There are various natural causes, such as gases from volcanoes. However, it is thought that air pollution by people now causes most acid rain. People started producing more acidic gases when they started building factories and power stations. These buildings as well as houses and vehicles burn coal or oil that have sulphur in them. This releases gases into the air that produce acid rain. Governments have tried since the 1970s to reduce the amount of sulphur being released into the Earth's atmosphere, and have had good results so far. However, it is expensive to clean the smoke from factories and power stations. In 2001 Great Britain still produced about five million tons of these gases every year; and China produced 18 million tons. The United States produced more than 20 million tons then, which declined to 8.1 million in 2010.