Air pollution is now second only to smoking as a killer in the UK, yet as a nation we are failing to meet targets on reducing emissions. The team investigate what air pollution is, where it comes from, and what it is doing to us. Jem Stansfield reveals that cleaner-looking air doesn't necessarily mean healthier air; and he demonstrates why modern diesel engines, in spite of their growing green credentials, may be bigger polluters than petrol. Maggie Philbin follows some typical commuters to reveal how much pollution we all actually breathe in, and she also finds out how a soap powder additive could turn all our clothes into personal pollution filters. And while people continue to worry about the health of their lungs, Liz Bonnin investigates new research that suggests it may be our hearts that are more at risk from air pollution.