I found this post on a 2009 forum, which seems to shed some light on the subject.
"I watched a video clip recently with Jay Leno and Bruce Porter from Arai. Bruce was asked the question about a helmet being dropped. His response surprised me but makes sense. Basically he said that the key to a helmets performance is the liner and that the shell is to spread the load more effectively. The design is intended to protect with an approx 12lb object in the helmet (head) and that a minor drop of an empty helmet would have minimal to no effect on the protective qualities of the helmet as the liner would have had no compaction. Cosmetic damage only in most cases."
And this...
"Ride magazine took four identical brand new HJC helmets to the BSI labs and subjected each to a BSI impact test. Just prior to the test one was dropped from 3feet one from 6 and one from 9. They could find no discernable difference in the test results between a non dropped and a dropped from 9 feet lid. you work it out."
Advice from Snell
"I dropped my helmet! Do I have to go buy a new one?"
Generally the answer is probably not. Helmets are one-use items, but are quite durable otherwise, at least the ones we certify. Frequent dropping or spiking a helmet on the ground, or other hard surfaces may eventually degrade the helmet's performance. Similarly if the helmet falls to the ground at highway speeds unoccupied, the owner must be aware that some degradation may have occurred. In general, the real damage comes when the helmet contacts an object with a head inside. The Foundation recommends that if you are participating in an activity that requires that you wear a helmet, you avoid hitting stuff with your head. It can be difficult to readily determine if a helmet has been damaged, and the protective capabilities compromised without a thorough inspection by a trained professional. Some manufacturers may provide this service or direct you to these others that can perform these inspections. The Foundation recommends that if you suspect your helmet may be compromised, then replace it. If the helmet has been involved in an impact while in use, replace it.