Had a doctor who also rode share a story with me. He and a friend had finished a ride and were in the car park, friend took off helmet as the chatted and cooled down, friend then remounted and began riding to his car, slipped on debris at 5 mph, fell, stuck his head, dead before the ambulance arrived.
Studies in USA show automobile drivers perceive helmet less bicycle riders to be more dangerous so they give them more space. Therefore it is safer to not wear a helmet, up until the point your head makes contact with something. That said I always wear a helmet. Been hit by cars twice, twice cracked my “styrofoam hat” but not my skull.
Have a cousin who is a nurse, she mentioned seeing an anti-helmet protest on the news, amongst the protesters were some of here dialysis patients. I guess one way to speed up the transplant queue.