Smog

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Smog

Introduction

Smog is a particularly potent type of air pollution. It is usually highly visible as a brownish or yellowish haze in the air, and although smog is often associated with large cities, it occurs around the world. Smog occurs when emissions from factories and cars mix with air under certain atmospheric conditions that trap pollutants near to the ground. Heavy traffic, warm weather, and still air tend to contribute to smog build up in cities such as Los Angeles, California. Ground level ozone is often a component of smog, and it is created by a complex series of chemical reactions catalyzed by sunlight.

Smog has a number of damaging impacts on health. The most obvious effects in a thick urban smog would be watering eyes and coughing. More serious impacts include triggering of asthma and heart attacks. Smog also reduces visibility, which can be dangerous when driving. Environmental legislation promoting cleaner air has dramatically reduced the incidence of smog in many countries, but it continues as a worldwide issue.

Historical Background and Scientific Foundations

In 1952, around 4,000 people in London, England, died as a result of the notorious Great Smog. Emissions from factories combined with smoke from people’s chimneys to create a great cloud of pollution that enveloped the city. The term smog, which means fog intensified by smoke, was coined in 1905, although the problem of the so-called “pea-souper” fogs occurring in urban areas dates back much further, and is mentioned in the work of nineteenth-century British writer Charles Dickens (1812–1870).

Smog is visible as a brownish haze polluting the air, particularly in cities and especially in the summer. It comprises an aerosol of noxious droplets and particles. One major component of smog is ozone, which is formed when nitrogen oxides and unburned hydrocarbons from motor vehicles react in the presence of sunlight to create a photochemical smog. Ozone in the stratosphere is helpful, as it absorbs ultraviolet (UV) light from the sun. Ground level ozone is a powerful pollutant that affects the eyes and lungs.

Los Angeles is a classic example of a city where climate and geographical factors create conditions for the formation of photochemical smog. The city is surrounded by mountains on three sides and has a dry, sunny climate. At night, the sky is usually clear and the ground cools fast. Air near the surface is cool, while upper layers are relatively warm. Lack of mixing of the layers forms an inversion and traps pollution from traffic close to the ground. Morning sunshine triggers a number of photochemical reactions, creating ozone. This photochemical smog is sometimes clearly visible as a brown haze by the afternoon.

Another example of smog is the Asian Brown Cloud, which is produced by burning of agricultural wastes and increases in the use of fossil fuels. The cloud covers the entire Indian sub-continent for much of the year and drifts out over the Indian Ocean at the end of the monsoon season.

Impacts and Issues

The World Health Organization (WHO) says that air pollution, including smog, claims five to six million lives per year. Fine particles in smog are linked with heart attacks, worsening asthma, lung cancer, and immune suppression. Ozone and formaldehyde cause eye irritation, coughing, chest pain, and attacks of asthma and bronchitis. Meanwhile, smog reduces visibility. If all sources of air pollution were removed from the atmos-

WORDS TO KNOW

AEROSOL: Liquid droplets or minute particles suspended in air.

INVERSION: A type of chromosomal defect in which a broken segment of a chromosome attaches to the same chromosome, but in reverse position.

OZONE: An almost colorless, gaseous form of oxygen, with an odor similar to weak chlorine, that is produced when an electric spark or ultraviolet light is passed through air or oxygen.

PHOTOCHEMICAL SMOG: A type of smog created by the action of sunlight on pollutants.

phere, people in cities could see an estimated ten times farther than they can today.

There are various legislative measures in place in the United States and elsewhere designed to reduce smog and air pollution. The 1990 Clean Air Act covers ozone, nitrogen oxides, and particles in smog. States must meet the criteria that the Environmental Protection Agency (EPA) lays down for these pollutants or take measures to clean up. Generally, this involves applying new, cleaner technology to vehicles and tackling traffic congestion. Some states such as California have gone one step farther by enacting tougher vehicle emission standards than are required by federal law.