We have now come full circle. Having seen how air pollutants are produced and dispersed, and the effects that result from their presence in the air and deposition to receptors, we will review in this Chapter the national and international limits that have been set. Air pollution standards are emission or concentration or deposition rates that are designed to protect people and the environment from harm. If there were precise concentrations at which such harm was observed to occur, then setting the standards would be straightforward. Unfortunately life is never that simple.