Bump caps designed for 'industry' ( I use that word loosely, anyone wearing a bump cap doesn't do a proper job) are ok for stuff falling on your head but don't have chin straps to stop it coming off should you fall over.
With chin straps, I'd rather smash my head on concrete with a bump cap on than no helmet at all, but I'd hazard a guess and say they don't absorb as much energy as a traditional foam lined helmet, potentially leading to brain-bouncing-around-skull injuries.