How is this measured exactly? Does every part of the country have to meet the standard, or is it an average? Most of the countries seem to be small island natio

How is this measured exactly? Does every part of the country have to meet the standard, or is it an average? Most of the countries seem to be small island nations or very sparsely populated, but I'd be surprised if the large Australian cities have air cleaner than that of any of the many tiny island nations that didn't make this very short list.