Denver Post: Requiring cleaner-running lawn mowers — as the federal government did this past week — might seem to be a small step toward reducing air pollution. But it’s one of the many kinds of changes — both small and large — that must be undertaken if this country is to make significant gains in the areas of ozone pollution, global warming and fossil-fuel dependency. The need for continued reductions in air pollution came into sharp focus here in recent days. While the Denver metro region …