Smog, a term coined from “smoke” and “fog,” describes a visible, hazy mixture of air pollutants hanging over urban and industrial areas. This complex cocktail of gases and microscopic particles is generated primarily by human activities, such as vehicle emissions and industrial processes. Smog occurs in two major forms, photochemical and industrial, each having a distinct chemical signature and source. This persistent air pollution poses a systemic threat, affecting the health of living organisms and destabilizing delicate ecosystems worldwide.
The Chemical Composition of Smog
Smog contains toxic compounds produced through atmospheric reactions. Photochemical smog, common in sunny, dry climates, is a secondary pollutant formed when primary emissions react in sunlight. The key ingredients are nitrogen oxides (\(\text{NOx}\)) and volatile organic compounds (\(\text{VOCs}\)), which create ground-level ozone (\(\text{O}_3\)) and peroxyacetyl nitrates (\(\text{PAN}\)).
Ground-level ozone is a highly reactive molecule and the most abundant toxic component of photochemical smog, peaking during warm, sunny periods. Industrial smog is characterized by high concentrations of sulfur dioxide (\(\text{SO}_2\)) and particulate matter (PM), often resulting from burning high-sulfur coal and oil. This type of smog, historically prevalent in industrialized cities, is a mix of soot and sulfur compounds combined with atmospheric moisture.
Particulate matter is a universal component in both smog types, categorized by size. \(\text{PM10}\) refers to particles less than 10 micrometers in diameter, and \(\text{PM2.5}\) refers to fine particles less than 2.5 micrometers. The \(\text{PM2.5}\) fraction is particularly concerning because its small size allows it to bypass the body’s natural defenses, penetrating deep into the lungs. \(\text{NOx}\) and \(\text{SO}_2\) are also present as primary pollutants from combustion sources, contributing to the formation of both ozone and fine particulate matter.
Physiological Effects on Human Health
Smog immediately affects the respiratory system. Ground-level ozone, a strong oxidant, irritates and inflames the airways upon inhalation. This oxidative stress damages the lining of the lungs, similar to a sunburn, which can lead to coughing, throat irritation, and difficulty breathing.
Exposure to ozone and particulate matter exacerbates pre-existing respiratory conditions like asthma and chronic obstructive pulmonary disease (COPD). Chronic exposure can permanently reduce lung function and increase the risk of developing bronchitis. The tiny \(\text{PM2.5}\) particles travel past the bronchi and bronchioles to settle in the deepest parts of the lungs, the alveoli.
Once lodged in the lungs, ultrafine particulate matter can cross the alveolar membrane and enter the circulatory system. This entry triggers a systemic inflammatory response that negatively impacts the cardiovascular system. The inflammation contributes to the hardening and narrowing of arteries (atherosclerosis) and increases blood pressure.
Smog exposure is directly linked to an elevated risk of acute cardiovascular events, including heart attacks and strokes. Pollutants can destabilize plaques in the coronary arteries, causing blockages that lead to ischemic heart disease. Long-term exposure to \(\text{PM2.5}\) is also associated with increased risk of heart failure and other cardiovascular diseases.
Vulnerable Populations
Certain populations face a disproportionately higher risk from smog exposure. Children are susceptible because their lungs are still developing, and they breathe more air relative to their body weight. The elderly and individuals with chronic diseases like diabetes or heart disease are also highly vulnerable, as systemic inflammation places additional strain on compromised organs. Air pollution has also been linked to broader systemic effects, including cognitive impairment, neurological diseases like dementia, and metabolic disorders.
Ecological Damage to the Environment
Smog inflicts widespread damage on natural systems. Ground-level ozone is highly phytotoxic (poisonous to plants). This gaseous pollutant enters plant leaves through the stomata, interfering with the cellular processes required for photosynthesis.
The disruption of photosynthesis leads to visible injuries, such as stippling or chlorosis (yellowing of green tissue). This damage results in stunted growth, reduced biomass, and diminished reproductive capacity. In agriculture, this translates into significant reductions in crop yields for sensitive staples:
- Soybeans
- Wheat
- Cotton
The presence of nitrogen oxides (\(\text{NOx}\)) and sulfur dioxide (\(\text{SO}_2\)) contributes to acid deposition. When these gases react with water vapor, they form sulfuric and nitric acids that fall to earth as rain, snow, or dry particles. Acid deposition changes the chemical composition of soil by leaching essential nutrients (like calcium and magnesium) and mobilizing toxic metals (such as aluminum), which harms tree roots and soil microorganisms.
In aquatic environments, acid rain causes the acidification of lakes and streams, devastating fish, amphibians, and invertebrates. This chemical alteration disrupts the food web and reduces biodiversity, as many aquatic organisms cannot tolerate lower pH levels. Smog also causes indirect harm to wildlife through habitat degradation, such as the loss of sensitive plant species used for food or shelter.
Smog also affects physical systems:
- It reduces atmospheric visibility, creating the characteristic brown haze that obscures scenic vistas and affects tourism.
- It diminishes solar radiation reaching the ground, reducing the efficiency of solar energy generation.
- The acidic and oxidative nature accelerates the corrosion and degradation of physical materials, including buildings and urban infrastructure.