Factual error: Lex crashes and his car lands on its roof, but in the span of a few seconds it has sunk straight to the bottom, and turned the right way up. Cars don't sink that quickly. (00:17:25)
Factual error: Lana says the theme of her mother's prom was Saturday Night Fever, but she was in the class of 1977. Prom would take place sometime before graduation (which would be in May, maybe June), likely February or March. Filming for "Saturday Night Fever" didn't even begin until March 1977.
Factual error: When the bug kid crashes his car, he's driving a VW Beetle, which has the motor in the rear, therefore would not have shut off in a head on collision.
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