When we were working on a friend's pool that had black algae (BA) and it was somewhere near shock level, my husband went in with mask and snorkel to attack some pits that had algae. After maybe 10 minutes his lips and nose were slightly burning. I sat at the edge and scrubbed with a pole and SS brush and managed to get wet when the FC was closer to mustard algae shock levels, my bikini parts were unhappy for days.