Air pollution is defined simply as the presence of substances in the atmosphere at concentrations that are detrimental to the health of living organisms or the environment. These airborne substances, which can be gases or tiny solid and liquid particles, are often the result of human activity, such as burning fossil fuels. Because the air we breathe has a direct and immediate impact on public health, governments worldwide monitor and regulate a specific group of these common pollutants intensely. This focused regulatory approach helps to track air quality trends and enforce limits designed to safeguard populations.
Why These Six Pollutants Are Prioritized
This specific group of six substances is designated as “Criteria Pollutants” because they are common across the country and pose significant threats to public welfare and human health. Regulators establish air quality standards for these six based on scientific criteria that detail the pollutants’ effects. These standards are divided into two main types: primary standards focus on protecting human health, especially sensitive populations. Secondary standards are established to protect public welfare, including preventing damage to crops, vegetation, and buildings, as well as addressing visibility impairment. The regulatory focus on these six pollutants provides a measurable framework for reducing the most common forms of air contamination.
Defining the Characteristics and Sources
Ground-level Ozone (\(\text{O}_3\))
Ground-level ozone is chemically distinct from the protective ozone layer high in the stratosphere. This pollutant is not emitted directly from any source but is formed through a complex chemical reaction in the atmosphere. Sunlight acts on precursor chemicals, primarily nitrogen oxides (\(\text{NO}_x\)) and volatile organic compounds (VOCs). Ozone pollution is typically worse during warmer summer months in urban and downwind areas.
Particulate Matter (\(\text{PM}_{2.5}\) and \(\text{PM}_{10}\))
Particulate matter refers to a mixture of solid particles and liquid droplets suspended in the air. It is categorized by size, with \(\text{PM}_{10}\) being inhalable particles 10 micrometers and smaller, and \(\text{PM}_{2.5}\) being fine particles 2.5 micrometers and smaller. Sources are diverse, ranging from construction sites and dust from unpaved roads to combustion sources like power plants, industrial furnaces, and vehicle exhaust. The smaller \(\text{PM}_{2.5}\) particles are especially concerning because their microscopic size allows them to penetrate deeply into the lungs.
Carbon Monoxide (\(\text{CO}\))
Carbon monoxide is a colorless, odorless, and highly toxic gas that results from the incomplete combustion of carbon-containing fuels. The majority of \(\text{CO}\) emissions come from mobile sources, particularly cars, trucks, and other vehicles. Other significant sources include non-road engines, industrial processes, and poorly vented furnaces or stoves. \(\text{CO}\) can build up to dangerous levels, especially in areas with heavy traffic congestion.
Nitrogen Dioxide (\(\text{NO}_2\))
Nitrogen dioxide is one of a group of highly reactive gases known as nitrogen oxides (\(\text{NO}_x\)). \(\text{NO}_2\) forms during high-temperature combustion processes, mainly in vehicle engines and electric power generation facilities. This gas is a reddish-brown color, which contributes to the visible haze over urban centers. \(\text{NO}_2\) is a major precursor chemical for the formation of both ground-level ozone and acid rain.
Sulfur Dioxide (\(\text{SO}_2\))
Sulfur dioxide is a highly reactive gas primarily emitted when sulfur-containing fossil fuels are burned. The largest industrial source is the burning of coal and oil in power plants and industrial boilers. Processing of sulfur-containing ores and the operation of diesel engines are also contributors. \(\text{SO}_2\) plays a significant role in the formation of acid deposition, which can affect terrestrial and aquatic ecosystems.
Lead (\(\text{Pb}\))
Lead is a heavy metal that can exist as particles or compounds in the air. While its presence in the atmosphere has decreased dramatically since the phase-out of leaded gasoline, current sources still include ore and metals processing, waste incinerators, and the manufacturing of lead-acid batteries. Once emitted, lead particles can settle out of the air onto soil and water, where they persist. Its accumulation in the body poses a long-term health risk.
Specific Health and Environmental Consequences
Exposure to these six pollutants leads to a wide range of adverse health outcomes, particularly impacting the respiratory and cardiovascular systems. Ground-level ozone and fine particulate matter are strongly linked to respiratory distress, including the aggravation of asthma, bronchitis, and reduced lung function. Particulate matter is small enough to enter the bloodstream, triggering nonfatal heart attacks and irregular heart rhythms.
Sulfur dioxide and nitrogen dioxide can cause irritation of the airways, leading to wheezing and shortness of breath. The compounds formed by \(\text{SO}_2\) and \(\text{NO}_x\) are the primary drivers of acid deposition, which damages forests, acidifies lakes and streams, and harms aquatic life.
Carbon monoxide poisoning occurs because the gas binds to hemoglobin, displacing oxygen, which can cause vision problems, reduced mental alertness, and death at high concentrations. Lead exposure is associated with neurological damage, particularly in children, resulting in developmental effects and lowered intelligence quotient.