Skinwalker - S2-E10
Visible crew/equipment: After Joseph gets locked out of the construction site, the worker, who locked him out, walks away. All the other workers at the site are gone, but when the shot returns to Joseph you can see the shoulder of someone standing between Joseph and the gate even though there should be nobody there.
Skinwalker - S2-E10
Visible crew/equipment: After Kyla dies the scene goes to the construction site as Martha and Lionel get out of a SUV. As Martha gets out you can see the camera and film crew reflected in the window of the SUV.
Skinwalker - S2-E10
Visible crew/equipment: While Clark and Pete are riding dirt bikes, you can see a crew-member in a green jacket in the bottom-right-hand corner of the screen in the shot where they go off the jump. In the next shot he has disappeared.
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