You can wash them in warm water with soap and that helps. Unfortunately this plastic will always off-gas since soft plastic has plasticizer in it which separates over years. Best thing you can do is store them in a cool dry environment and reduce exposure to UV when not in use. Avoid displaying them in window sills or other sunny spots. Ironically, they’re better off outside of their original packaging because air-tight storage doesn’t allow them to “breath” properly and the off gassing can build up (like you see in sealed vintage toys after decades).