In most states it is against the law to switch from unleaded to leaded fuel, or to tamper with catalytic converters. Thanks to these different kinds of emission controls, the air in the United States is better than it used to be. Amounts of most major air pollutants have gone down. For example, between 1975 and 1984 amounts of carbon monoxide and sulfur dioxide in the air dropped an average of more than 30 percent. The amount of lead dropped 70 percent.
Scientists now believe that some problems are more serious than they first thought.
For example, researchers are studying harmful effects of acid rain . When some emissions from factories and cars mix with sunlight and vapor in the air, they change into acidic compounds. These compounds can travel long distances in the air. Then they can fall to earth with rain, snow or dust. When they fall on lakes, they can turn the water acidic, like vinegar.