So I googled and apparently mythbusters has done some testing on this and this is taken from another motorcycle forum. Seems a simple drop of a helmet is generally not enough to cause any major issues with its intended purporse its generally cosmetic.

'Taken from mythbusters

The Mythbusters are not DOT, SFI, SNELL or FIA compliance engineers. They would not know when a helmet is out of compliance.

A rated helmet is typically a fiber composit shell with an internal styrofoam (destructive) interior, with a gelcoat exterior. The gelcoat simply gives a good surface and protects the composit shell from damage via sunlight, oils, water, etc. A scratch to the gelcoat isn't much of an issue. The composit shell can crack when dropped but is USUALLY notiable under observation. The interior styrofoam should be NON resilant. (IE: It can crush under force and will NOT retain it's original properties.) This is most usually why a helmet is replaced early. If you are in an accident and the styrofoam is dented/crushed at all, the helmet is out of compliance. This is far more of an issue than just dropping the helmet. A rated helmet can almost always be sent back to the manufacture for a free or cheap evaluation. If interior damage is noted they can replace the interior.
I would rather wear a cracked fiberglass shell than one with an interior out of compliance.
The shell ONLY keeps the interior stuff together. In a serious head hit, it has little value other than to keep the styrofoam cushion in place so that it can absorb the blow.

When SNELL, etc. does testing they do make sure that the shell is adequate, but they are MOST interested in how many G's the head inside the helmet is protected to. The shell is ALMOST a cosmetic issue. On Snell 90 and above there was also some fire testing done. (Not on DOT MC helmets) "