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The word 'smog' has been around since the early 20th century and was shorthand for 'smoky fog'. The first smog was recorded in London in 1873. These original smogs came from industrial pollution, such as burning coal. Today's smog is more likely to be caused by vehicle exhausts in places where smoky factories are a thing of the past. But in the developing world, industry as well as traffic, may result in the creation of smog. Whatever the cause, smog is a significant health risk to those exposed to it.

Frequently Asked Questions

Smog is a type of air pollution. The physical and chemical composition of smog is complex and variable. What all smogs have in common is that they are visible as a low-hanging cloud, often brownish or yellowish in colour and occurring usually over an urban area. Put simply, smog is a fog mixed with smoke. Fog is a visible collection of liquid water particles and or tiny ice crystals. Smoke is the visible product of combustion, consisting of carbon and other solid and liquid particles suspended in the air. Combine smoke and fog and you have smog.

Today, the most significant type of smog is photochemical smog, which is formed by the interaction of nitrogen oxides and hydrocarbons from vehicle exhausts and industrial sources in the presence of sunlight. A complex series of chemical reactions result in the formation of ground-level ozone, as a significant component of photochemical smog. Volatile Organic Compounds (VOCs) can also form photochemical smog when they react with nitrogen oxides. Industrial sources of VOCs include paints and solvents.

Photochemical smog is really a cocktail of noxious ingredients. Besides ozone, it may also contain nitrogen oxides, hydrocarbons and VOCs because these components may not react completely with one another in the ozone-forming reactions, as well as aldehydes (including formaldehyde) and peroxyacyl nitrates. Some of these components are so-called secondary pollutants because they are not themselves emitted from sources like traffic or factories. Instead, they are formed by a reaction between primary pollutants. Smog can linger long after the sun has gone in, and it can also travel through great distances. For instance, pilots flying in the high Arctic have reported layers of a dense reddish-brown haze over the Arctic atmosphere. Smog has been found to consist of aerosols, sulphates, dust, soot and toxic heavy metals that have drifted to the polar region from heavily industrialised parts of Russia and Europe.

Ozone is the most-studied component of smog as far as the impact on health is concerned. It is well known that exposure to ground-level ozone in photochemical smog causes chest pain, burning, or discomfort when breathing in, as well as coughing, eye burning, itching and irritation and sore throat. Ozone is not very soluble in water, so it readily reaches the lower part of the respiratory tract, rather than being removed in the secretions of the nose and throat. Children and the elderly are most susceptible to the adverse effects of ozone.

Research has also shown that high ozone days in cities are associated with a peak in asthma attacks, heart attacks and accompanying hospital admissions the next day. A recent study highlights the danger of chronic exposure to even relatively low levels of ozone. People living in cities with the highest ozone levels had three times the risk of dying of lung disease, compared with those living in cities with the lowest levels. In general, ozone exposure has been linked to excess mortality, with the risk being greater among the elderly and in the summer months. Globally, smog and air pollution, in general, is a major killer. For instance, it is estimated that around 2 million people die each year in India because of exposure to air pollution.

Ozone is especially dangerous to people with asthma. It makes the airways contract more, increases airway inflammation, and makes asthma attacks worse. It can increase the impact of pollen on the nose and airways. Scientists continue to study the impact that ozone has on the lungs, down to the cellular and molecular level.

Other components of smog, like nitrogen dioxide, particulate pollution and aldehydes are similarly noxious and harmful to health. Broadly speaking, inhaled pollution sets up inflammation within the lungs, and may also enter the bloodstream. Inflammation sets the scene for heart disease as well as decreasing lung function in many ways. Those with pre-existing heart and lung disease, such as asthma sufferers, are most at risk. There is a great deal still to learn about the long-term health effects of exposure to smog pollution but it is likely that the death toll from its effects will continue to increase, particularly in less developed countries, where environmental policy is still evolving.

Smog is present in all cities to a greater or lesser extent. It is more likely to form in fine, hot weather when the sky is clear and when there is little or no wind. Smog is more common in cities that are surrounded by hills or mountains and when vertical circulation of the air is inhibited by high temperatures. In other words, in conditions where the pollution cloud gets trapped near ground level.

Here are some notorious incidences of smog from around the world:

London was greatly affected by winter smogs in the 19th and the first half of the 20th century (they were nicknamed 'pea soupers' because of their density). The Great London Smog of 1952 killed 4,000 people in just four days (many thousands more died in the months to follow). The scandal led to the Clean Air Act and the introduction of smokeless fuel. The Asian Brown Cloud is a thick smog made up of ash, acids, aerosols, dust and photochemical smog components which are capable of cutting the amount of solar energy reaching the Earth's surface by as much as 15%. It covers the whole of the Indian subcontinent during most of the year. In December 2011, smog caused the delay or cancellation of hundreds of flights at Beijing Capital Airport. Environmentalists and citizens were angered when the government referred to the problem as 'fog' while pollution monitors at the US Embassy in Beijing showed soaring levels of PM2.5 pollution during the smog incidents. Mexico City is located in the Valley of Mexico, which is 7,300 feet above sea level and surrounded by mountains that are even higher. This geographical location leads to a thermal inversion- with cold mountain air trapping warmer air near the surface of the city. Add to these millions of motor vehicles emitting exhaust and, in the festive season, multiple firework displays and you have a recipe for a thick smog that descends over Mexico City every December, despite efforts by the city's politicians to control air pollution. Los Angeles, long notorious for its smogs, has just been rated the United States' most ozone-polluted city, according to the American Lung Association's State of the Air report. Geographical and climatic factors make the city smog-prone – but its dependence on the automobile does not help. The city's latest anti-pollution effort is the introduction of smog-eating roof tiles which are chemically treated to destroy nitrogen oxides in the air before they can form smog.

First, know when smog levels are high. The UK Daily Air Quality Index lists levels of:

Ozone PM2.5 PM10 Nitrogen dioxide Sulphur dioxide

All of these are components of smog. The daily readings come with colour codes that tell you how dangerous the levels are to your health. Follow the advice given, for instance, by avoiding strenuous physical activity on high smog days, so you breathe in less pollution. If you have asthma, be prepared to use more of your reliever inhaler, even if you are indoors or in your car on such days.

Simple precautions can help too. On a high smog day:

Stay indoors with external doors and windows closed if you can Don't walk or exercise on or near roads with heavy traffic

Ways to reduce smog include:

Car sharing, cycling, walking or using public transport wherever possible Drive a low pollution car Don't have smoky bonfires or barbeques on hot summer evenings Respect your city's environmental policies

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