Londoners are being warned not to breathe too deeply when they go outside. A toxic fog is hanging over the streets, threatening the health and 1WELL of the capital. It is small 2CONSOLE to know that this has been the state of the city’s air for more than 200 years. London is in a natural basin surrounded by hills and its air generally holds 3MOIST because of the river running through it, so it has always had a natural fog problem. Then came the industrial 4EVOLVE, with coal fires powering steam-driven factory machines and being used to heat homes. As the city’s industry and population grew 5PACE from the 1820’s onwards, smoke mixed with the moist air and on cold days produced a particularly nasty atmosphere. It was thick, yellow, and had a 6SULPHUR smell. What was worse, it became trapped in London’s narrow roads and 7ALLEY. It was only when gas and electricity became less 8AFFORD that steps could be taken to alleviate the situation.