Air pollution


Air pollution is a broad term applied to all chemical and biological agents
that modify the natural characteristics of the atmosphere.

Some definitions also consider physical perturbations such as noise
pollution, heat, radiation or light pollution as air pollution. Some
definitions include the term harmful as a requisite to consider a change to
the atmosphere as pollution.

Causes

The sources of air pollution are divided in two groups: anthropogenic
(caused by human activity) and natural.

Natural sources include:

   * Volcanic activity
   * Dust from natural sources, usually large areas of land with little or
     no vegetation
   * Gases, such as methane, emitted by the digestion of animals, usually
     cattle.
   * Smoke from wildfires.

Anthropogenic sources are mostly related to burning different kinds of fuel.
They include:

   * Dust and chemicals from farming, especially of erodible land, see Dust
     Bowl
   * Industrial activity in general.
   * Vehicles with internal-combustion engines.
   * Stoves and incinerators, specially coal ones.
   * Paint fumes, or other toxical vapors.
   * Aerosols.

Contaminants

Contaminants of air can be divided in particles and gases.

Particles are classified by their sizes. A usual division is in PM10 and
PM2.5. PM10 are particles whose size is less than 10 microns (0.01 mm); they
are dangerous to humans because they can be breathed and reach the lungs.
PM2.5 are particles whose size is less than 2.5 microns (0.0025 mm), and
they are even more dangerous because they can pass the alveoli and reach the blood.

Important pollutant gases include:

   * Carbon monoxide.
   * Nitrogen oxides.
   * Hydrocarbons.
   * Ozone in the lower part of the atmosphere (in the upper part, it helps
     to reduce the amount of ultraviolet radiation from the sun that reaches
     earth).
   * Sulfur dioxide, which causes acid rain.

The worst single incident of air pollution to occur in the United States
occurred in Donora, Pennsylvania in late October, 1948