According to Spanish law, yes it is compulsory to wear a helmet when cycling in Spain. The fine for not wearing a helmet is €150 + 3 point licence deduction! How they can translate the fine from a bicycle onto your driving license is a little cheeky, and theres a thread about it on another spanish cycling forum here.
However they do make three or four exceptions for the following situations:
- Prolonged ascents
- Conditions of extreme heat
- Urban streets
- Medical reasons
Now in my opinion, Tenerife is one big long ascent. That doesn't excuse you from riding back down without a helmet downhill though.
The coast of Tenerife now has what can only be described as one continuous urban zone. So unless you have the great misfortune of getting a ticket from an officer for the little gaps of 5-20m between the signs at the start and finish of some towns, I'd say it's ok to ride around without a helmet in those places.
There are sometimes very hot days here called calimas.
I can't think of any medical reasons not to wear a helmet.
By the way, I want to make it perfectly clear that I don't condone riding without a helmet. I always have to provide the option of a rental helmet for those cyclists that don't bring their own or forget/lose their own bicycle helmets. Whether you decide to ride with or without a helmet is obviously your choice.
The only time I ride without a helmet is for journeys under 5km where I don't exceed jogging pace (when delivering a rental bike for example). During these occasions, I have passed both the civil guard police (on local streets) and the local police... they could easily have stopped me, but they didn't. I think the reason is because they're too busy fighting organised crime in Tenerife (yes there is a big hash & cocaine problem here).
I will say that I have noticed that when cars do see you, they steer WELL clear of this "crazy cyclist riding without a helmet". The trouble is when they don't see you & you fall off and crash into the road surface, splitting your head open. I have been unlucky enough to crash twice while riding in Australia with a helmet. Lucky for me, I didn't land on my head anyway.
Incidentally, one of my clients did crash while wearing one of my rental helmets. I never saw it, but yes the helmet was totalled, and yes he was also naturally very grateful for wearing it.
"Los conductores y ocupantes de bicicleta deberán utilizar cascos de protección homologados o certificados cuando circulen en vías interurbanas, salvo en rampas ascendentes prolongadas, por razones médicas y en condiciones extremas de calor.
Los ciclistas en competición y los profesionales durante los entrenamientos o en competición, se regirán por sus propias normas"
If you have any other questions about road rules in Spain, just ask.