Researchers have found that 13 out of 29 UV filters used in sunscreens in the US and Europe can disrupt the function of human sperm cells - with some even mimicking the effect of the female hormone progesterone, and messing with sperm mobility.

So far, the UV filters have only been tested in human sperm cells in the lab, and not in live males, so there's no need to freak out, but it's a link that's worth investigating further. "These results are of concern and might explain in part why unexplained infertility is so prevalent," said lead researcher Niels Skakkebaek from the University of Copenhagen in Denmark.

It's also worth mentioning that while the link between some sunscreen ingredients and sperm function is just now emerging, a link that is already established is the one between UV exposure and skin cancer, so you definitely shouldn't go without sunscreen.

But there are two types of sunscreen - physical sunscreens, which reflect UV rays with ingredients like zinc, and chemical ones, which contain ingredients that absorb UV rays to stop them reaching your skin. This new research suggests that those UV absorbers in chemical sunscreens might also have an effect on sperm cells.

It's already been established that UV filters in sunscreen can be readily absorbed through our skin, and studies have found these ingredients in human blood samples and in 96 percent of urine samples in the US.

But it's not yet clear this is a health issue, which is why the Danish team decided to test 29 of the 31 UV filters approved for use in sunscreen in the US and the European Union on live, healthy, donated sperm cells in the lab.

The cells were tested in a buffer solution that mimicked the conditions in female fallopian tubes. The researchers were looking at how the UV filters affected calcium signalling, which is a pathway that's essential for sperm cells to be able to swim healthily.

These calcium signalling pathways are usually triggered by the hormone progesterone, but the team found that 13 out of the 29 UV filters - or 45 percent - also kicked off calcium signalling. "This effect began at very low doses of the chemicals, below the levels of some UV filters found in people after whole-body application of sunscreens," said Skakkebaek.

They also showed that nine of the UV filters specifically triggered the calcium signalling channel that's stimulated by progesterone, suggesting that it was mimicking its activity - and this stopped sperm cells from swimming properly, which could have an impact on human fertility.

The results have been presented at the 98th annual meeting of the Endocrine Society in Boston, but haven't been peer-reviewed as yet, so they need to further scrutinised before we can say for sure that a link could exist between sperm cell function and these sunscreen ingredients.

But the findings suggest that we don't know as much about the things we rub all over our bodies as we probably should - and there's definitely room for more research on this topic.

"Our study suggests that regulatory agencies should have a closer look at the effects of UV filters on fertility before approval," said Skakkebaek.