outdoor air pollution

Air pollution is a serious problem in the United States and many other industrialized countries. In the United States, the Environmental Protection Agency is charged with identification and regulation of pollutants in the ambient air that may cause adverse health effects.

Despite federal and state regulations, many cities and regions in the United States currently do not meet these primary standards. Epidemiologic research, human clinical studies, and animal toxicologic studies continue to provide evidence for adverse health effects of ambient air pollutants, even at exposure levels below the current standards.

The major sources of ambient air pollutants are:
Combustion of fossil fuels. These are divided into mobile sources such as motor vehicles, stationary sources such as power plants and factories, and other sources such as barbecues and fireplaces.

Tailpipe emissions from motor vehicles are a complex mixture of carbon monoxide, oxides of nitrogen, hydrocarbons, diesel exhaust particles, and other particulates including lead oxide from tetraethyl lead contained in leaded gasoline.
Photochemical reactions. Oxides of nitrogen and volatile hydrocarbons interact in the atmosphere to produce ozone (O3) as a secondary pollutant.

Power plants. These release sulfur dioxide (SO2) and particulates into the atmosphere. Coal and oil contain sulfur, leading to atmospheric formation of sulfates. Automobiles release oxides of nitrogen, leading to atmospheric formation of nitrates. Aerosolized acid sulfates contribute to acid rain.

Waste incinerators, industry, smelters. These point sources release acid aerosols, metals, mercury vapor, and organic compounds that may be hazardous for human health.

One example of the numerous hazardous chemicals emitted by these sources is methyl isocyanate that was accidentally released at Bhopal in India in 1984, resulting in 3000 deaths due to pulmonary edema. Some of the air toxins, such as polycyclic aromatic hydrocarbons, are known carcinogens.

Lungs are the major target of common outdoor air pollutants; especially vulnerable are children, asthmatics, and people with chronic lung or heart disease. The serious toxicity associated with lead exposure is discussed subsequently under Industrial Exposures. The major air pollutants and the mechanisms responsible for their adverse health effects are summarized briefly.

Ozone

Ozone is a major component of smog that accompanies summer heat waves over much of the United States. Exposure of exercising children and adults to as little as 0.08 ppm produces cough, chest discomfort, and inflammation in the lungs. Asthmatics are especially sensitive and require more frequent visits to emergency rooms and more hospitalizations during smog episodes. It is not known whether these acute changes lead to chronic, irreversible lung injury.

Ozone is highly reactive and oxidizes polyunsaturated lipids to hydrogen peroxide and lipid aldehydes. These products act as irritants and induce release of inflammatory mediators, cause increased epithelial permeability and reactivity of the airways, and decrease ciliary clearance. The highest inhaled dose is delivered at the bronchoalveolar junction; however, ozone also causes inflammation of the upper respiratory tract.

Nitrogen Dioxide

Oxides of nitrogen include NO and NO2. These have lower reactivity than ozone. Nitrogen dioxide dissolves in water in the airways to form nitric and nitrous acids, which damage the airway epithelial lining. Children and patients with asthma have increased susceptibility to nitrogen dioxide; there is a wide variation in individual responses to this pollutant.

Sulfur Dioxide

This pollutant is highly soluble in water; it is absorbed in the upper and lower airways, where it releases H+, HSO3- (bisulfite), and SO3- (sulfite), which cause local irritation.

Acid Aerosols

Primary combustion products of fossil fuels are emitted by tall smoke stacks at high altitudes and are transported by air. In the atmosphere, sulfur and nitrogen dioxide are oxidized to sulfuric acid and nitric acid, respectively, which are dissolved in water droplets or adsorbed to particulates. These acid aerosols are irritants to the airway epithelium and alter mucociliary clearance. Asthmatics have decreased lung function and increased hospitalizations when exposed to acid aerosols, although there is a wide variation in airway responses.

Particulates

The deposition and clearance of particulates inhaled into the lungs depend on their size. Ambient particulates are highly heterogeneous in size and in chemical composition.

It is uncertain which characteristics of ambient particulates contribute to their adverse health effects. Recent epidemiologic and toxicologic studies suggest that ultrafine particles (less than 0.1 μm in aerodynamic diameter) are more hazardous. They contribute to increased morbidity and mortality, especially among infants, the elderly, and people with chronic cardiopulmonary disease.

The mechanisms responsible for these adverse health effects are suspected to involve:

 (1) systemic cytokine release associated with pulmonary inflammation; - (2) increased blood viscosity;
 (3) autonomic changes associated with variable heart rates and arrhythmias.

See also

 indoor air pollution