tunathatpeopleeat. Theselimitsareespeciallyimportantforchildrenandpregnantwomen,
since mercury particularly affects young people. If the mercury just stayed in fat, it would
not be harmful, but that fat is used when a woman is pregnant or nursing a baby, or when
she burns the fat while losing weight. Methyl mercury poisoning can cause nervous system
or brain damage, especially in infants and children. Children may experience brain damage
or developmental delays. Like mercury, other metals and VOCS can bioaccumulate, causing
harm to animals and people high on the food chain.
Acid Rain
Acid rain is caused by sulfur and nitrogen oxides. These pollutants are emitted into the
atmosphere from power plants or metal refineries. The oxides come out of smokestacks that
have been built tall so that pollutants don’t sit over cities. The high smokestacks allow the
emissions to rise high into the atmosphere and travel up to 1000 km (600 miles) downwind.
As they move, these pollutants combine with water vapor to form sulfuric and nitric acids.
The acid droplets form acid fog, rain, snow, or they may be deposited dry. Most typical is
acid rain(Figure22.8).
Figure 22.8: How acid rain is formed. Anthropogenic pollutants are those that are human-
made. Deposition of a pollutant occurs when it is placed on a surface. Rain can bring wet
deposition or a pollutant can be blown onto the ground for dry deposition. ( 4 )
Acid rain water is more acidic than normal rain water. To be called acid rain, it must have
a pH of less than 5.0. Acidity is measured on thepH scale, which goes from 1 to 14. A
value of 7 is neutral. Lower numbers are more acidic and higher numbers are less acidic (also
called morealkaline). The strongest acids are at the low end of the scale and the strongest
bases are at the high end. Natural rain is somewhat acidic with a pH of 5.6. The acid comes
from carbonic acid that forms when CO 2 combines with water in the atmosphere. A small