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In today's lecture I'm going to talk about changes in air pollution since the middle of the last century and what has created these changes. So, um — by the 1950s, air pollution was very visible with frequent thick black fogs known as 'smogs' in many large cities around the world. The main source of this pollution was from factories and it caused severe health problems. For example, a particularly severe smog in London in 1952 caused over four thousand deaths. Obviously something had to be done and in 1956 a Clean Air Act was introduced in Britain. This addressed the pollution from factories and the smogs soon disappeared. However, as you know, these days air pollution is still a big issue. The main difference between now and the 1950s is that you can't see it — it's invisible. Also, the main source of pollution now is from cars and lorries, and although these don't produce visible signs, this air pollution is still a significant risk to health. And one of the key factors in the rise of this type of pollution is that we have all become much more vehicle-dependent. There are far more cars and lorries, trains and planes than in the 1950s and this is now the main source of air pollution around the world.