Head Forester from Newcastle, UK, comparing healthy (large) & damaged (small) branch of a sitka spruce Picea sitchensis, both representing five years growth. The damage is associated with acid rain pollution & is expressed in terms of stunted overall growth, yellowing of needles & premature fall of needles. Acid rain is the lowering of the pH levels (7 is normal) in rainfall by the addition of manmade pollutants into the atmosphere. These include sulphur dioxide & oxides of nitrogen resulting from the burning of fossil fuels. The impact of acidification on soils, plant & animal life & bodies of water is of growing concern.