Watch Horizon Season 2 Episode 9 Online

Nine years after the passing of the Clean Air Act, where do we stand? Scientists are gradually finding out why dirty air Is so harmful to ill persons with Dr. P. J. Lawther of Air Pollution Research Centre at St. Bartholomew's Hospital. Whenever the things they study are too big, too far off, or too hot to handle, scientists can make a model of these-but can they be sure their models truly represent reality?