The term “smog” was first used in London during the early 1900’s to describe the combination of smoke and fog that often blanketed the city. Smog is a type of air pollution which is composed of nitrogen oxides, sulfur oxides, ozone, smoke or particulates among others (less visible pollutants include carbon monoxide, CFCs and radioactive sources). Man-made smog is derived from coal emissions, vehicular emissions, industrial emissions, forest and agricultural fires and photochemical reactions of these emissions.

Modern smog, as found for example in Los Angeles is a type of air pollution derived from vehicular emission from internal combustion engines and industrial fumes that react in the atmosphere with sunlight to form secondary pollutants that also combine with the primary emissions to form photochemical smog. In certain other cities, such as Delhi, smog severity is often aggravated by stubble burning in neighbouring agricultural areas. The atmospheric pollution levels of Los Angeles, Beijing, Delhi, Mexico City, Tehran and other cities are increased by inversion that traps pollution close to the ground. It is usually highly toxic to humans and can cause severe sickness, shortened life or death.

Air Pollution is defined as “Chemicals added to the atmosphere by natural events or human activities in high enough concentrations to be harmful”. Two categories of air pollutant are: (1) Primary Air Pollutant: Harmful substance that is emitted directly into the atmosphere, (2) Secondary Air Pollutant: Harmful substance formed in the atmosphere when a primary air pollutant reacts with substances normally found in the atmosphere or with other air pollutants. The 20th century has been characterised by a transition from urban air dominated primary pollutants to one where secondary pollutants are very important. Three major outdoor air pollution problems are industrial smog from burning coal, photochemical smog from motor vehicle and industrial emissions, and acid deposition from coal burning and motor vehicle exhaust.

Photochemical smog is a unique type of air pollution which is caused by reactions between sunlight and pollutants like hydrocarbons and nitrogen dioxide. Although photochemical smog is often invisible, it can be extremely harmful, leading to irritations of the respiratory tract and eyes. In regions of the world with high concentrations of photochemical smog, elevated rates of death and respiratory illnesses have been observed. Photochemical smog is therefore considered to be a problem of modern industrialisation. It is present in all modern cities.

The initial step in photochemical reactions is the absorption of a photon by an atom, molecule, free radical or ion. The result of this absorption is strongly dependent on the energy, in other word the wavelength of the photon. Photochemical smog can occur in any environment where there are large and continuous emissions of primary air pollutants. However specifics of climate and geography play an important role in the persistence and severity of the photochemical smog. Two factors influencing the formation of photochemical smog (a) Topography: Wind moves air masses both horizontally and vertically, dispersing pollutants and reducing concentrations with distance from the source. The amount of dispersion which occurs is directly related to the stability of the air or how much vertical air movement is taking place. (b) Temperature Inversion: Thermal inversions worsen any type of smog .It is abnormal arrangement of air masses. A warmer layer of air is trapped between two layers of colder air. This causes pollutants to be trapped near the earth’s surface can form when hills or mountains stop horizontal winds, causing pollutants to collect over a city. Warm air collects over the polluted air, acting as a lid to stop the pollutants from being dispersed. Major Air Pollutants responsible for photochemical smog are Carbon oxides (CO, CO2), Nitrogen oxides and nitric acid (NO, NO2, HNO3), Sulphur dioxide and sulphuric acid (SO2, H2SO4), Particulates (SPM), Ozone (O3), Volatile organic compounds (VOCs).

The reactions that lead to the formation of photochemical smog are irritated by sunlight and involve hydrocarbons and nitrogen oxides emitted from automobiles. Photochemical smog (mainly automobile pollution) from the burning of coal and oil contains sulphur dioxide, sulphur trioxide, soot and ash (particulate matter) and sulphuric acid. It can cause breathing difficulties in humans, plus acid rain damage to plants, aquatic systems, and metal or stone objects.

The time of day is a very important factor influencing on the amount of photochemical smog:(a) Early morning traffic increases the emissions of both nitrogen oxides and no methane hydrocarbons (NMHC) a type of VOCs - as people drive to work. (b) Later in the morning, traffic reduces and the nitrogen oxides and volatile organic compounds begin to react to form nitrogen dioxide and increase its concentration. (c) As the sunlight becomes more intense later in the day, nitrogen dioxide is broken down and its by-products from increasing concentrations of ozone. (d) At the same time, some of nitrogen dioxide can react with the volatile organic compounds to produce toxic chemicals like PAN.

Vegetation is easily harmed main agents of damage are ozone and PAN. Sensitive crops, trees and other vegetation are harmed at lower ozone concentrations than is human health. Ground level ozone can damage leaves, and reduce growth, productivity and reproduction. It can cause vulnerability to insects and disease, and even plant death. When ozone levels are fairly high over a long period, agricultural crops can suffer significant harm.

Smog can also accelerate the deterioration of rubber, plastics, paints and dyes. Damage to metals, stone, concrete, clothing, rubber and plastic is directly related to contaminants in the air. The typical culprits are sulphur dioxide, sulphuric acid, ozone (photochemical smog), and nitric acid (HNO3).

Anyone who engages in strenuous outdoor activity—from jogging to manual labor—may suffer smog-related health effects. Physical activity causes people to breathe faster and more deeply, exposing their lungs to more ozone and other pollutants. Four groups of people are particularly sensitive to ozone and other air pollutants in smog:

• Children—Active children run the highest risks from exposure to smog, as children spend a lot of time playing outside. As a group, children are also more prone to asthma—the most common chronic disease for children—and other respiratory ailments than adults.

• Adults who are active outdoors—Healthy adults of any age who exercise or work outdoors are considered at higher risk from smog.

• People with respiratory diseases—People with asthma or other chronic respiratory diseases are more sensitive and vulnerable to the effects of ozone. Typically, they will experience adverse effects sooner and at lower levels of exposure than those who are less sensitive.

• People with unusual susceptibility to ozone—some otherwise healthy people are simply more sensitive to the pollutants in smog than other people, and may experience more adverse health effects from exposure.

Elderly people are often warned to stay indoors on heavy smog days. Elderly people are probably not at increased risk of adverse health effects from smog because of their age. Like any other adults, however, elderly people will be at higher risk from exposure to smog if they already suffer from respiratory diseases, are active outdoors, or are unusually susceptible to ozone.

In addition, the numbers of road crashes with high mortality and morbidity rates have also been reported. The road, the air and the coastal traffic may be effected.

Don’t take chances with smog on days when air quality is poor. The best approach is to spend less time outdoors and replace vigorous activities, like running or biking, with gentler options, such as walking. You can also schedule your outside activities for the early morning or evening, when ozone levels are low. These simple steps can help protect you and your family on smoggy days, whether you live in a major city or you’re just passing through.