I think I found the answer. Google to the rescue
Major city defined by population:
major city means a zone with a population concentration in excess of 250 000 inhabitants or, where the population concentration is 250 000 inhabitants or less, a population density per km2 which justifies the need for ambient air quality to be assessed and guaranteed;
I think that opens it up to a lot of cities across the US that I'm sure have been open for quite some time now.