My guess would be moisturizing lotions.
Several years ago, about a dozen of us rented a vacation house in Lake Tahoe - with a hot tub.
The air up there is really dry, some people were really slathering themselves with lotion. After skiing, we went to soak while the snow fell. After dinner, some of us went back. Gross. A layer of foam with scum clumps on the surface. We left the filter on all night. It looked better the next day. Until one individual hopped in for a while. This one was the major moisturizer, and sadly, it was a guy... Instant foam.