Michael Portillo ventures deep underground onto London's newest railway - the Elizabeth Line - enjoying a smooth ride from Paddington to Tottenham Court Road. Emerging into the sunshine in Bedford Square, Michael recalls the choking smogs which plagued the capital during the 1950s. He learns how smoke-free zones were introduced to tackle the problem and, high above the capital at the top of the BT Tower, discovers how the capital's air quality is monitored today.