Overview: Reporter Marcel Theroux and director Kate Hardie-Buckley travel to Mongolia's capital Ulaanbaatar, where the air quality can reach more than 100 times the accepted limit and is causing a public health disaster. A layer of smog caused by coal smoke blankets the city. The smog is full of floating soot particles, some of which are small enough to bypass the body's defences, causing fatal illnesses including respiratory disease, heart disease and cancer. According to Unicef, cases of respiratory infections have tripled here over the last 10 years as pollution surges. The team visit a children's hospital where senior paediatrician Dr Oyenbileg tells Theroux that respiratory illness is the biggest killer of children under five, as they are more susceptible to respiratory diseases such as bronchitis and pneumonia.