There's only one answer really.
I took a low-speed tumble not long ago. My arm took the brunt of the fall, but I did end up bumping my head on the ground. I didn’t hit it very hard, so can I reuse my helmet?
The straight answer to that is no. I mean, technically speaking, yeah, you can wear the helmet again; it really all comes down to how much you value your head. Even if the lid shows little to no exterior damage, even just a light impact on the ground with your head in it has compromised the helmet’s integrity.
The reason a helmet becomes obsolete after a crash has nothing to do with the exterior appearance of the shell. It’s all about the shock-absorbing compound found inside, between the interior and exterior layers of the lid. Protection is usually provided by expanded polystyrene (EPS), more widely known under the trademarked name “styrofoam”. The material is used in a number of applications for its energy absorbing and diffusing properties, including in car bumpers and, of course, in motorcycle helmets.
If you are wearing the helmet and hit your head on the ground, the EPS absorbs and disperses part of the energy of the impact so your head doesn’t. Even in low-speed or low-impact situations, the force of the impact, however gentle it is, combined with the weight of your head, crushes the polystyrene beads of the EPS. Once crushed, the beads do not regain their shape which means that the lining has lost its efficiency.
If you wear a compromised helmet and take another tumble, the crushed EPS won’t be able to absorb as much energy, which means the impact to your head will be more significant.
That’s why as frustrating and as heartbreaking as it might be, no matter how hard you hit your head, the best call to make for your own safety is to buy a new helmet.
Source: Helmet Facts