I doubt the author can pinpoint his health issues to the effects of NOx. Particulate matter can also cause asthma attacks for instance. I'm not saying that Diesel cars (and buses, trucks etc.) are not a factor in pollution, but the article is focusing solely on them and describing Seattle as a comparative paradise, which I find pretty laughable. Ok, Seattle probably has better air circulation than London, but I hope you'll agree Diesel engines can't be blamed for that ;)