Continuity mistake: Season 7, episode "Persona". When Lana walks in from the balcony her hair swaps from being in front of her shoulder to behind it, then back again. Then when she sees Bizzaro's face in the reflection of the glass, her hair is styled differently again.
Continuity mistake: When Lana comes in with her brown bag of groceries, she puts the vegetables and wine bottle on the counter. During the rest of the scene these items move their positions a few times.
Continuity mistake: When Phantom Clark finds Lana on the balcony with her phone pressed against her ear, she lowers the phone down to her waist twice between shots.
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