The term acid rain has already become a household word. But, what exactly does it mean? Although many people know this word, not many fully understand its meaning. Here's how rain becomes acid rain: First, we know that sea water and lake and river water evaporates into the atmosphere, forming clouds, and comes back to the ground in the form of rain. In other words, rain is distilled water and neutral (pH of 7), and its conductivity is close to that of pure water. But, when the atmosphere is polluted with sulfur oxide () and oxides of nitrogen (), the rain goes through oxidation with ozone () or hydrogen peroxide () before falling to the ground, and forms some and . These are contained in the rain that pounds your umbrella. These compounds and are sulfuric acid and nitric acid, respectively. When these acids fall to the ground--and they do--it's a serious problem.

The definition of acid rain is not very clear, but generally rainwater with pH below 5.5 or 5.6 is called acid rain. The pH alone, however, does not tell us the actual quantity of pollutants contained in rain.

Some acid rain with pH of 5 has a conductivity of 50 µS/cm, while other acid rain with pH of 5 can have conductivity of 100 µS/cm. Naturally, the acid rain of 100 µS/cm has a higher concentration of pollutants. Incidentally, in the case of rain with very small content of pollutants and conductivity of around 10 µS/cm, reliable measurement of pH is difficult, and in some cases it may show a pH of around 5, caused by aspects other than pollutants.

When measuring acid rain, therefore, we recommend that you first measure conductivity in order to determine the density of pollutants before measuring pH.

In Japan, an acid rain incident occurred in 1973 where people complained of irritation to the skin or eyes, and this occurred with a drizzling rain.

The relationship between large-scale logging and acid rain was brought to the forefront especially in Europe and North America, and now investigations are being undertaken worldwide regarding the chronic effects of acid rain.

Have you ever turned on a water faucet in some remote place and been surprised with strange smell coming from the water? Perhaps a musty or chlorine odor? In supplying tap water, the water is taken from a lake or river, run through a treatment process and then a small amount of chlorine is added to kill germs before it is sent through pipes to faucets in the area. Therefore, the taste and smell can vary, depending upon the origin of water. The cleaner the original water and the more upstream its source, the more delicious the water. Does this mean that pure water, which contains no impurities at all, is delicious? No. If there are no minerals in the water, it has no taste. The water which we think is tasty is natural spring water. That's why you see bottled mineral water in the stores. Those minerals are substances such as calcium, magnesium, sodium, potassium and iron, which are dissolved in water.
Let's look at the conditions for good-tasting water specified by the Ministry of Health & Welfare in Japan.

When conductivity of tap water and natural water is measured, the results of measurement are sometimes just the reverse of what we might expect. Tap water of high conductivity contains a lot of such components as chlorine that take away from the taste of water. Mineral water contains minerals that make water delicious. (Incidentally, tap water is 100 to 200 µS/cm, and good-tasting water is often between 400 to 700 µS/cm.)

To make tap water delicious, there are various water purifiers on the market. Some remove chlorine with activated charcoal, and others use calcium carbonate to help dissolve calcium. As long as you understand the system of purifiers, you can determine whether a water purifier is working properly by measuring the conductivity before and after passing water through the purifier.