I visited the Giro facilities here in Santa Cruz a few weeks back, and was treated to a demonstration in their helmet testing lab. They have a large apparatus that drops a weighted helmet with a spherical metal fixture to approximate a human skull in it, on top of a collection of different shaped objects to duplicate rock, curb stone, flat street impacts, etc.... A gauge in the metal sphere in the helmet measures the amount of g-forces over time transfered through the helmet into the human skull and displays it on a screen.
In the demo I saw, the first impact of a helmet on a flat surface was well under the US CPSC guidelines for helmets. The force was spread out over safe period of time (milliseconds), and the peak force was safely buffered. The helmet looked fine to the eye after the first impact.
However, each subsequent impact in the test apparatus had much higher g-forces transfered through the helmet, and the duration of the impact shortened with a sharper ramp, meaning that the helmet's ability to protect was diminished after each hit. The second impact on the Giro helmet was still under CPSC guidelines, but the amount of force transfered thorugh to the skull fixture had nearly doubled over the first impact.
The lesson here? Giro builds really safe helmets, that do a great job of protecting your noggin. They beat the safety guidelines by a big margin. But each impact significantly reduces the helmet's ability to do it's intended job, and there is no real way to tell how badly your helmet was affected in your accident. Replace the helmet after that first impact. Hang the old one up on the wall and tell your grandkids about it some day...
Cheers,
