Major cities, including Beijing and Hong Kong, have adopted the stringent standards set by the European Union, Euro IV, for passenger and heavy-duty vehicles, with the rest of China set to follow over the coming years (<a href=http://ehp03.niehs.nih.gov/article/fetchArticle.action;jsessionid=38EE274BD0D5242525BEEE8B97B9C908?articleURI=info%3Adoi%2F10.1289%2Fehp.1003024#r31>Walsh 2007</a>).</p>