Smog, a term coined from “smoke” and “fog,” represents a severe form of air pollution that significantly reduces visibility and poses widespread health risks. While historical smog episodes were linked to specific industrial pollution events, modern smog is a complex atmospheric phenomenon driven primarily by emissions from vehicles and energy production. This pervasive haze is a major global environmental challenge, particularly in densely populated urban centers. Addressing this pollution is a public health concern due to its systemic impact on human physiology.
Primary Components and Types of Smog
The term “smog” encompasses two distinct categories: industrial smog and photochemical smog. Industrial smog, also called London-type or sulfurous smog, historically dominated urban areas where high-sulfur coal and oil were burned for heating and power generation. This pollution is primarily composed of sulfur dioxide (\(\text{SO}_2\)), sulfuric acid droplets, and particulate matter like soot.
Photochemical smog is the dominant form of air pollution in modern, sunny, and heavily motorized cities. It is characterized by a brownish-yellow haze and is highly oxidizing. This smog forms from the reaction of precursor pollutants, namely nitrogen oxides (\(\text{NO}_x\)) and volatile organic compounds (VOCs), which are emitted from vehicle exhaust and industrial processes. These are primary pollutants because they are emitted directly from a source.
The subsequent chemical reactions produce secondary pollutants that form in the atmosphere. The most abundant secondary pollutant in photochemical smog is ground-level ozone (\(\text{O}_3\)), along with compounds like peroxyacyl nitrates (PANs). Particulate matter (PM), specifically \(\text{PM}_{2.5}\) and \(\text{PM}_{10}\), is also a significant component, originating from both direct emissions and atmospheric transformation.
Atmospheric Reactions Driving Smog Formation
The formation of photochemical smog depends entirely on solar energy, which acts as the catalyst for the chemical reactions. The process begins when nitrogen dioxide (\(\text{NO}_2\)) absorbs ultraviolet (UV) radiation from sunlight. This absorption causes the \(\text{NO}_2\) molecule to break apart, yielding nitric oxide (NO) and a free oxygen atom (O).
The free oxygen atom quickly combines with molecular oxygen (\(\text{O}_2\)) to create ground-level ozone (\(\text{O}_3\)). Normally, this ozone would react with nitric oxide (NO) to reform nitrogen dioxide (\(\text{NO}_2\)), establishing a balance. However, the presence of VOCs disrupts this balance by reacting with the nitric oxide, preventing it from consuming the ozone.
This chemical interference allows ozone concentration to build up to unhealthy levels near the ground. Warm temperatures and stagnant air masses, often stabilized by a temperature inversion, accelerate these reactions and trap pollutants close to the surface. The resulting mixture includes ozone, various aldehydes, and peroxyacyl nitrates (PANs), which contribute to the visible brown haze.
Physiological Effects on Human Health
Exposure to smog introduces a complex cocktail of irritants and toxic compounds that affect nearly every organ system. The respiratory system is the main entry point, where pollutants cause inflammation and oxidative stress. Short-term exposure to ground-level ozone, a potent oxidant, can cause chest pain, coughing, and throat irritation, and aggravate pre-existing conditions like asthma and bronchitis.
Particulate matter, particularly fine \(\text{PM}_{2.5}\) particles, is dangerous because its small size allows it to penetrate deep into the lung’s gas exchange regions. Once in the lungs, \(\text{PM}_{2.5}\) can be absorbed into the bloodstream, causing systemic inflammatory responses. Long-term exposure to these particles is associated with chronic effects, including reduced lung function, chronic obstructive pulmonary disease (COPD), and an increased risk of lung cancer.
The cardiovascular system is also affected, as smog exposure increases the risk of heart attacks, strokes, and blood pressure changes. The systemic inflammation and oxidative stress triggered by circulating pollutants impair the function of the endothelium, the inner lining of blood vessels. This dysfunction contributes to the hardening of arteries (atherosclerosis) and greater susceptibility to acute cardiovascular events.
Certain populations face a heightened risk from smog-related illness. Children, whose lungs are still developing and who breathe more air per body weight, are more susceptible to long-term damage. The elderly and individuals with pre-existing conditions like diabetes or cardiopulmonary disease are also at increased risk for heart attacks and respiratory mortality following exposure.
Air Quality Index and Personal Safety Measures
The Air Quality Index (AQI) is the primary tool used by environmental agencies to communicate daily air pollution levels and associated health risks. This color-coded scale converts concentration measurements of major pollutants, including ozone and particulate matter, into a single number between 0 and 500. The six categories range from Green (Good, 0-50) to Maroon (Hazardous, 301+), each providing specific health advisories.
When the AQI reaches the Orange level (101-150), air quality is unhealthy for sensitive groups, such as children and people with respiratory illnesses. Once the index hits Red (151-200), the air is unhealthy for all populations, and everyone is advised to limit prolonged outdoor activity. In the most severe categories, Purple (Very Unhealthy) and Maroon, all individuals are advised to remain indoors and minimize physical exertion.
Individuals can take several steps to mitigate exposure during periods of elevated AQI. The most effective measure is to check the daily AQI forecast and reduce time spent outdoors, especially during peak pollution hours in the afternoon. If remaining indoors, using an air purifier equipped with a High-Efficiency Particulate Air (HEPA) filter can help reduce indoor concentrations of \(\text{PM}_{2.5}\). While cloth or surgical masks provide minimal protection, wearing an N95 or P100 respirator offers better defense against inhaling fine particulate matter when outdoor activity cannot be avoided.