[originaltext] According to a recent report by the UNEP, if car numbers keep

According to a recent report by the UNEP, if car numbers keep increasing at the present rate, there will be more than a billion on the road by 2025. Today, motor vehicles put out 900 million tones of carbon dioxide a year--about 15 percent of our total output. More vehicles will mean more global warming. Also by 2025, two-thirds of the world’s people will live in cities, so traffic jams and pollution will loom large in most people’s lives.

We live in a society in which the medicinal and social use of substances is pervasive: an aspirin to quiet a headache, some wine to be sociable, coffee to refresh oneself in the morning. When do these socially acceptable and apparently constructive uses of a substance become misuses? Most substances taken in excessively will produce negative effects such as poisoning. Repeated use of a substance can also lead to addiction or substance dependence. Dependence is marked first by an increased tolerance, and then by the appearance of withdrawal symptoms when the substance is discontinued.