Continuity mistake: At the end of the episode, the doctor takes the mouthpiece out of Lex's mouth, then the scene cuts to Clark. When it cuts back to Lex, the mouthpiece is back in his mouth.
Continuity mistake: After Clark gets his powers back the cut on his cheek heals and leaves only blood on his cheek. While still at the power generator he goes to wipe this blood off of his face with the back of his hand only in the shot just before you see his hand go to wipe, a second before his hand actually appears on screen, the blood that was on his cheek is now gone as if it was already cleaned.
Continuity mistake: As Clark stands outside the room where Lex has just got his shock treatment the blood on his cheek is back even though his cheek wound was healed and he wiped any residual blood off in the previous scene at the power generator.
Continuity mistake: When Lex first tries to break out of Belle Reve, he stabs a security guard in the leg with a pencil deep enough to draw a large amount of blood on the guy's pants leg. But when Lex is trying to incapacitate the man you can see the pencil flopping around on the guards leg and not sticking inside the guy's leg.
Answer: The kryptonite only creates mutations when it reacts with the environment and an individual. Remember, the FDA and several environmental groups did check out Smallville after the meteor shower, but found no danger in the rocks. Obviously they're not always emitting mutating rays, or all the flora and fauna of the surrounding area would be horribly mutated.
Phoenix