Continuity mistake: When Pete and Clark go to Byron's house to find him, Clark finds the hidden locked door to the cellar. Clark then breaks the locks and uses both his hands to pull on the door. Cut to a different angle (from above Clark) but now he is just using his left hand.
Continuity mistake: After Byron transforms, he pushes Pete through the windshield of an old car. The impact causes the front passenger door of the car to open. Yet later when Clark check up on Pete, the front passenger door is now closed.
Continuity mistake: Clark arrives at the Talon after Lex reads the mysterious love letter for Lana. Lex the gives back the letter to Lana twice between shots.
Continuity mistake: Byron mom buys him a book by Edgar Allan Poe at the used bookstore. Byron takes it with his right hand and looks at it. Cut to a wider angle (as Byron stands up) but now he is holding the book in his left hand.
Continuity mistake: When Lex has the envelope and pulls it away from Lana as she grabs for it, the flap is opened, so the wax seal is on top. In the next shot the flap is folded down.
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