It seems as though the sentence should read:
Because their skin is so thin, frogs are also sensitive to the effects of (lower, low) atmospheric ozone levels caused by pollution from industry and cars.
Either "low" or "lower" could be correct, depending on the context. If a standard of ozone level has been mentioned previously, then "lower" would be logical. If the low level is an absolutely low level without comparison to any other level, then "low" would be appropriate.