Urban Air Quality: Practical Steps for Cleaner Neighborhoods

Air pollution is the presence of substances in the air that are harmful to humans, other living beings or the environment. Pollutants can be gases, like ozone or nitrogen oxides, or small particles like soot and dust. Both outdoor and indoor air can be polluted. (EPA Smart Growth)

Outdoor air pollution comes from burning fossil fuels for electricity and transport, wildfires, some industrial processes, waste management, demolition and agriculture. Indoor air pollution is often from burning firewood or agricultural waste for cooking and heating. Other sources of air pollution include dust storms and volcanic eruptions. Many sources of local air pollution, especially burning fossil fuels, also release greenhouse gases that cause global warming. However, air pollution may limit warming locally. (HUD Community Planning)

Sources

Burning fuel to generate electricity causes air pollution; lignite and coal produce the most air pollution, followed by oil, and then by fossil gas and biomass. Methane leaks are common in oil and gas production, and oil refineries emit a wide range of pollutants. Some hazardous air pollutants are produced in plastic and rubber production, whereas chloroform can be produced during water chlorination, and arsenic is found in the mining industry. Many polluting industries have been pushed out of richer nations, and China too has started to push its most polluting industries out of the country. (USDA National Agriculture Library)

Construction and demolition produces dust, but also other pollutants. The direct particles from construction and demolition are relatively coarse. Construction also has an indirect impact on air quality, as cement production is one of the main sources of particle pollution. Though banned in many countries, asbestos persists in older buildings, where it poses a risk of lung disease when disturbed. Building materials including carpeting and plywood emit formaldehyde, a gas which can cause difficulty breathing and nausea. (EPA Environmental Resources)

Road vehicles produce a significant amount of all air pollution. For instance, they may be responsible for a third to half of all nitrogen dioxide emissions, and are a major cause of climate change. Vehicles with petrol and diesel engines produce about half of their emissions from their exhaust gas, and the other half from non-exhaust emissions (tire and brake wear and erosion or disturbance of the road surface); electric vehicles produce no tailpipe emissions, but still produce the other emissions. Diesel trains, ships, and planes also cause air pollution. (University of Minnesota Extension)

Agricultural emissions, both from crops and from animal agriculture, contribute substantially to air pollution. For instance, methane is emitted by the digestion of food by cattle, causing ground-level ozone. Agriculture is also a major source of ammonia, which can form fine particulate matter. Practices like slash-and-burn in forests like the Amazon cause large air pollution alongside deforestation. (Penn State Extension)

Major pollutants

Air pollutants can be tiny solid or liquid particles dispersed in the air (called aerosols), or gases. Pollutants are classified as primary or secondary. Primary pollutants are produced directly by a source and remain in the same chemical form after they have been emitted into the atmosphere. Examples include carbon monoxide gas from car exhausts and sulfur dioxide from factories. Secondary pollutants are not emitted directly. Rather, they form in the air when primary pollutants react with each other or with other parts of the atmosphere. Ground-level ozone is one example of a secondary pollutant. Some pollutants may be both primary and secondary — both are emitted directly and formed from other primary pollutants. (EPA Smart Growth)

Ammonia (NH3) is emitted mainly by overuse of synthetic nitrogen fertilizers on farmland, and from manure and urine from livestock. At typical concentrations in the air, it is not harmful to health directly. However, ammonia can react with other pollutants in the air to form ammonium sulfate or nitrate salts, contributing to particulate matter pollution. Furthermore, when ammonia is deposited onto the soil, it can harm ecosystems via eutrophication. (HUD Community Planning)

Carbon dioxide (CO2) is mainly emitted by the burning of fossil fuels. CO2 is sometimes called an air pollutant, because it is the main greenhouse gas responsible for climate change. Although the World Health Organization recognizes CO2 as a climate pollutant, it does not include the gas in its Air Quality Guidelines or set recommended targets for it. This question of terminology has practical consequences, for example, in determining whether the U.S. Clean Air Act (which is designed to improve air quality) is deemed to regulate CO2 emissions. The Inflation Reduction Act of 2022 amended the Clean Air Act to define CO2 from fossil fuel burning explicitly as an air pollutant. (USDA National Agriculture Library)

Carbon monoxide (CO) is a colorless, odorless, and toxic gas. It is a product of combustion of fuel such as natural gas, coal, or wood. In the past, emissions from vehicles were the main source of CO, but modern vehicles do not emit much of it. Now, wildfires and bonfires are the main source of outdoors CO. Indoors, CO is a larger problem and mainly comes from cooking and heating. In poorly ventilated spaces, CO can accumulate to dangerous levels, and exposure may cause people to lose consciousness and die. When CO is destroyed in the atmosphere, it can raise levels of CO2 and .mw-parser-output .template-chem2-su{display:inline-block;font-size:80%;line-height:1;vertical-align:-0.35em}.mw-parser-output .template-chem2-su>span{display:block;text-align:left}.mw-parser-output sub.template-chem2-sub{font-size:80%;vertical-align:-0.35em}.mw-parser-output sup.template-chem2-sup{font-size:80%;vertical-align:0.65em}CH4. (EPA Environmental Resources)

Exposure

Exposure to air pollution varies widely across the world and across groups. Children, for example, are more exposed because they breathe more rapidly than adults and closer to the ground, where pollution from vehicle exhaust and dust is more concentrated. Similarly, people engaging in strenuous exercise inhale more pollutants than those at rest. People can reduce their exposure by wearing high-quality face masks or by using air purifiers. (University of Minnesota Extension)

For some pollutants, low exposure can be seen as safe, whereas other pollutants have negative health effects even at low levels. As evidence has grown that even very low levels of air pollutants hurt human health, the WHO halved its recommended safe limit for particulate matter from 10 μg/m3 to 5 μg/m3 in 2021. Under the new guideline, nearly the entire global population—97%—is classified as exposed to unsafe levels of fine particles (PM2.5). The new limit for nitrogen dioxide (NO2) became 75% lower. For all pollutants together, the World Health Organization concluded that 99% of the world population is exposed to harmful air pollution. (Penn State Extension)

For some pollutants such as black carbon, traffic related exposures may dominate total exposure despite short exposure times, since high concentrations coincide with proximity to major roads or participation in (motorized) traffic. A large portion of total daily exposure occurs as short peaks of high concentrations. (EPA Smart Growth)

While air pollution affects a variety of populations, some groups are more exposed. In many regions, there are disparities in exposure to pollution by race and income. This is especially true in countries with high inequalities in incomes and healthcare, such as the United States. Polluting industries and roads are more likely to be placed in poorer communities, and people in these communities are more likely to work outdoors, leading to additional exposure. Residents in public housing, who are generally low-income and cannot easily move to healthier neighborhoods, are highly affected by nearby refineries and chemical plants. Additionally, lower-income communities more often perform polluting activities, such as using solid biofuels for cooking. In the United States, Blacks and Latinos generally face more pollution than Whites and Asians. (HUD Community Planning)

Health effects

Air pollution is an important risk factor for various diseases, such as COPD (a common lung disease), stroke, heart disease, lung cancer, and pneumonia. Indoor air pollution is also associated with cataract. According to the WHO, 99% of the world's population lives in areas with air pollution that exceeds WHO recommended levels. Even at very low levels (under the World Health Organization recommended levels), fine particulates can continue to cause harm. (USDA National Agriculture Library)

Pollutants strongly linked to ill health include particulate matter, carbon monoxide, nitrogen dioxide (NO2), ozone (O3), and sulfur dioxide (SO2). Fine particulates are especially damaging, as they can enter the bloodstream via the lungs and reach other organs. Air pollution causes disease by driving inflammation and oxidative stress, suppressing the immune system, and by damaging DNA. (EPA Environmental Resources)

People living in poverty, babies, and older people are disproportionately affected by air pollution; pregnancy is also more risky when exposed to air pollution. Communities with a low socioeconomic status and minority groups are more vulnerable to pollution than more privileged communities. Lower-income groups might for instance have less access to healthcare. (University of Minnesota Extension)

Estimates of deaths due to air pollution vary. The 2024 Global Burden of Disease Study estimates that air pollution contributed to 8.1 million deaths in 2021, which is more than 1 in 8 deaths. Outdoor particulate pollution (PM2.5) was the largest cause of death (4.7 million), followed by indoor particulate pollution (3.1 million) and ozone (0.5 million). (Penn State Extension)

Social and environmental impacts

Naturally, water in the atmosphere is slightly acidic. Some pollutants can form strong acids, making rainwater much more acidic. Key acids that cause acid rain are nitric acid (HNO3), sulfuric acid (H2SO4) and hydrochloric acid (HCl). HCl comes from coal combustion. H2SO4 forms from SO2, which comes from the burning of coal and oil and from some industrial processes like smelting. HNO3 forms from NO2, which is formed during high-temperature combustion. The term acid rain not only refers to rain, but also to pollution from hail, fog, and snow. (EPA Smart Growth)

Acid rain caused substantial damage in the 1970s, including lake acidification and forest diebacks in Northern Europe. Due to the changed acidity in water bodies and soils, essential nutrients such as magnesium and calcium became soluble and could be washed away. Other elements, such as aluminium, which were toxic to vegetation, became available for the roots to absorb. Acid rain also impacts buildings and statues made of specific stones (e.g. marble, calcite or freestone), as the stone reacts chemically with the acid in the water and erodes. (HUD Community Planning)

Air pollution can settle (deposit) on the soil or in water, causing various problems. For instance, ammonia and nitric acid in the air can contribute to nutrient pollution in water, a process called eutrophication. At first, the extra nutrients help plants grow, but dense plant growth blocks sunlight from reaching the bottom. Plants in the lower layers then die, and with fewer plants producing oxygen, the oxygen level drops. This harms organisms that need oxygen to live, and can lead to the loss of sensitive species. (USDA National Agriculture Library)

Various studies have estimated the impacts of air pollution on agriculture, especially ozone. Ozone acts as an oxidant and reduces photosynthesis. One study estimated that for a 1% increase in ozone concentrations, there would be a global economic loss of $10 billion each year. For PM2.5, a 1% increase in pollution levels would lead to about $5 billion in losses, especially in colder climates. After air pollutants enter the agricultural environment, they not only directly affect agricultural production and quality, but also enter agricultural waters and soil. Air pollution further decreases the productivity of laborers via health impacts. (EPA Environmental Resources)

History of air pollution

Mummified remains of people in Peru, Egypt and Britain show that ancient people in these regions suffered from blackening of the lungs caused by open fires in poorly ventilated homes. Recorded complaints of air pollution go back to the Greek and Roman period. Outdoor air pollution became a problem with the rise of cities, caused by household smoke and by early industrial activities (such as smelting and mining). In particular, lead levels, found in Arctic ice cores, were about ten times higher in the Roman period than in the period before. (University of Minnesota Extension)

During the Industrial Revolution, outdoor air pollution started to rise strongly, mostly due to the large-scale burning of coal. This occurred first in Britain, then in the rest of Northern Europe and the United States. By the 19th century, buildings around industrial plants started to blacken, while plants and trees in public parks started to wither. Smoke-induced fogs reduced the amount of sunlight city-dwellers got, contributing to cases of rickets, a childhood disease caused by lack of sunlight and poor diet. (Penn State Extension)

However, the business and political leadership of the industrial cities were enthusiastic backers of the industry: heavy black smoke meant prosperity, high profits, and high wages. (EPA Smart Growth)

The miasma theory was a prominent idea in the 18th and 19th centuries that gave a false explanation of how deadly epidemics like cholera, yellow fever, and malaria ("bad air") originated and spread. It said that illnesses were caused by breathing in a mysterious "miasma," a harmful vapor that arose from decaying organic matter. Epidemics often came in the summer because that is when people spent more time outside. The theory motivated an enormous emphasis on public sanitation in major cities to remove smelly pollution, especially human and animal excrement, from streets and back alleys. The theory collapsed when physicians accepted the new germ theory of disease in the late 19th century. Germs coughed up by an infected person or spread by certain types of mosquitoes or hookworms were the real reason people caught an infectious disease. (HUD Community Planning)

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Frequently Asked Questions

What is the most important thing to know about Air quality?

The most important factor is starting with an honest assessment of your current situation and available resources. Effective implementation depends on matching the approach to your specific context — climate, scale, community, and goals all matter. (USDA National Agriculture Library)

Conclusion

Urban Air Quality: Practical Steps for Cleaner Neighborhoods represents an important dimension of the larger shift toward sustainable, ecologically grounded ways of living. Whether you are just beginning or deepening existing practice, the resources and knowledge are increasingly accessible. The steps taken today — however modest — contribute to a compounding body of change that matters both locally and globally. (EPA Environmental Resources)

Additional reference: Wikipedia — Air quality

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