Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

It is easy to forget that bicycle helmets are only designed to protect
in minor impacts:

“In cases of high impact, such as most crashes that involve a motor
vehicle, the initial forces absorbed by a cycle helmet before breaking
are only a small part of the total force and the protection provided
by a helmet is likely to be minimal in this context. In cases where
serious injury is likely, the impact energy potentials are commonly of
a level that would overwhelm even Grand Prix motor racing helmets.
Cycle helmets provide best protection in situations involving simple,
low-speed falls with no other party involved. They are unlikely to
offer adequate protection in life-threatening situations.“

Dr Carwyn Hooper from St George’s University in London reports:

“Looking at evidence, it does not matter if people are wearing a
helmet or not, any serious accident on a bike is likely to kill them”

Helmets increase the volume of the head, thus the chance of the head
hitting the ground in an accident. Helmets also increase the risk of
neck injuries, and can aggravate brain injuries. There is no guarantee
that a fall with a helmet will result in less severe head, neck or
brain injuries.

It is natural to assume a helmet saved us. But that doesn’t mean it is
true. We don’t know what would have happened without it. Cyclists,
with and without helmets, get hit by cars; the survival rates are
identical. Most bicycle accidents do not result in serious head
injuries, with or without helmets. We tend to overlook this, and
attribute a lack of head injury to the helmet:

https://crag.asn.au/the-fallacy-of-the-cracked-helmet/