Scientists at Missouri University of Science and Technology studied how sunscreen compounds react to light.
From the test results, they believe that zinc oxide, a staple ingredient, releases free radicals when it undergoes a chemical reaction sparked by bright light.
The free radicals bond with other molecules, but while doing so they can damage cells or the DNA within cells, increasing the risk of skin cancer.
Some sun exposure can be beneficial, for example by helping the body produce Vitamin D, but UVA and UVB radiation from the sun can cause sunburn, skin ageing or suppress the immune system.
But, ironically, the tests suggested that UV light causes the strongest reaction with compounds in sunscreen.
The lead researcher, Dr Yinfa Ma, still advised people to take the usual precautions in the sun, saying he needed to take further tests to see if zinc oxide really does generate free radicals, as he suspects.
He said: “I still would advise people to wear sunscreen.”
“Sunscreen is better than no protection at all.”
