When her black cat rapidly dropped from a healthy 14 pounds to a skeletal five pounds, it was natural for Arlene Blum to investigate whether a toxic chemical in her home might be to blame. The veterinarian’s diagnosis raised that possibility, and Blum had expertise in the harm that chemicals can cause. Her research as a chemist in the 1970s helped reveal the possible health hazards posed by flame retardants used in children’s sleepwear. What surprised Blum, executive director of the nonprofit Green Science Policy Institute in Berkeley, Calif., was the chemical she discovered in Midnight’s blood, in the foam of her couch and in dust throughout her house. It was a substance only slightly different than the one that, decades earlier, she encountered in kids’ pajamas, leading to a federal ban on the compound for that sort of use.