Coral reefs are home to more than 25 percent of all marine life.

We thought we knew everything about choosing the right sunscreen: We can recite the recommendations of the American Academy of Dermatology in our sleep. Pick a product with an SPF of 30 or higher that offers broad-spectrum protection against both UVA and UVB rays and don’t be fooled into thinking your tinted moisturizer is an adequate sun shield (the thin layer you apply is too scant for anything more than an incidental walk); apply sunscreen 15 to 30 minutes before venturing outdoors, even on a cloudy day.

While that’s all solid advice, it turns out there’s an environmental consideration as well: If you’re going to be spending your summer days on the beach, be sure to choose a sunscreen that won’t harm coral reefs. According to researchers from Polytechnic University of the Marche in Ancona, Italy, many common sunscreens contain chemicals that activate viruses in the symbiotic algae called zooxanthellae, which feeds the coral through photosynthesis and contributes to its dazzling colors. The chemicals octinoxate, oxybenzone, 4-methylbenzylidene camphor and butylparaben cause the virus to replicate until its algae host explodes, spilling viruses into the seawater where they can infect neighboring reef communities. Without zooxanthellae, the coral bleaches and dies. The irony is that those of us who especially treasure underwater life, snorkeling and scuba diving to experience its splendor up close, may be contributing to the reefs’ demise. The study’s researchers estimate that 4,000 to 6,000 metric tons of sunscreen wash off swimmers annually in oceans worldwide, threatening 10 percent of the coral reefs with sunscreen-induced bleaching and death.

We learned all of this while chatting with reps from MyChelle Dermaceuticals, a natural paraben-free skincare line. And we also learned, after our jaw dropped open in horror, that there’s an easy solution: When you’re likely to be swimming in the ocean, choose a biodegradable sunscreen with physical filters that block and scatter ultraviolet radiation (look for the ingredients titanium oxide or zinc oxide) rather than a sunscreen that absorbs UV rays and is likely to contain at least one of the four coral-damaging ingredients. Consider MyChelle Sun Shield SPF 28 ($19.19), a reef-safe sunblock with zinc oxide that doesn’t leave behind that telltale white mask, or Soléo Organics All-Natural SPF 30 Sunscreen ($21.99), a chemical-free, eco-friendly brand from Australia.

We just read the labels of our stash of sunscreens and were shocked to see that they do contain coral killers. Do you have a favorite chemical-free sunscreen?

Tags: American Academy of Dermatology, Ancona, Australia, chemical, chemicals, Chemistry, Coral, Coral reef, Electromagnetic radiation, favorite chemical-free sunscreen, Hiking equipment, Italy, Marche, Octyl methoxycinnamate, Oxybenzone, Polytechnic University, Polytechnic University of the Marche, radiation, Sun tanning, Sunscreen, Sunscreening agents, Ultraviolet, USD, Zinc oxide