Causes and effects of photochemical smog

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Photochemical smog is usually associated with warm, sunny, urban areas, particularly those with a large amount of industry and heavy traffic. This type of smog has become a particular problem with increasing human reliance on motor vehicles.

Photochemical smog can have a range of negative effects on health and the environment.


Photochemical smog often has a brown, hazy, foglike appearance, and it usually occurs in urban areas. Although the word "smog, first used for the type of smog that originated during the Industrial Revolution, is derived from "smoke" and "fog," neither of these is present in photochemical smog. Photochemical smog results from a reaction between sunlight and chemicals that are in the air because of pollution.


The combustion of fossil fuels, particularly in industry and car engines, is the major cause of air pollution that contains chemicals essential for the formation of photochemical smog. This use of fossil fuels causes nitrogen oxides and volatile organic compounds to pollute the atmosphere. The chemical reactions that cause photochemical smog to form can take place at temperatures of more than 17.8 degrees Celsius, with sunlight as a catalyst.

Conditions that aid photochemical smog formation

Specific conditions are necessary for photochemical smog. One of these is weather. Dry, windless days are ideal for the formation of photochemical smog, as rain and wind can prevent air pollution from remaining in one area for long. Geographical areas that are sheltered by hills, for example, are also more prone to this type of smog because they are exposed to less wind that would carry pollution away. Urban areas also provide suitable conditions for photochemical smog, as they have a high concentration of motor vehicles and industry.

Effects on humans

The effects of photochemical smog on humans are mainly related to the respiratory system. The people most at risk are infants, the elderly, and people with underlying health conditions, particularly health conditions of the respiratory system. Symptoms can include a tight chest, coughing, inflammation of the lung, heightened symptoms of asthma in those who already suffer from that condition, and a greater susceptibility to developing other lung diseases. Other effects include headaches and sore or watery eyes. Photochemical smog contains peroxyacetyl nitrates and ground-level ozone, which cause the majority of smog's health effects and effects on the environment.

Effect on the environment

Photochemical smog also has an impact on the environment. Low-level ozone in this type of smog can affect the growth and reproduction of plants as damage and discolouration of leaves affects the rate of photosynthesis. Insects are also exposed to health effects from photochemical smog, and damage to plants affects a range of animals who feed from them, limiting their source of food.