I’m betting your husband is right that it’s laundry detergent.
That said, There are two different kinds of mange. Sarcoptic, and Demodectic. . Sarcoptic is the kind with the mites. Demodex, usually in my experience, is autoimmune and triggered by stress. I’ve seen it quite a bit in the rescue dogs that I’ve had come through over the last 20 years or so. The demodex usually starts around the eyes and the nose you start to see some weird hair loss there if you’re not really looking for it you might miss it. But it’s easily treated with a course of ivermectin, which is not expensive and easily administered.
If you do think it may be the detergent, give them both a good thorough bath with a mild shampoo like baby shampoo or ivory liquid and then towel will dry them well and see what happens a mild oatmeal shampoo or an unscented puppy type shampoo usually works well also. I would probably still do this anyway no matter what it is.