Found this:
Care of your Helmet
Do not paint it or attach decals to it unless you have the go-ahead from the manufacturers. The chemicals used in glues and paints can damage the helmet.
Look after your helmet like gold. Store it in a soft, protective helmet bag. Do not bang it, scratch it or drop it as it is designed to self-destruct on impact, thus protecting your head and neck. If you do, have it check out professionally.
Keep it away from strong chemicals. This means all those household cleaners under the kitchen sink. Cleaning it with plain toilet soap and warm water is always best.
Do not store it in direct sunlight. If the inner gets very wet during a thunder storm allow it to dry naturally.
Replace it every two to five years depending on its condition and the amount of riding you do.
The helmet inner can become smelly over a period of time. Some helmets have a system whereby the soft inner lining of the helmet can be removed for cleaning. This is a major advantage in a country like ours where it is so hot. One can also wear a type of head bandana made out of a thin cotton under you helmet (try a flee market for one). This can easily be washed thus keeping your inner, mould-free! When you do need to wash your helmet inner ordinary toilet soap and warm water does the trick. As helmets are all-weather items a little water cannot harm them so do not be shy with it when washing the inside. Then allow it to dry naturally.