In 2007, high levels of bromate—a carcinogen formed when bromide and chlorine react with sunlight—were found in Los Angeles’s Ivanhoe Reservoir. Today three million black plastic balls help deflect UV rays.

In 2007, high levels of bromate—a carcinogen formed when bromide and chlorine react with sunlight—were found in Los Angeles’s Ivanhoe Reservoir. Today three million black plastic balls help deflect UV rays.