Still safe to use a helmet when it's dropped?

Probably... but not advisable. The manufacturer refuses any liability if it's ever been dropped once because the impact sets up tiny hairline cracks in the plastic, so the helmet won't ever quite be as good as it was before it was dropped.

The job of a helmet is to protect a rider's head from injuries. And how does it do it? the helmet has many layers/shells inside it. When you suffer a crash, the helmet absorbs the impact/shock by breaking the outermost shell and eventually nullify the effect to the maximum by the time the shock reaches your head. Which is why helmets are not made of metal or any hard material as such. So in other words, it is made to break itself so that your head does not. A helmet which fell once may not prove to be that useful anymore as the nullifying may not be that effective. Though it might still protect you from many scars. But that depends ultimately on the intensity of the fall (you falling from the bike and the fall your helmet recently had). So is that a risk worth taking? A helmet really saves lives.

The "Don't drop your helmet" warning that comes with the helmet is pretty scary, but from the seat to the ground is a pretty short drop. If you were not riding on the freeway at the time, then it is unlikely you damaged the helmet.

In "Motorcycle Accident Cause Factors and Identification of Countermeasures: Technical report (1981)", Harry Hurt thoroughly investigated 900 motorcycle accidents, and noted that "Very few protection failures were encountered. The only failures involved poly-carbonate shells which had deteriorated at areas of high residual stress and stress concentration, due to stress crazing and cracking." Rough handling is not mentioned as a problem, and he does note that "Damage to the styrofoam liners from sizzy bars and mirrors was noticed but never contributed adersely in impact attenuation."

Harry Hurt also notes that most of the helmets he studied were built before 1974, (the study was performed around 1980) and I haven't seen a cracked plastic helmet since the 70's, so that seems to be a solved problem. If your helmet doesn't have a crack in it (and there is no reason it should,) it's fine.

Most likely, your helmet is fine—especially if it was dropped from a very short distance. Were you riding your bike at the time? I think the warning is generally just a liability thing. If it can't handle a 2-foot drop, it's probably not gonna do much for you in a crash either. Make sure that the helmet you have is DOT and ECE certified.

My wife dropped her helmet once... from waist height to cement pad. There was no sign of damage on the surface of the helmet, but I had it x-rayed and it showed serious spider-webbing throughout the shell. NEVER, EVER TRUST A HELMET THAT HAS BEEN DROPPED, even from a short height!

I know this is an old thread, and there have been a number of responses to it already, but I thought I'd chip in with something positive on the subject.

This is assuming your helmet has received a minor impact, such as dropping it from the seat or the bars, and we're not talking about obvious damage sustained from a more serious impact. We probably all know if it's the latter then the helmet has done it's job and it's time to retire it.

Up to now, it's not been possible to perform an effective visual check on the outer shell of a helmet, and indeed even an x-ray cannot provide a conclusive answer, especially for minor drops where there could be hairline cracks in the composites. This is based on research conducted by the University of Queensland.

We're using laser-based technology that's up to 100,000x more sensitive than the human eye to identify any integrity issues with the outer shell of a helmet which has been manufactured from composite materials.

Another point to highlight: Even a minor drop can deform the outer side of the EPS liner, and if the EPS liner has taken an impact, the effect of that impact could act in a similar way as a pre-loaded spring on any subsequent impact(s).

In summary, up to now the answer to your question has been very subjective and people have been led by other's opinions. Our service removes doubt and ambiguity, and provides a Yes/No answer based on scientific evidence.

As always, however, if you have any nagging doubt - retire the helmet.