Unpolluted rain has a pH value between 5 and 6. Rain is always slightly acidic due to oxidation, but it becomes dangerous when its pH level reaches 4 or lower.
Normal rain at the 5 or 6 pH value is considered almost neutral on the pH scale, therefore negative effects of this rain are not measured. However, when sulphur dioxide and nitrogen oxides increase acidity, rain can become known as acid rain. Acid rain typically has a pH value of 4, although it has been recorded as being as low as pH2.
While the strongest acid rain has shared a pH level near that of vinegar (2.2) and lemon juice (2.3), acid rain is dangerous for the environment. It begins to dissolve the nutrients in the soil that plant life needs, introduces toxic substances into the soil such as aluminium, as well as threatens the fragile nature of aquatic-based ecosystems. Some fish, for example, will suffer mutations, which in turn declines the population of their species. This may lead to an increase in the population of the larger insects these fish feed on, thereby decreasing the population of smaller insects or plankton which the larger insects eat. The decline of the larger insect's food supply could cause the population of larger insects to drop rapidly, followed by a sudden increased fall in the already declining population of the fish.