Impacting the asphalt from the height of a bike will break the skull at zero bike-speed according to some literature I read some years ago. That was due to just the acceleration of gravity (falling).

I had a near head-cracker in the 70's. We were on a family ride and stopped at a railroad crossing for some reason. I got back on my bike and was just getting started when the front wheel slide out on a rail. I hit that rail with my head. I was stunned for 10 to 15 seconds and I was wearing a state of the art helmet for the times.

There is no valid technical argument for not wearing a helmet. The risk is well defined/documented. One can't predict the future and therefore the nature/severity of an accident. Head injuries have a higher risk of serious disabilities.

Helmets are light and very well vented especially if one is willing to buy the mid to higher-end models.

There's also a responsibility, much ignored to keep one's self fully functional to support one's self and his family. There's also a responsibility, again much ignored to not become a burden on society. As the old saying goes, one's rights end where another's begins. It's a balancing act concerning the personal freedom/rights thing.

Al