2.3 Air pollution trends and control

Trends in levels of traditional air pollutants

US trends

The graphs below are from the EPA 2024 air quality report^[1] and show trends in the average US criteria air pollutant levels relative to the national standards and in terms of mass; of these pollutants, only PM2.5, fine particulates, shows signs of increasing, and all are below the current national standard (Figs 1, 2). These are national averages; several areas of the US violate standards for one or more criteria pollutants.^[2]

Nationally, concentrations of air pollutants have dropped significantly since 1990:

• Carbon monoxide (CO) 8-hour,  down 79%
• Lead (Pb) 3-month average,  down 87% (from 2010)
• Nitrogen dioxide (NO₂) annual, down 62%
• Nitrogen dioxide (NO₂) 1-hour,  down 55%
• Ozone (O₃) 8-hour,  down 18%
• Particulate matter 10 microns (PM₁₀) 24-hour,  down 29%
• Particulate matter 2.5 microns (PM_2.5) annual,  down 37% (from 2000)
• Particulate matter 2.5 microns (PM_2.5) 24-hour,  down 29% (from 2000)
• Sulfur dioxide (SO₂) 1-Hour,  down 92%
• Numerous hazardous air pollutants, or air toxics, have declined with percentages varying by pollutant

Despite increases in air concentrations of pollutants associated with fires (carbon monoxide, particle pollution, and ozone), national average air quality concentrations remain below the current, national standards.

Emissions of most key air pollutants continue to decline from 1990 levels:

• Carbon monoxide (CO),  down 71%
• Ammonia (NH₃),  up 15%
• Nitrogen oxides (NO_x),  down 73%
• Direct particulate matter 2.5 microns (PM_2.5),  down 28%
• Direct particulate matter 10 microns (PM₁₀),  down 27%
• Sulfur dioxide (SO₂),  down 93%
• Volatile organic compounds (VOC),  down 46%

In addition, from 1990 to 2017 emissions of air toxics declined by 74 percent, largely driven by federal and state implementation of stationary and mobile source regulations, and technological advancements from American innovators.

Global trends

Worldwide, mortality associated with air pollution is declining (Fig 3).  But international progress in reducing air pollution varies considerably by region.

At present, the most polluted cities (Fig 4) and countries are concentrated in Southeast Asia and Africa, but instances of poor air quality can be found much more widely. Within regions, air temperature, weather, agricultural activity, home heating, and local phenomena such as wildfires and dust storms can cause seasonal changes in air pollution, as well. Temperature inversions, which occur when warm air at low elevation becomes trapped by a higher cold-air layer that prevents the warm air from rising, can trap air in valleys, or in cold, surface air layers in winter, concentrating local pollutants for several days at a time and significantly increasing pollution concentrations and resulting health impacts.

International air-quality rankings such as those shown in Figure 4 may rely solely on measurements of PM2.5, which are easy and inexpensive to obtain. Nations have not agreed upon an international air-quality index, but international reporting of useful component measurements is becoming more common. Although measurements are not universally available for all the major air pollutants, observations such as the improvements in the ozone hole, and reduction in acid rain provide evidence of the global trends in some air pollutants.

Control techniques for traditional air pollution

Air-pollution control is achieved by modifying processes that produce air pollutants and by cleaning dirty air. Reductions in use of fossil fuels and emission controls have reduced CO₂, CO, NO_x, SO₂, and VOCs, which has led to reductions in ground-level ozone, and particulates. Changes to refrigerants, construction materials and home furnishings have reduced levels of chemicals that deplete stratospheric ozone. Restoration to repair degraded land can reduce dust (particulates). At mining sites, roads may be wetted daily or more often to reduce dust, where control is required. Cover crops – plants raised during the non-growing season for crops, to protect soil – can reduce dust from fallow agricultural lands.

Air-pollution control technologies to reduce particulates in industry and transportation use static electricity, strong air currents, protective bags, and filters to capture the particles where they are produced. Scrubbers use liquid or treated surfaces (activated charcoal is an example) to wash out or trap polluting chemicals from flue-gas stacks (“smoke stacks”). Air-polluting substances can be oxidized – burned – using catalysts (substances that help to speed up chemical reactions), converting them to CO₂.

Effectiveness of air pollution control has varied among air pollutants, even in developed countries with strong laws to protect air quality. The differences between reductions in SO₂ and reductions in NOx demonstrate some of the issues involved. Both pollutants result primarily from burning of fossil fuels, but SO₂ originates more from stationary industrial sources and less from mobile vehicular sources than NOx (3.1, Fig 2). Scrubbers and similar technology used to remove pollutants from flue-gas stacks work more effectively on SO₂ than on NOx. Increases in numbers of vehicles on the road and in number of miles driven further increase NOx emissions in many countries. Air pollution control of both these emissions has resulted in substantial declines in acid rain – one of the primary effects of both pollutants – but whereas the EPA reported a 92% drop in SO₂ between 1990 and 2023, NOx levels in the same period declined by 55% (Fig 1, 1-hour standards).

Air pollution policies and standards

US air pollution regulation

In the US, criteria air pollutants are regulated by the Clean Air Act (CAA), as are toxic air pollutants and pollutants that deplete stratospheric ozone. The Environmental Protection Agency is tasked with implementing the Clean Air Act, which includes permitting of major sources and some area sources of air pollution, requirements for air-pollution control technology used to reduce air pollution at major sources, and limits on the levels of certain chemicals in products such as construction materials, solvents, paints, etc. that can contribute to air pollution. Many of these chemicals are “novel entities” – synthetic materials – whose production has put the planetary boundary in novel entities into the high-risk zone.^[3]

The EPA sets National Ambient Air Quality Standards (NAAQS) for each criteria air pollutant. State agencies responsible for air quality develop State Implementation Plans to indicate how they will stay within the standards and monitor pollutants to ensure standards are met. Areas that exceed the NAAQS are labeled non-attainment areas; state and local agencies that manage air quality are required to develop plans to bring non-attainment areas into compliance with NAAQs.

Major new stationary sources of air pollution must meet high standards of pollution control technology in order to obtain permits to emit air pollution, particularly if they will be built in non-attainment areas. Manufacturers of new chemicals that may be air pollutants are required, under the Toxic Substance Control Act (TOSCA) to notify EPA about new chemicals or new uses of existing chemicals that may contribute to air pollution. If the EPA finds that new chemicals or new uses pose unreasonable threats to human or environmental health, the agency can require additional testing to provide further information, and can impose restrictions on use.

International air pollution regulation

International treaties cover some air pollutants, particularly pollutants that deplete stratospheric ozone and cause the “ozone hole” over the South Pole (Fig 5). The Montreal Protocol on on Substances That Deplete the Ozone Layer  (1987; usually simply the Montreal Protocol) was a binding implementation agreement that grew out of the Vienna Convention on the Protection of the Ozone Layer (1985). It was the first of only a handful of UN treaties to achieve ratification by all nations, and has been called the most successful environmental agreement. The Protocol sets limits on production and use of ozone-depleting substances. Most of these chemicals are early refrigerants, aerosols, and related chemicals – chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs) – but the list of controlled chemicals is updated regularly to ensure ongoing protection of the ozone layer. The Protocol includes a so-called “ratcheting” mechanism that decreases allowable emissions over time – an important reason for its success.

Ozone depletion over the South Pole continued to worsen during the early years of implementation of the Montreal Protocol, with the largest thinned area occurring in the 2000s with a total area of the ozone hole of 28.4 million square kilometers or more than 4 times the size of the US. In 2024, the maximum extent of the ozone hole was 21.9 million square kilometers – still a significant proportion of the all-time maximum. Nevertheless, the Protocol has led to reductions of emissions of ozone-depleting substances by more than 99% and production of many of them has ceased. Scientists now predict that the ozone hole could “heal” completely well before the end of the century.

An earlier, and less heralded air pollution treaty, the 1979 Convention on Long-Range Transboundary Air Pollution paved the way for the Montreal Protocol. It was the first multilateral agreement to address the problem of air pollution crossing international boundaries, and allowed Europe, Russia, and North America to work together to understand and reduce long-range air pollution. It not only reduced air pollution emissions as intended but also began the process of uncoupling air pollution from economic growth, and important step for better air quality in the long term^[4].

Not all air pollution treaties involve many nations. The US-Canada Air Quality Agreement (1991) is a binational treaty, similar to the Convention on Long-Range Transboundary Air Pollution, to address air pollutants that contribute to acid rain and smog. By 2007, both nations had reduced ozone levels sufficiently to meet the main requirement of the treaty, and a 2023 review and assessment^[5] recommended updating the agreement to develop and implement strategies for additional air pollutants such as PM2.5.

National air pollution standards very widely among the nations of the world, due, in part, to technological capacity. Whereas Figure 5 demonstrates observed levels of PM2.5 around the world, Figure 6 shows the standards set by nations for PM2.5. Because pollutants are not measured uniformly, comparing national standards and national pollution levels ranges can be difficult. For example, Figure 6 shows the PM2.5 standard for a 1-year averaging period; a map of the PM2.5 standards for a 24-hour averaging period would rank nations differently. WHO guidelines are available for both measurement frameworks, which allows comparison to a standard, but comparisons between countries can vary depending on the specific measurement. Similarly, observed pollution levels may be measured with different instrumentation, at different elevations off the ground, with different densities of measurements, at different time intervals, etc.

Regulation and control of greenhouse gases

The UN Framework Convention on Climate Change (UNFCCC; 1992) was the first international treaty to address greenhouse gas (GHG) emissions that cause global warming. The Paris Agreement, adopted in 2015, built on the UNFCCC to create a binding agreement requiring nations to set firm goals (nationally determined contributions: NDCs) to reduce GHG emissions. Like the Montreal Protocol, it has a ratcheting mechanism that requires nations to be more and more ambitious in their reductions over time. However, the Paris Agreement has not enjoyed the success of the Montreal Protocol; emissions of GHGs continue to increase.

Developed countries bear most of the blame for the lack of progress: the US and the European Union are the 2nd and 4th greatest emitters and have the technology and the greater economic stability needed to undertake deep cuts and have not done so. Since 2025, the US is not even participating in the treaty – the only major emitter to withdraw from the Paris Agreement (twice – the US withdrew from 2017-2021, as well). The Paris Agreement explicitly encourages developed countries to take the lead in emissions reductions. It acknowledges that transitional and developing countries need time to balance decreased emissions with economic growth.

Emissions from China (the largest emitter by a considerable margin) and India (3rd greatest emitter) demonstrate the air-quality outcomes of a slower transition to clean energy. China now produces more GHGs than the developed countries, combined. However, in recent years, China has led the world in installing renewable energy; in 2023, China’s additions were over 60% of world additions in renewable capacity. Globally, however, national goals of developed, transitional, and developing countries are insufficient to reduce warming, policies and actions typically lag behind national goals, worsening the outlook for success.

The “promises” that nations make in their NDCs become actual reductions in GHG emissions and GHG levels through a variety of mechanisms. Cap-and-trade agreements, also called emission trading systems, create an economic market by allowing potential GHG emitters to emit a certain amount of GHG, and allowing fast-reducing emitters to sell credits to emitters that are slower to reduce their emissions. Like the ratcheting mechanism used to reduce sulfate air pollution in the US, cap-and-trade agreements generally tighten emissions limits over time, creating pressure on all emitter to find efficiencies and move away from GHG-emitting energy sources. The EU uses cap-and-trade systems, as does China. In the US, a regional cap-and-trade system called RGGI – the Regional Greenhouse Gas Initiative – binds several eastern states, and California and Washington state have state-based systems.

Governments use subsidies, tax credits and other financial incentives to lighten the financial burden associated with transitions to clean energy. These may be aimed at individuals, corporations, local and state governments, and cooperatives. At the level of individuals, subsidies may support addition of solar panels, heat pumps, and electric vehicles. At higher levels, subsidies can support infrastructural development such as energy grid expansions and updates. Subsidies can also support research and investment in evolving technologies such as those involved in carbon capture and storage, which removes CO₂ already in the atmosphere, to hasten reductions in GHG levels over what can be achieved through reduced emissions.

Monitoring and reporting are important elements of achieving reductions in GHG emissions. An accurate understanding of changes in emissions is needed to ensure that government and corporate targets are being met and to allow appropriate enforcement of regulations. Monitoring is carried out at emissions sites, by ground-based stations across the land, and by remote sensing using satellites, aircraft, and drones. In some areas, so-called third parties – neither the emitters nor the enforcers – may undertake monitoring, often under stringent oversight to insure impartiality and accuracy.

Section 6.5 in the Energy chapter addresses the changes in energy production and use and in management of atmospheric CO₂ required to control GHG levels.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer back to the content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.