That's essentially it. You can get benefits from reducing air pollution wherever you reduce air pollution. We have not seen a threshold for the effects even in relatively clean communities. An emission reduction in a less polluted community can have, all other things being equal, the same impact as the same emission reduction in a polluted community. It sounds a little bit odd, but the reality is that the risks are more or less linear, as you reduce the concentrations.
On May 17th, 2007. See this statement in context.