Air Pollution: Understanding Its Many Forms

Air pollution is a dynamic phenomenon that encompasses a wide range of pollutants, each posing varying degrees of harm to human health and the environment. These pollutants can be classified into primary and secondary categories, with primary pollutants being directly emitted from sources such as vehicle emissions, industrial processes, and natural occurrences. Secondary pollutants, on the other hand, are formed through chemical reactions between primary pollutants and other atmospheric compounds. The most common air pollutants include particulate matter (PM), which consists of solid and liquid particles suspended in the air, with PM2.5 being the most harmful due to its ability to penetrate deep into the lungs. Ground-level ozone (O3), a secondary pollutant, poses significant health risks and is a major contributor to smog in cities like Los Angeles. Other prominent pollutants include carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2), lead, and hazardous air pollutants (HAPs) such as benzene and asbestos. Understanding the diverse nature of these pollutants is crucial for developing effective strategies to mitigate their impact on human health and the environment.

Particulate matter (PM)

PM10 refers to particles with a diameter of 10 micrometres or less, which are inhalable and can enter the lungs, potentially causing adverse health effects. These particles are often referred to as inhalable coarse particles. PM10 has been linked to the worsening of respiratory diseases, including asthma and chronic obstructive pulmonary disease (COPD), leading to hospitalizations and emergency department visits.

PM2.5, on the other hand, refers to fine particles with diameters of 2.5 micrometres or less. These particles are the main cause of reduced visibility (haze) in many parts of the world. They pose a significant risk to human health and are associated with the greatest proportion of adverse health effects related to air pollution. Short-term exposures to PM2.5 can lead to respiratory issues, while long-term exposure has been linked to premature death, particularly in individuals with chronic heart or lung diseases. According to the 2019 World Air Quality Report, Los Angeles county is among the most polluted areas in the United States when it comes to PM2.5 levels.

The sources of PM10 and PM2.5 particles differ, and they also have distinct chemical compositions. PM2.5 particles are emitted directly from sources such as construction sites, unpaved roads, fields, smokestacks, and fires. Most particles, however, form in the atmosphere as a result of complex reactions involving pollutants such as sulfur dioxide and nitrogen oxides. Particulate matter can also originate from both outdoor and indoor sources. Indoor activities such as smoking tobacco, cooking, and burning candles or incense can generate particulate matter. Additionally, particles can form indoors from gaseous pollutants emitted by household cleaning products and air fresheners.

To protect public health, the Environmental Protection Agency (EPA) has established regulations and standards for particulate matter pollution. The Air Quality Index (AQI) provides daily information on outdoor air quality and associated health risks. This helps individuals take necessary actions to safeguard their health during periods of harmful air pollution levels.

Ground-level ozone

To address ground-level ozone pollution, the US Environmental Protection Agency (EPA) works with states and local governments to reduce emissions of pollutants that contribute to its formation. EPA regulations help states manage ozone levels in outdoor air, and states are required to draft implementation plans to improve air quality in areas that do not meet national standards.

Ozone levels have declined in some regions due to improvements in vehicle emissions, cleaner fuels, and emission-reducing requirements. However, ground-level ozone continues to be a concern in highly industrialised and urban areas, such as Los Angeles, where it often exists at high concentrations due to high temperatures and basin-like landscapes that trap the ozone and prevent its dispersal by wind.

It is important to distinguish ground-level ozone from stratospheric ozone, which occurs naturally in the upper atmosphere and forms a protective layer that shields the Earth from harmful ultraviolet radiation. Stratospheric ozone is considered "good" ozone, while ground-level ozone is a significant health and environmental concern.

Carbon monoxide

CO is a harmful air pollutant that can have serious health effects when inhaled in large amounts. It reduces the amount of oxygen that can be transported in the bloodstream to critical organs like the heart and brain. Exposure to high levels of CO can cause dizziness, confusion, unconsciousness, and even death. People with heart disease are particularly vulnerable to the effects of CO, even at lower levels, and may experience chest pain, reduced exercise capacity, and other cardiovascular issues with repeated exposure.

Indoor sources of CO include gas stoves, malfunctioning or improperly vented gas appliances (such as water heaters, furnaces, and clothes dryers), space heaters, fireplaces, tobacco smoke, and car emissions that enter the home. The highest levels of indoor CO typically occur during colder months when inversion conditions trap air pollution near the ground.

CO also contributes to climate change. It participates in chemical reactions in the atmosphere that produce ozone, a climate change gas. While CO has a weak direct effect on the climate, its indirect contribution through ozone formation has led to its classification as a short-lived climate forcing agent. As a result, reducing CO emissions is considered a potential strategy to mitigate the effects of global warming.

To protect public health and safety, the U.S. EPA sets and reviews standards for CO in outdoor air under the Clean Air Act. These standards help state, tribal, and local agencies ensure that CO levels are maintained within safe limits.

Nitrogen dioxide

The TEMPO (Tropospheric Emissions: Monitoring of Pollution) instrument has been used to measure nitrogen dioxide air pollution over North America. TEMPO scans the continent and measures sunlight reflected and scattered off the Earth's surface, clouds, and the atmosphere. Gases in the atmosphere absorb the sunlight, and the resulting spectra are used to determine the amounts of several gases, including nitrogen dioxide. TEMPO has shown that in polluted regions, most of the nitrogen dioxide is located near the ground. Visualizations from TEMPO have also revealed high levels of nitrogen dioxide over multiple urban areas across the US, Canada, Mexico, and the Caribbean. The measurements also show that pollution levels tend to rise during rush hour and dissipate throughout the day.

Radon

Photochemical smog, which is formed from interactions between particulates, nitrogen oxides, ozone, and other air pollutants, is also a significant issue in many cities. Smog can irritate the eyes and respiratory system and is particularly problematic in cities with a lot of industry and traffic, especially those situated in basins, as the landscape can trap the smog and prevent its dispersal by wind.

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