Smog, a term coined from combining “smoke” and “fog,” represents a complex mixture of airborne pollutants that reduces visibility and poses a significant health risk. The distinct odor associated with smog is not a singular scent but a constantly shifting cocktail of chemicals released into the atmosphere. Because smog forms from various sources—ranging from industrial emissions to vehicle exhaust—its specific smell depends entirely on the dominant mix of pollutants present. The sensory experience is a direct reflection of the underlying chemical composition.
Describing the Complex Scent of Smog
The immediate sensory experience of smog is often physical irritation. Many people report a sharp, acrid, and metallic sensation that scratches the throat and stings the eyes. This is due to highly reactive compounds stimulating the common chemical sense, which is distinct from the sense of smell. The irritation triggers discomfort, sometimes causing coughing or wheezing as the body reacts to the airborne irritants.
The general smell is frequently described as something burnt or stale, similar to the lingering scent of spent fireworks or an engine running in an enclosed space. In certain areas, a faint, almost chlorine-like or bleach-like smell can be detected, which signals the presence of powerful oxidizing agents. This complex scent rarely smells like simple smoke; instead, it carries a synthetic and processed quality that indicates chemical reactions are taking place in the air. The odor is not uniform and changes in intensity and character depending on the pollutant concentration and local weather conditions.
The Chemical Components Driving Smog Odors
The sensory characteristics of smog are directly attributable to a few specific gaseous pollutants that are highly detectable by the human nose. One of the most common odor contributors in urban smog is ground-level ozone (\(\text{O}_3\)), a secondary pollutant formed when nitrogen oxides and volatile organic compounds react in sunlight. Ozone is a potent oxidizing agent that is often described as having a sharp, metallic, or electrical odor, similar to the air after a lightning strike. Even at low concentrations, this gas is highly irritating and can cause respiratory discomfort.
Another major source of odor, particularly in areas with heavy industrial activity or coal burning, is sulfur dioxide (\(\text{SO}_2\)). This colorless gas has a distinctly pungent, choking odor, which is sometimes compared to the smell of a recently struck match. When \(\text{SO}_2\) is released from combustion sources, it can react with moisture in the air to form sulfuric acid aerosols, contributing to the choking sensation and the heavy odor.
Volatile organic compounds (VOCs) also contribute significantly to the smell profile. These compounds are emitted from sources like vehicle exhaust, solvents, and gasoline vapors. VOCs often carry a hydrocarbon or fuel-like scent, and their partial combustion products contribute the “burnt” or “chemical” note defining the smog experience. When these components mix with fine particulate matter, the resulting odor is a pervasive, heavy, and complex chemical miasma.
How Smog Type Changes the Smell
The specific blend of pollutants determines which of the chemical odors is dominant, creating two broadly recognized types of smog with distinct sensory profiles. Photochemical smog, commonly known as Los Angeles-type smog, forms primarily in warm, sunny climates and is dominated by ozone and nitrogen oxides. The resulting odor profile is highly sharp and acidic, characterized by the metallic tang of ozone and the irritating bite of oxidized compounds.
Industrial smog, sometimes called London-type smog, is historically linked to the burning of high-sulfur coal and is dominated by sulfur dioxide and soot. This creates a much heavier, more visibly dense, and often grayish air pollution event. The smell of industrial smog is overwhelmingly sulfurous and smoky, described as a dense, choking odor that hangs low to the ground. The dominance of sulfur compounds and particulate ash gives this air a suffocating quality, contrasting with the high-pitched sharpness of its photochemical counterpart.