Continuity mistake: In the shot of Lana being stepped on by the horse, her right arm is out by her side and her hair is covered with the wood shavings lining the stall. In the next shot of her, her right arm is up so that her right hand is by her head and her hair isn't covered with the shavings. Her arm is back out by her side in the next shot when Clark arrives.
Continuity mistake: When Lex and Clark go to where Lex saw Morgan and it's a bunch of people sewing, Lex grabs a man's hand and puts it on the table. On the table is a piece of fabric that the hands are on, but in the close up shot, there is no fabric. It's back in the next shot.
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