, that are associated with health effects such as infertility.
The researchers tested 29 of the 31 UV filters approved for use in the United States and Europe by dissolving and applying them to sperm samples provided by healthy volunteers.
Almost half of the filters tested were found to stop sperm from functioning properly.
"These results are of concern and might explain in part why unexplained infertility is so prevalent," Skakkebaek said in a statement.
The team's previous research
discovered that a range of endocrine disruptors could impact the functionality of human sperm. UV filters, which were found to be easily absorbed through the skin, were among them.
"When you put sunscreen on the skin, some of the UV filters can penetrate the skin and go into the bloodstream," said Skakkebaek.
In previous studies, UV filters were also found in more than 95% of urine samples
tested as part of trials in the United States, Spain, France and Denmark, according to Skakkebaek's research
and a report by the Danish Environmental Protection Agency
. The accuracy of urine as a measure for this is, however, limited when compared with blood samples.
These filters were found to affect human sperm by mimicking the effects of the female hormone progesterone. This mimicry then altered signaling inside of the sperm cells.
"If chemicals can work like progesterone, they can potentially affect the sperm function," said Skakkebaek.
Progesterone plays a vital role in a sperm's ability to mature and fertilize a female egg. It controls the attraction of a sperm cell toward a female egg, as well as the sperm's ability to move and break through barriers to reach the egg.
When progesterone binds to a sperm cell, this leads to calcium signals being sent within the sperm, caused by changes in the concentration of calcium ions. By mimicking this effect, chemicals like the UV filters disrupt this process and change the inner workings of the sperm, the researchers explained during their presentation
At low quantities, the UV filters were found to have an additive effect, meaning that despite being present at lower levels, they still had an impact on sperm samples when different filters were combined together.
"If you test them in a mixture you get a small calcium signal. ... This means that at very low levels there would still be an effect," said fellow researcher Anders Rehfeld
during his presentation of the work at the conference.
The results remain limited as they provide evidence of what happens in the lab, not necessarily in humans.
"These studies were done by incubating ejaculated sperm with these compounds in the laboratory. But this is a million miles from what might happen to sperm in a guy who puts on sunscreen," said Allan Pacey, a professor of andrology at the University of Sheffield.
More research is needed on the effects of sunscreen on sperm, other researchers said.
"Although providing compelling evidence, we need to further our understanding through (human) studies and epidemiologic studies on these chemicals," said Russ Hauser,
professor of reproductive physiology at the Harvard School of Public Health.
Other ways to reduce sun exposure
In the meantime, people should continue to control their sun exposure to prevent other health risks, such as skin cancer.
"The risks of skin cancer are far better understood, and until more data is available I think it's more important that men continue to use sun protection creams as appropriate," said Pacey.
Skin can also be protected from sun exposure in other ways.
"My advice would be to minimize sun exposure by reducing exposure during midday hours, wear hats and clothing to cover skin," Hauser said.
Skakkebaek acknowledged that more research is needed, especially as UV filters are found not only in sunscreens.
"UV filters are used in many products apart from sunscreens, including creams and lotions that are used daily," said Hauser. "This has resulted in widespread general population exposure."
Many other environmental and lifestyle factors are also thought to be affecting the decline in sperm quality worldwide. These include smoking -- particularly exposure while in the womb -- marijuana
, excessive alcohol and obesity, in addition to certain chemicals in everyday consumer products, according to Skakkebaek. He remains sure the environment has a key role to play in the decline seen in fertility worldwide, but further research is needed to provide the evidence he needs.
"There's no clear answer," said Skakkebaek. "(But) this is an important medical area that's greatly overlooked."