Air Pollution: Slow Decline Or Stagnation?

Despite improvements in air quality over the past few decades, air pollution remains a pressing issue, with progress in reducing pollution levels exhibiting signs of stagnation. This is particularly evident in the United States, where the rate of improvement for key air pollutants such as nitrogen oxides and carbon monoxide has significantly slowed down in recent years. This trend raises concerns about the nation's ability to meet its goals for decreasing ozone pollution, which continues to pose a health hazard to millions of Americans. The COVID-19 lockdowns provided a unique opportunity to study the impact of reduced car traffic on air pollution, yet the declines observed during this period were modest, indicating that vehicles may not be the primary contributors to air pollution in certain regions. As researchers strive to understand the reasons behind the slowing progress in air pollution reduction, it is evident that continued efforts and innovative solutions are necessary to address this persistent global challenge.

Ozone pollution

Ozone (O3) is a gas molecule composed of three oxygen atoms. While ozone in the upper atmosphere shields us from the sun's ultraviolet radiation, ozone at ground level is a harmful air pollutant. Ground-level ozone is formed when oxides of nitrogen (NOx) and volatile organic compounds (VOC) react in the presence of sunlight. These precursor gases are emitted from sources such as car tailpipes, power plants, industrial boilers, refineries, and chemical plants. Ground-level ozone, often referred to as smog, poses serious health risks, especially on hot sunny days in urban areas. It can cause respiratory issues, particularly for vulnerable groups such as people with lung diseases or metabolic disorders. Studies have also linked ground-level ozone exposure to an increased risk of premature death.

In recent years, there has been a slowdown in the progress made towards reducing ground-level ozone pollution in the United States. This unexpected finding has raised concerns about the nation's ability to achieve its goals for decreasing ozone pollution. The slowdown in emission reductions has been significant, with nitrogen oxide concentrations dropping by only 1.7% yearly from 2011 to 2015, compared to a 7% yearly decrease from 2005 to 2009. This slowdown has been observed even during periods of reduced car traffic, such as lockdowns, suggesting that vehicles may not be the primary source of air pollution in certain areas.

The challenges in reducing ozone pollution have been attributed to various factors. First, pollution from burning coal can affect a larger area than vehicle emissions, and some regions may be downwind of coal-fired facilities, impacting their air quality. Second, ground-level ozone can be transported long distances by wind, affecting not just urban but also rural areas. Additionally, the formation of ground-level ozone is influenced by complex chemical reactions involving multiple pollutants, making it challenging to control and regulate.

To address the issue of ozone pollution, the Environmental Protection Agency (EPA) in the United States has designated areas as attainment or nonattainment based on their air quality standards. Nonattainment areas are required to develop state implementation plans (SIPs) outlining measures to improve air quality. These plans aim to reduce emissions of pollutants that contribute to ground-level ozone. The EPA also provides air quality forecasts and notifications to help individuals take precautionary measures to protect their health.

While the slowdown in emission reductions is concerning, ongoing research combines EPA inventories with advanced satellite instruments to better understand pollution changes and improve emission controls. By studying the complex interactions between various emission sources and atmospheric conditions, scientists and policymakers can work towards more effective strategies to combat ozone pollution and improve air quality.

Nitrogen oxide and carbon monoxide levels

Nitrogen oxides (NOx) and carbon monoxide are two major air pollutants that are emitted by cars, trucks, and other vehicles, as well as by industrial sources such as power plants. They are key ingredients in ground-level ozone and smog formation, which have detrimental effects on human health and the environment.

Nitrogen oxide, often referred to as nitrogen dioxide (NO2), is a reddish-brown gas that is soluble in water and a strong oxidant. It is produced through the high-temperature combustion of fuels in processes used for heating, transportation, industry, and power generation. Household sources of nitrogen oxides include equipment that burns fuels, such as furnaces, fireplaces, and gas stoves. Exposure to nitrogen dioxide can irritate airways, exacerbate respiratory diseases, and is closely linked to asthma and other respiratory conditions.

Carbon monoxide (CO) is a harmful pollutant that can provoke direct poisoning when inhaled. It diffuses across lung tissues and into the bloodstream, hindering the body's cells from binding to oxygen. This oxygen deprivation damages tissues and cells, leading to symptoms such as breathing difficulties, exhaustion, dizziness, and even death in cases of high exposure.

In recent years, the decline in nitrogen oxide and carbon monoxide levels has slowed down significantly. While nitrogen oxide concentrations dropped by 7% yearly from 2005 to 2009, they only declined by 1.7% per year from 2011 to 2015, indicating a 76% slowdown. Similarly, carbon monoxide levels have not decreased as quickly as anticipated, with satellite data showing a slower improvement than EPA ground-based inventories. This slowdown in emission reductions has contributed to stalled progress in reducing smog and improving air quality.

The impact of vehicle emissions

The environmental impact of automobiles is extensive, from tailpipe emissions to road infrastructure. The production of cars also leaves a large carbon footprint as materials like steel, rubber, glass, plastics, and paints must be created before a car is manufactured. Similarly, when a car reaches the end of its life, its impact on the environment does not end. Plastics, toxic battery acids, and other products may stay in the environment long after the car is discarded.

Vehicle emissions are not limited to tailpipe emissions but also include the emissions produced during the production of vehicles and their fuel. The burning of gasoline produces emissions, and there are additional emissions associated with the extraction, refining, distribution, and burning of the fuel. Electric vehicles (EVs), on the other hand, do not produce tailpipe emissions when running on electricity. However, the production of EVs and their batteries contributes to emissions, and the environmental impact of EVs depends on the energy sources used to charge their batteries.

The transportation sector has the highest contribution to global warming in the US, with vehicles emitting about 20% of the country's total GHG emissions. The impact of vehicle emissions is not limited to the US, as the US is the second-largest emitter of GHGs globally. The growth of the EV sector and other technological advancements offer opportunities to reduce vehicle emissions and mitigate their impact on the environment and human health.

Air pollution in the US

The "State of the Air" 2025 report by the American Lung Association revealed that 46% of Americans (approximately 156.1 million people) live in areas with failing grades for unhealthy levels of ozone or particle pollution. This issue disproportionately affects communities of color, who are more vulnerable to the health impacts of air pollution due to higher rates of chronic conditions such as asthma, diabetes, and heart disease.

Several factors contribute to the persistence of air pollution in the US. Despite reductions in vehicle emissions, other sources, such as power plants and industrial facilities, continue to emit significant amounts of pollutants. For example, the Houston metro area, home to a large concentration of petrochemical facilities, experienced only a slight decrease in air pollution during the lockdown, despite a 40% reduction in local traffic.

Additionally, extreme weather events, including heatwaves, droughts, and wildfires, are exacerbating air quality issues. The changing climate is making it more difficult to maintain progress in improving air quality, as seen in the western and eastern states affected by the 2023 heatwave and wildfires.

To address these challenges, further efforts are needed to reduce emissions and mitigate the impacts of climate change. The United States has taken steps towards this by signing the Kigali Agreement in 2022, committing to reducing the use of hydrofluorocarbons (HFCs), powerful greenhouse gases found in air conditioners and refrigerators.

Air pollution and health

Air pollution is a major threat to global health and prosperity. It is responsible for more than 6.5 million deaths each year globally, a number that has increased over the past two decades. Air pollution exposure is associated with oxidative stress and inflammation in human cells, which may lay a foundation for chronic diseases and cancer. The International Agency for Research on Cancer of the World Health Organization (WHO) has classified air pollution as a human carcinogen.

Short-term exposure to higher levels of outdoor air pollution is associated with reduced lung function, asthma, cardiac problems, and emergency department visits. Long-term exposure to air pollution increases a person's risk for diseases with a longer onset, such as stroke, heart disease, chronic obstructive pulmonary disease, and cancer.

The health burden of air pollution is not evenly shared. Some people are more at risk of illness and death from air pollution than others. Several factors affect an individual's level of risk, including exposure and susceptibility. People who are pregnant, children, older adults, and those living with chronic conditions are more susceptible to the health impacts of air pollution.

Particulate matter (PM) is a major source of health risks from air pollution. PM refers to a mixture of tiny bits of solids and liquids in the air we breathe. The smaller the particles, the more dangerous they are as they can penetrate deep into the lungs and even enter the bloodstream, travelling to other organs and causing systemic damage. Sources of particle pollution include motor vehicles, factories, power plants, equipment, wood burning, and wildfires.

Ozone is another significant air pollutant that affects human health. Ground-level ozone, or smog, is created when pollutants emitted by cars, power plants, industrial boilers, refineries, and other sources chemically react in the presence of sunlight. Ozone is a powerful lung irritant and can cause inflammation and other damage to the respiratory system, even within hours of exposure.

While air quality in the US has improved over the past few decades, recent research shows that improvements for key air pollutants have slowed down significantly in recent years. This slowdown indicates that it may be more challenging than previously thought for the nation to achieve its goals of reducing ozone pollution.

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