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 As the name suggests, acid rain is just rain which is acidic. The rain becomes acidic because of gases which dissolve in the rain water to form various acids. Rain is naturally slightly acidic because of the carbon dioxide dissolved in it (which comes from animals breathing), and to a lesser extent from chlorine (which is derived from the salt in the sea). This gives rain a pH of around 5.0, and in some parts of the world it can be as low as 4.0 (this is typical around volcanoes, where the sulphur dioxide and hydrogen sulphide form sulphuric acid in the rain). About 70 percent of acid rain comes from sulphur dioxide (SO2), which dissolves into the water to form sulphuric acid. The rest comes from various oxides of nitrogen (mainly NO2 and NO3, collectively called NOx). (These figures are for Scandinavia - Scotland has a very similar ratio, while the north-eastern USA has 62 percent sulphuric acid, 32 percent nitric acid and 6 percent hydrochloric acid). These gases are produced almost entirely from burning fossil fuels, mainly in power stations and road transport:
 Acid rain causes lakes and rivers to become acidic, killing off fish - all the fish in 140 lakes in Minnesota have been killed, and the salmon and trout populations of Norway's major rivers have been severely reduced because of the increased acidity of the water. Short-term increases in acid levels kill lots of fish, but the greatest threat is from long-term increases, which stop the fish from reproducing. The extra acid also frees toxic metals which were previously held in rocks, especially aluminium, which prevents fish from breathing. Single-celled plants and algae in lakes also suffer from increased acid levels, with numbers dropping off quickly once the pH goes below 5, and by the time the pH gets down to 4.5, virtually everything is dead. A very highly publicised problem is the effect of acid rain on trees. Conifers appear to be particularly affected, with needles dropping off, and seedlings failing to produce new trees. The acid also reacts with many nutrients the trees need, such as calcium, magnesium and potassium, which starves the trees. The trees are then much more susceptible to other forms of damage, such as being blown down, or breaking under the weight of snow.
 Rather surprisingly, the effects of acid rain on trees have overshadowed the effects on people. Many toxic metals are held in the ground in compounds. However, acid rain can break down some of these compounds, freeing the metals and washing them into water sources such as rivers. In Sweden, nearly 10,000 lakes now have such high mercury concentration that people are advised not to eat fish caught in them. As the water becomes more acidic, it can also react with lead and copper water pipes, contaminating drinking water supplies. In Sweden, the drinking water reached a stage where it contained enough copper to turn the hair green! The worst part is that much copper can also cause
diarrhea in young children, and can damage livers and kidneys.
A less serious problem is the damage acid rain causes to certain materials, particularly limestone and marble. The acid dissolves the calcium carbonate in the stone, and this solution evaporates, forming crystals within the stone. As these crystals grow, they break apart the stone, and the structure crumbles. The best approach to acid rain is to reduce the amount of NOx and SO2 being released into the atmosphere. Fitting a catalytic converter to a car can reduce the emissions of NOx by up to 90 percent, but they are very expensive, and cause more carbon dioxide to be released, which contributes to the greenhouse effect. All these methods for reducing acid gases are expensive, and have drawbacks, so laws have been passed to force businesses to use them.
 The best way to reduce them is not to use as much energy in the first place. You can help in the following ways:
- Turn off lights when you leave a room
- If you have a car, don't use it for short journeys
- Basically, anything at all that uses less energy
Liming is a temporary fix to this major problem. Lime buffers the acid, and lakes where there is a lot of lime in the ground are found to have been less affected by acid rain. However, liming, the pouring of huge quantities into water, is very expensive. It also does not completely undo the damage caused by acid rain, and within a few years the lime will be dissolved by the acid and need to be replaced. Another idea is to try and prevent the air pollution that causes acid rain. One way is to "wash" the coal so that when it burns, it will pollute the air less. The coal is crushed, sprayed with water, and filtered to remove dirt and other impurities that can increase the amount of pollutants in the smog that comes from burning coal.
 Another method is called scrubbing. Scrubbers "clean" the gases from coal fires by spraying the gases with water and lime. This mixture combines with the sulfur dioxide to form sludge. However, many companies complain that the scrubbers are far too expensive to buy (they're $150 million a piece!) as well as to operate, since many people are needed to operate them. Also, what to do with the sludge that scrubbers produce is a problem because so far it has no known use. A researcher and microbiologist named Henry Tsuchiya discovered a bacterium that eats the sulfur on raw coal. If this sulfur is removed before the coal is burned, then sulfur dioxide won't form when the coal is burned, or at least not as much will be produced. However, this preventative measure is still being researched.
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