Helmets can be damaged by certain chemicals, and the problem generally isn't obvious to the untrained eye, which makes it even more dangerous. The outer shell on most motorcycle helmets is pretty durable, but the inner, the very important impact-absorbing polystyrene layer and the visor can both be damaged seriously by a slight exposure to certain chemicals. Any aerosol with butane as propellant can cause damage, and a variety of other chemicals, especially solvents, can also.
Most motorcycle helmet manufacturers recommend using nothing other than water and their own cleaning products to clean their helmets. A spokesperson from a leading helmet manufacturer, said: 'We found some other helmet cleaners will not harm the shell but when they are used on a visor or leak into the internal visor parts on a helmet, the petrochemicals contained in these solutions make them brittle and thus easily breakable. You would be surprised by the small amount of solution that will do this damage.' A small proportion of helmets, typically the cheaper ones, are made from injection-molded (polycarbonate) plastic – you'll notice a thin line running front to back on the outer shell of these types. They are more susceptible to chemical damage than lids made from composites (high-tech fiberglass or carbon-fiber etc...).
So...aerosol sprays like spray paint, plastic cleaners and even air fresheners? Avoid it, they can seriously damage your favorite helmet. Specialized helmet cleaning products should be okay, but the only safe answer is water. Stickers are fine on fiberglass helmets but, if you've got an 'cheap' injected-molded polycarbonate plastic helmet, be wary as solvents in the glue can easily damage the outer shell unless there's a coating to prevent it.