The “optimal” level of POD emissions that will maximize social welfare by balancing the costs of emission control against the benefits of decreased environmental damage cannot be determined, because of pervasive uncertainty about the likely extent of future ozone depletion, its relationship to the quantity of potential ozone depleters emitted, the effects of ozone depletion in the biosphere, and the appropriate valuation of these consequences. Moreover, we can expect to learn more about each of these areas of uncertainty through continuing scientific research, and through observation of atmospheric and biospheric responses to past and current POD emissions.