Matt Zwolinski has an article on libertarianism.org about the alleged impracticability of the non-aggression with respect to pollution in a modern economy.
He seems to think that the non-aggression principle implies that an instance of aggression warrants any degree of retaliatory coercion. For example, if carbon dioxide is a pollutant, then it would be within moral bounds to stop people from exhaling, basically killing them. Really!
In referencing Murray Rothbard’s defence of the non-aggression principle, Zwolinski drops Rothbard’s elaboration in the The Ethics of Liberty of the principle of proportionality on the maximum, not mandatory, degree of acceptable punishment. If exhaling were an act of pollution, it wouldn’t warrant killing someone. Additionally, if industrial production were of such a value (and I think it is), community norms would develop what, if any, level of aggression was tolerable before legal force or social pressures was acceptable.
That is assuming exhaling carbon dioxide was polluting, a form of initiating force to interfere with the value a person achieves from property. Since exhaling carbon dioxide under normal conditions is imperceptible to human senses, how could someone’s value he or she acts to gain or to keep be interfered with? Under normal conditions, a person’s exercise of property rights is not interfered with by exhaling carbon dioxide.