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Air pollution and the businesses trying to clean it up: In today's programme we hear from a "fun" run in car-choked Cairo. Meanwhile presenter Ed Butler takes a ride across London to meet fellow cycling enthusiast and air pollution monitor Professor Andrea Sella of University College London.

Dr Audrey de Nazelle of Imperial College explains why the air in some developing country cities such as Delhi (pictured) is so dirty that doing such outdoor activities is actually bad for your health.

Ed also speaks to two businesspeople using innovative technologies to tackle this ubiquitous urban blight - Allison Dring of Elegant Embellishments who put titanium dioxide on a wall in Mexico to destroy airborne pollutants, and Dutch entrepreneur Daan Roosegaarde who has invented a giant vacuum filter.

(Picture: Runners take part in the New Delhi 10K Challenge amid heavy smog in New Delhi; Credit: Dominique Faget/AFP/Getty Images)