Per-capita numbers are useful if the measure you're interested in is how naughty each of us are in terms of contributing to the problem as individuals, but those numbers don't necessarily tell you where the problem is most acute. If the question is 'Which country has the most polluted air' the answer isn't the United States. When figuring per-capita numbers, don't forget that China is now home to almost 1.3 billion people. As for where the gunk actually is, here is an interesting link to study.