I researched as well, and found the following informations:

A fall into a chair creates about 10 g and is sufficient to cause a concussion.

Sneezing and coughing, like we all do everyday, creates about 3 g.

I also saw the research results about nfl players. They said that helmet collissions, which create about 50-60g can cause concussions. But the helmet acts like a second skull, providing additional protection to the head. So this number can probably not be taken for impacts whithout helmets.

(I cannot add links as my number of posts is less than 10)

My second concussion was caused by a fast tennis ball that hit me straight on the head.
According to calculation in a physics forum, a tennisball served at 200km/h creates a force of 3g(the same as sneezing). I don't understand it. How can this tennis ball cause a concussion, when it has the same force on the body as a simple sneeze?