Deucer said:
The foam inside the helmet is designed to permanently compress once it takes a hit. during an accident, this impact comes from your head compressing against the padding on the inside of the helmet, which compresses against the foam, which presses against the inside of the outer shell, which is pressed against asphalt.
In order for a drop to affect the foam permanently, the drop would have to be from a height sigificant enough to distort the outer shell of the helmet. But there is still nothing inside the helmet to compress against, so it's not the same as taking a hit while wearing the helmet.
The bottom line is you never know for sure without sending the helmet in, but I would think that it would be pretty hard to damage the foam due to a drop without screwing up the finish pretty bad.
+1, it most likely didn't affect anything, but it should be inspected inside and out. Even big impact damage is not usually very visible on the outside, though. Without a head mass in the helmet though, the actual impact energy is minimal within the working range of the shell or liner, especially if the surface it fell on was flat and carpteted. etc.
Here's from Snell:
Mishaps
If an empty helmet is dropped a few feet from a table top or the back of a bike onto a hard floor or pavement, the impact management is likely unaffected. The shell may be marred, even chipped but, in our experience, there would be no detectable effect in test results. Unless the manufacturer advises otherwise, one such simple fall is no reason to mistrust a helmet. Even so, such mishaps are to be avoided. Helmet damage is cumulative. A history of clumsy handling will destroy a crash helmet eventually.
Deliberate abuse is another matter. Crash helmets are inherently fragile, they protect by taking damage. An intemperate act may render a helmet useless. Anyone who abuses a helmet should be responsible to replace it.
Helmet Damage
Assessing impact damage is much more difficult. Cosmetic chips and dings start to appear on many helmets almost as soon as they’re taken out of the box but need not suggest degradation of protective capability. However, broad areas of deep parallel scratches and any broadly distributed pattern of cracks suggests some sort of head impact. If a Snell certified helmet has involved in head impact, it should be retired and replaced.
Sometimes, the impact managing liner of a crashed helmet will feel spongy to the touch. Particularly if it is made of expanded polystyrene (EPS). When the helmet shell strikes an impact surface, it stops moving immediately but the head inside the helmet remains in motion crushing the liner between itself and the inner surface of the helmet shell. As it is crushed, the liner applies controlled braking forces to the head slowing it to a relatively gentle stop. But it takes permanent damage doing so. This damage may be detectable. If an EPS liner feels spongy in some areas and firm in others, the liner has likely been compromised, the helmet should be retired and replaced.
Unfortunately, many helmets will not be visibly affected by impact. Some helmet shells will flex considerably without cracking or splitting. The crush damage to an EPS liner may be at the outer surface, just under the shell so that the inner surface remains deceptively firm. If the helmet was truly impacted, its capabilities have almost certainly been compromised and it should be replaced. Who ever was wearing it knows but there may be no practical way for anyone else to determine its condition conclusively.
