In reply to Kenny:
No, are you mad? Only the appropriate helmet should be used for each activity, EU stamped after inspection less than 12 months ago.
All helmets should be discarded after exposure to sunlight, rain, the slightest knock (this can cause micro delamination which is visible only with an ultra-sonic scanner) or, of course, after any injunction to do so by the authorities. Can't be too careful, you know, if you are hurt it will cost the community more than you and such behavior is really both foolhardy and anti-social.
If in doubt, the best thing is to show your proposed helmet to the nearest chemist, as one does for wild mushrooms, they'll issue the required chit for road use after inspection. While you're there I would advise you to have the length of your bicycle pump checked at the same time (all done for the standard 14€ cover charge, so why hesitate?); there have been a spate of nasty accidents lately due to non-standard pumps. Be careful to walk to the chemist pushing the bike after removing the chain to avoid any risk of accusation of unsafe, anti-social pedaling.
Come to think of it, it's probably better advised to stay in bed, provided you don't use an electric blanket, of course….