Smog is a type of large-scale outdoor pollution. It is caused by chemical reactions between pollutants derived from different sources, primarily automobile exhaust and industrial emissions. Cities are often centers of these types of activities, and many suffer from the effects of smog, especially during the warm months of the year.
For each city, the exact causes of pollution may be different. Depending on the geographical location, temperature, wind and weather factors, pollution is dispersed differently. However, sometimes this does not happen and the pollution can build up to dangerous levels. A temperature inversion occurs when air close to the earth is cooler than the air above it. Under these conditions the pollution cannot rise and be dispersed. Cities surrounded by mountains also experience trapping of pollution. Inversion can happen in any season. winter inversions are likely to cause particulate and carbon monoxide pollution. summer inversions are more likely to create smog.
 Great Smog of 1952
Early in December 1952, a cold fog descended upon London. Because of the cold, Londoners began to burn more Coal than usual. The resulting Air pollution was trapped by the inversion layer formed by the dense mass of cold air. Concentrations of pollutants, coal smoke in particular, built up dramatically. The problem was made worse by use of low-quality, high-sulphur coal for home heating in London in order to permit export of higher-quality coal, because of the country's tenuous postwar economic situation. The "fog", or smog, was so thick that driving became difficult or impossible. The extreme reduction in visibility was accompanied by an increase in criminal activity as well as transportation delays and a virtual shut down of the city. During the 4 day period of fog, at least 4,000 people died as a direct result of the weather.
 Formation of Smog
Photochemical smog (or just smog for short) is a term used to describe air pollution that is a result of the interaction of Sunlight with certain chemicals in the atmosphere. One of the primary components of photochemical smog is ozone. While ozone in the stratosphere protects earth from harmful UV radiation, ozone on the ground is hazardous to human health. Ground-level ozone is formed when vehicle emissions containing nitrogen oxides (primarily from vehicle exhaust) and volatile organic compounds (from paints, solvents, and fuel evaporation) interact in the presence of sunlight. Therefore, some of the sunniest cities are also some of the most polluted.
Smog, a hybrid of the words "smoke" and "fog," is a term for air pollution. It creates a hazy, fog-like atmosphere, typically around a major urban area that has many pollutants present. Some effects of smog are fairly immediate, and over time it can produce some nasty results.
 Bad Ozone
One component of smog is ozone, a gas that occurs naturally in upper elevations but which, in lower elevations, often has harmful effects. A number of polluters such as cars, power and chemical plants and petroleum refineries all contribute to form the ozone in smog when their pollutants react to sunlight.
 Health Impact
Everyone has a different level of susceptibility to the negative effects of smog, but many people begin to feel effects immediately after being outside in a smoggy area. The first health effects you may experience are throat irritation, coughing and possibly even chest pain, depending on your personal tolerance. These low-grade symptoms may disappear when you leave a smoggy area, but frequent exposure to smog can lead to long-term health problems.
Smog can have devastating effects for people with existing health problems. If you have a respiratory illness, such as Asthma or emphysema, you put yourself at greater risk of attacks every time you are exposed to smog. The ozone in smog does damage to the lining of the lungs, a superficial problem if exposure is limited to a few days. However, if exposure constinues over a prolonged time, your lungs might be unable to keep recovering in the same, normal way. Smog can cause onsets of colds or even pneumonia, and, in cases of extremely regular exposure, it has even been known to lead to cancer.
According to the American Lung Association, human lungs and heart can be permanently affected by air pollution and smog. While the young and the elderly are particularly susceptible to the effects of pollution, anyone with both short and long term exposure can suffer ill health effects. Problems include shortness of breath, coughing, wheezing, bronchitis, pneumonia, inflammation of pulmonary tissues, heart attacks, lung cancer, increased asthma-related symptoms, fatigue, heart palpitations, and even premature aging of the lungs and death.
Smog is not only dangerous to the health of humans but also to the general well-being of entire ecosystem. In addition to its harmful effects on humans, smog has far-reaching effects at the bottom of the food chain. The ozone that smog contains often prevents plants from growing to their potential, causing damage to crops. Some crops with a strong penchant for smog-induced infection include lettuce, tomato, soybean and peanut.
 Agriculture and Forests
Plants are perhaps more sensitive to air pollutants than humans. In particular, acid rain has left areas barren or with severely damaged vegetation . Yet, perhaps the greatest damage has been from ground-level ozone and PAN. Entering leaves of plants from the stomata during normal gas exchange, both ground-level ozone and PAN can cause discoloration, damage, and loss of leaves-reducing photosynthesis by as much as 50%. Plants also become more vulnerable to attacks by pests, disease, and other environmental disasters. Consequently, the plant's ability to store food, grow, and reproduce is hindered.
In numerical terms, ground-level ozone, alone, has been estimated to cause 10% to 40% growth loss, premature aging, and a decrease in pollen lifespan. Losses in crop yields were estimated to be 20% to 30% between 1989 and 1992 . In Ontario alone, smog was attributed to reduce crop yields equivalent to $70 million per year.
In a forestry aspect, smog incurs a cost on the existence value of trees and wild plants. In Los Angeles, smog was attributed to the deaths of 50% of trees in nearby areas. Similarly, ground-level ozone from the Central Valley and San Francisco-Oakland metropolitan areas was responsible for increasing stress and vulnerability on the ponderosa and Jeffrey pines in the Sierra Nevada. An attack by western pine beetles subsequently diminished the number of these trees.
As it perhaps can be predicted, the monetary costs of the loss of forests are difficult to measure and can have direct economic costs.
 Materials and Aesthetics
It is said that cleaning is just as destructive as it is costly. Perhaps this is even more so when considering the material and aesthetic aspects of smog. Besides the fact that most people derive a psychological benefit of seeing a clear sky and a clean surrounding, the costs of smog can be millions of dollars.
The most visible characteristic of cities smothered by smog is perhaps the black and soot-covered windows, walls, drapes and curtains, and other exposed surfaces. Yet, other damages can be seen. Sulfur dioxide corrodes metal and stone-damaging machinery and industrial instruments, as well as destroying buildings, statues, and monuments . Ground-level ozone, destroying synthetic materials, can cause leather to become brittle and rubber to lose its elasticity, resulting in cracks. Moreover, ground-level ozone has been found to damage cotton, acetate, nylon, polyester, and other textiles, while bleaching dyes, paints, and coatings.
While it is uncertain as to how much is exactly spent on the cleaning or replacement of materials, a couple of million dollars is considered to be a reasonable estimate. Canada, alone, estimates that the increase in ground-level ozone from the United States has cost it up to one billion dollars in material damages ("Smog"). Considering that cleaning and replacement costs do not include materials that are irreplaceable and the observation that people have actually spent more to move further away from cities, these costs of pollution most likely will be underestimates
 Ecological Systems
Sulfur dioxide and nitrogen oxides are largely responsible for the sources of acid precipitation. Because it results in acid rain with a pH of 5.5 or less, smog can have serious widespread ecological impacts on aquatic systems, forests, and on humans far away from its point of origin. As a basic biology course will explain, slight deviations from pH values in the environment can be critical to the proper functioning of enzymes, hormones, and other proteins. In aquatic systems with a normal pH of 6 to 8, a slight deviation in most cases will pose no threat, as organisms adapt . However, an organism's ability to successfully reproduce may be hindered, and in more extreme cases, a population of an organism may actually become extinct. In forests, acid precipitation not only damages trees and plants, but also affects soil contents, which can thwart growth towards acid-tolerant species. For humans, the effects of acid rain may vary from aesthetic values to the issue of clean water and air. In all of these cases, no exact monetary value can be assigned.
The fact that everyone and everything in the environment is interlinked in a chain demonstrates the difficulty in measuring an externality such as smog. Yet the simple recognition that such externalities exist can work wonders in policies attempting to ensure a more sustainable and healthier future.
 Minimize the Risk
To reduce your exposure to smog and its potential health effects:
- Check the Air Quality index in your community, especially during "smog season" from April to September. Tailor your activities accordingly;
- Avoid or reduce strenuous outdoor activities when smog levels are high, especially during the afternoon when ground-level Ozone reaches its peak. Choose indoor activities instead;
- Avoid or reduce exercising near areas of heavy traffic, especially during rush hour; and
- If you have a heart or lung condition, talk to your health care professional about additional ways to protect your health when smog levels are high.
- To help reduce the overall levels of smog in the air:
- When possible, use public transportation instead of your car. You could also walk or ride your bicycle, as long as smog levels are not too high;
- Look for alternatives to gas-powered machines and vehicles. Try a rowboat or sailboat instead of a motorboat or a push-type lawnmower instead of one that runs on gasoline;
- Consider fuel efficiency when you buy a vehicle. Keep all vehicles well maintained;
- Reduce energy use in your home. Learn more about alternative energy resources;
- Do not burn leaves, branches or other yard wastes;
- Consider joining a citizens' committee to advocate for cleaner air in your community; and
- Spend time talking with your children about the importance of a sustainable lifestyle.