Acid rain or acid precipitation is the rain, snow, fog hail which has pH levels below 5. The pH scale is used to measure the level of acidity by measuring the hydrogen ion concentration, were pH 1 is the strongest acid and pH 14 is the strongest base. Acids and bases of the same amount and strength can neutralise each other. Acid rain is caused by the release of oxides of sulphur (SO2) and nitrogen (NOx) which when combined with the moisture in the air, may fall as rain with acidity of sulphuric or nitric acids. In Europe, the main cause of the acid rain is caused by the release of sulphur oxides. These are released when burning fossil fuels. Cars, power-plants, industries and burning coal and oil are the main contributories to the release of sulphur oxides. Nitrogen oxide is a result of any combustion or burning processes. The main source of emissions of nitrogen oxides are vehicles, residential and commercial furnaces and engines. Some acid rain is caused naturally by volcanoes, forest fires and rotting vegetation. An aspect of acid rain that has made it an issue of great political controversy is that it can be transported by prevailing winds. Neighbouring or close-by countries may be polluting, but the precipitation may only occur in near countries. The sulphur and nitrogen oxides can also be moved and deposited without forming acid rain. This occurs when it doesn't combine with moisture in the air to make acids, so it is deposited often in the ground or in leaves. This is also potentially harmful we may breathe this in, and the moisture in our throats or lungs may turn them into acids.
The problem of acid rain has been greatly ignored until quite recently, even though it was detected in the industrial revolution. Only 1990 was it declared the worst environmental hazard Europe, about 100 years after it started affecting our environment. The problem is affecting Northern Europe, Northern USA and Canada.