Bobby's return to Yellowknife leaves him torn between money and old friendships.

Air pollution has become the world's greatest environmental health risk, and science is only beginning to reveal its wide-ranging effects. Globally, 19,000 people die each day from air pollution, killing more than HIV/AIDS, tuberculosis, malaria and car accidents combined. What happened to the air we breathe? Sustainability journalist Tim Smedley has travelled the world to try and find the answer, visiting cities at the forefront of the fight against...