Answer to Question 1
The Clean Air Act required that the most widespread pollutants be identified, ambient standardslevels that need to be achieved to protect environmental and human healthbe set, and control methods and timetables to meet the standards be established.
The six criteria pollutants that are covered by the National Ambient Air Quality Standards are carbon monoxide, nitric oxides, sulfur dioxide, particulates, ozone, and lead. The primary standards are quantities of the pollutants that can be found in air and are based on the presumed highest level that can be tolerated by humans without noticeable ill effects, minus a 10 to 50 margin of safety.
Answer to Question 2
The pH of an environment is extremely critical, because it affects the function of virtually all enzymes, hormones, and other proteins in the bodies of all organisms living in that environment. Ordinarily, organisms are able to regulate their internal pH within the narrow limits necessary to function properly. A consistently low environmental pH, however, often overwhelms the regulatory mechanisms in many life-forms, thus weakening or killing them. Most freshwater lakes, ponds, and streams have a natural pH in the range from 6 to 8, and organisms are adapted accordingly. Most are severely stressed, and many die, if the environmental pH shifts as little as one unit from the optimum. As aquatic ecosystems become acidified (pH 5 and below), more organisms die off, either because the acidified water kills them or because it keeps them from reproducing.
From the Green Mountains of Vermont to the San Bernardino Mountains of California, the die-off of forests trees in the 1980s caused great concern. Red spruce forests are especially vulnerable. In New England, 1.3 million acres of high-elevation forests were devastated. Commonly, the damaged trees lost needles as acidic water drew calcium from them, rendering them more susceptible to winter freezing. Sugar maples, important tress in the Northeast, have shown extensive mortality, ranging from 2080 of all trees in some forests. Much of the damage from acid precipitation to forests is due to chemical interactions within the forests soils. Sustained acid precipitation at first adds nitrogen and sulfur to the soils, which stimulate tree growth. In time, though, these chemicals leach out large quantities of the buffering chemicals (usually calcium and magnesium salts). When these buffering salts no longer neutralize the acid rain, aluminum ions, which are toxic, are dissolved from minerals in the soil. The combination of aluminum and the increasing scarcity of calcium, which is essential to plant growth, leads to reduced tree growth.
