by Mike Barrett, Natural Society:
A chemical commonly found in sunscreen breaks down into toxic compounds when exposed to sun and water, research shows.
Avobenzone is an oil-soluble compound that is notable for its ability to absorb ultraviolet (UV) light. It is most often an ingredient in sunscreen and cosmetics, including lip balms and moisturizers. The compound converts UV rays into safer wavelengths that don’t damage the skin, and transforms UV energy into thermal energy. 
The chemical was approved by the U.S. Food and Drug Administration (FDA) for use in commercial cosmetics in 1988. It is a known endocrine-disruptor.
According to researchers in Russia, exposure to water and sunlight causes avobenzone to break down into chemicals that are potentially dangerous to people.
When the team exposed avobenzone to a chlorinated water solution simulating that used in swimming pools, the ingredient broke down into a mix of aromatic acids, aldehydes, phenols, and acetyl benzenes, several of them extremely toxic. Researchers tracked the process using chromatomass spectrometry. 
Albert Lebedev, a chemist at Lomonosov Moscow State University, said in a news release:
“On the basis of the experiments one could make a conclusion that a generally safe compound transforms in the water and forms more dangerous products. In spite of the fact that there are no precise toxicological profiles for the most established products, it’s known that acetyl benzenes and phenols, especially chlorinated ones, are quite toxic.”