 A corollary of the non-excludability characteristic is that there are limitations to consumers’ consumption decisions regarding the public good, if it is produced. Individuals might have different preferences for the level of security provided by national defence, but once national defence is in place, they will consume the level that has been produced, not more or less of it. One cannot “opt out” of the consumption of a public good. Similarly, while everyone might like clean air, individuals will differ in their degree of tolerance of pollution. But once “clean air” has been produced, consumers must consume it independently of their preferences.