They could definitely blind you.
We went during the hot weather and it was unbearable with masks etc. One poor fella decided to ditch the mask and just use his shades as eye protection.
He got shot in the mouth and it broke his front tooth off.
Great fun though. The other team turned up all kitted out with sniper rifles, side arms and all sorts.
For the record there was no legal repercussions because it was in Essex and the bloke who lost his tooth said "My fault, I'm a silly c#$@ for taking off the mask, I might get the dentist to whack a bit of gold in there. Bosh."