ESS 8.3.1 Urban Air Pollution

Learning Objectives

Outline the meaning of “air pollution”
Outline two types of pollutants: primary and secondary
Describe the sources of air pollution
- wildfires
- chemical industries
- construction and demolition
- indoor air pollution
- PM

Facts About Air Pollution

Air pollution refers to the contamination of indoor or outdoor environments by chemical, physical, or biological agents that alter the natural composition of the atmosphere. A major component of air pollution is particulate matter (PM), consisting of extremely small solid or liquid particles suspended in the air. Pollutants originate from both natural and anthropogenic sources.

Health and Global Impacts of Air Pollution

Air pollution—comprising outdoor and indoor particulate matter and ozone—is a major risk factor for several leading causes of mortality, including heart disease, stroke, lower respiratory infections, lung cancer, diabetes, and chronic obstructive pulmonary disease (COPD). In 2019, air pollution accounted for approximately 11.65% of global deaths.

Pollution burdens are disproportionately high in low-income countries (LICs), primarily due to indoor pollution arising from the use of solid cooking fuels, and in middle-income countries (MICs) undergoing rapid industrialization, where outdoor pollution dominates. According to the World Health Organization (WHO), 2.5 billion people are exposed to air pollution levels seven times higher than recommended guidelines, and in LICs, over 90% of urban air pollution originates from poorly maintained, aging motor vehicles.

Types of Air Pollutants Based on Sources

Air pollutants are often classified as primary or secondary:

Primary pollutants

are emitted directly from a process. The most significant anthropogenic primary pollutants arise from the combustion of fossil fuels. These include:

carbon monoxide (from incomplete combustion)
carbon dioxide
unburned hydrocarbons
nitrogen oxides (NOₓ), particularly nitrogen dioxide
sulfur dioxide (from sulfur-rich coal)
particulate matter (PM)
mercury (from coal-fired power plants)
sulfur compounds (from high-sulfur coal)

Secondary pollutants

form when primary pollutants undergo chemical reactions in the atmosphere, sometimes facilitated by sunlight. Key secondary pollutants include:

tropospheric ozone (formed from hydrocarbons and NOₓ in sunlight)
particulate matter produced from gaseous precursors
peroxyacetyl nitrate (PAN), a stable ozone oxidant
nitrogen dioxide (formed from oxidation of nitric oxide)
sulfuric and nitric acids (formed when sulfur dioxide and nitrogen oxides react with atmospheric water), which contribute to acid deposition.

Other Sources of Air Pollution

Natural sources include volcanic eruptions, dust storms, wildfires, pollen, and mould spores. Anthropogenic sources encompass emissions from motor vehicles, industrial processes, biomass burning, and indoor cleaning products. Air pollution arises from chemicals, particulates, and biological materials emitted through a range of processes.

Wildfires

Wildfires—unplanned fires occurring in vegetated areas—are responsible for substantial atmospheric pollution. Approximately 90% are triggered by human activities such as agricultural burning, discarded cigarettes, unattended campfires, and arson, though lightning remains a natural cause. Wildfires emit soot, dust (PM), and smoke laden with toxic chemicals, which may persist for days. They also contribute to thermal pollution, raising local temperatures and increasing fire risk.

Chemical Industries

Industrial activities, including chemical and textile manufacturing, release various organic and inorganic compounds. Microbial decomposition of these substances by fungi and bacteria can generate methane, a harmful greenhouse gas.

Constructions and demolitions

The expansion of urban areas has increased pollution from construction and demolition, which release particulate matter and emissions from heavy fossil-fuel-powered machinery. These activities also contribute to noise and visual pollution.

Indoor Air Pollution

Indoor pollution stems from several sources, including:

inefficient combustion of fuels in poorly ventilated homes, producing smoke with PM concentrations up to 100 times higher than WHO safety limits
old or poorly maintained wood-burning stoves and open fires
tobacco smoke
volatile organic compounds (VOCs) released from soft furnishings, synthetic carpets, paints, adhesives, and certain cleaning products

Key WHO statistics include:

One-third of the global population relies on unsafe cooking fuels or technologies.
In 2020, household air pollution caused approximately 3.2 million deaths, including 650 daily deaths among children under five.
Combined ambient and household air pollution results in 6.7 million premature deaths annually.
Exposure to indoor pollutants increases the risk of stroke, cardiovascular disease, pulmonary illnesses, and lung cancer.
Women and children remain the most vulnerable groups.

Particulate Matter (PM)

Particulate matter has become an increasingly critical component of urban air pollution. PM consists of microscopic liquid droplets or solid particles that can be inhaled, with concentrations typically highest in urban and industrial settings.

PM₁₀ refers to particles ≤10 μm in diameter; these can damage lung tissue or enter the bloodstream.
PM₂.₅, with diameters ≤2.5 μm, poses greater health risks due to deeper penetration into the respiratory system and bloodstream.

The severity of health impacts depends on the particles’ chemical composition, physical properties, concentration, size, and duration of exposure. Adverse effects include:

absorption of toxic substances such as cadmium (from cigarette smoke) or lead (from fossil-fuel combustion), which can cause neurological or kidney damage
allergic responses, including sneezing and eye irritation
infections caused by biological PM such as bacteria and fungi
asbestosis from inhalation of asbestos fibres released during demolition of older buildings
irritation of mucous membranes
increased respiratory stress and exacerbation of asthma