Basically, he was harmed...
The visible effects are the blood dripping from his nose and his ears, but this is symptomatic of possibly more serious (although obviously not life threatening) internal injuries.
If you're asking why he wasn't as badly injured as the rest of Tuco's crew, it's because they had no idea what was about to happen and he was entirely aware (being the propagator of the attack)..
Walt wasn't disorientated, confused or alarmed, unlike everyone else in the room who just experienced the physical effects of a surprise explosion!
Not to mention he was all jacked up on adrenaline..this is the more or less the beginning of his Heisenberg persona.
I miss Walt ... I'm Breaking Sad.