Watch HDToday. Smog was the deadly downside of Britain's industrial might, as this powerful and revealing documentary spells out. In 1937, coal was Britain's lifeblood; it fuelled her industry and heated most homes. But coal was wasteful and dirty, and it had an unpleasant, even lethal by-product. Smog wasn't just nasty and disruptive, it took its toll on buildings, the economy, child development and adult health - and it was a killer, claiming scores of lives every year. By 1937, the battle against smog was already being waged: the film points to processing technology to convert raw coal into oil or smokeless fuel, the increasing use of cleaner energy from gas and electricity, and improved housing. But another 15 years of periodic outbreaks of smog still lay ahead, before London's Great Smog of 1952 finally spurred Parliament into action in the form of the Clean Air Act of 1956. -BFI