I wondered that as well. 0 tolerance means our entire planet should be condemned as uninhabitable, then. Clearly there must be some level of parts per 10 to t

I wondered that as well. 0 tolerance means our entire planet should be condemned as uninhabitable, then. Clearly there must be some level of parts per 10 to the N that is no more dangerous than a walk on the beach. 300 parts per million is probably bad. 1 part per trillion is probably cleaner than a random meadow.