A type of pollution which washes out of the atmosphere as dilute sulphuric and nitric acids. It tends to be a regional rather than a global phenomenon, linked to particular industrial activities and meteorological conditions. It includes rain, more than normally acidic snow, mist, sleet, fog, gas and dry particles. It upsets the balance of nature, disrupting ecosystems, and destroys forests and woodlands, plants and crops; kills aquatic life by altering the chemical balance of lakes and rivers and corrodes building materials and fabrics. The pollutants are caused principally by discharges from power station chimneys of sulphur dioxide and nitrogen oxides released by burning fossil fuels, coal and oil.