Air pollution, explained
Pollutants in the air aren't always visible and come from many different sources.
Despite decades of progress, the air quality in the United States has started to decline over the past few years, according to data in the Environmental Protection Agency's 2024 "Our Nation’s Air" report. The agency reported a nearly 38 percent increase in unhealthy air days nationwide, rising from 596 days in 2022 to 822 days in 2023.
The American Lung Association's 2024 "State of the Air" report shows people in the U.S. experienced the most days with "very unhealthy" and "hazardous" air quality due to particle pollution in 25 years. The report also found that 131 million people are living in areas with unhealthy levels of air pollution.
The reasons for the recent decline in air quality remain unclear, says the agency, but may be related to high numbers of wildfires, a warming climate, and increasing human consumption patterns driven by population growth and a strong economy. The long-term outlook also remains unclear, even as politicians debate air pollution standards.
What is air pollution?
Air pollution is a mix of particles and gases that can reach harmful concentrations both outside and indoors. Its effects can range from higher disease risks to rising temperatures. Soot, smoke, mold, pollen, methane, and carbon dioxide are a just few examples of common pollutants.
In the U.S., one measure of outdoor air pollution is the Air Quality Index, or AQI which rates air conditions across the country based on concentrations of five major pollutants: ground-level ozone, particle pollution (or particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide. Some of those also contribute to indoor air pollution, along with radon, cigarette smoke, volatile organic compounds (VOCs), formaldehyde, asbestos, and other substances.
A global health hazard
Poor air quality kills people. Worldwide, bad outdoor air caused an estimated 4.2 million premature deaths in 2016, about 90 percent of them in low- and middle-income countries, according to the World Health Organization. Indoor smoke is an ongoing health threat to the 3 billion people who cook and heat their homes by burning biomass, kerosene, and coal. Air pollution has been linked to higher rates of cancer, heart disease, stroke, and respiratory diseases such as asthma. In the U.S. nearly 134 million people—over 40 percent of the population—are at risk of disease and premature death because of air pollution, according to American Lung Association estimates.
While those effects emerge from long-term exposure, air pollution can also cause short-term problems such as sneezing and coughing, eye irritation, headaches, and dizziness. Particulate matter smaller than 10 micrometers (classified as PM10 and the even smaller PM2.5) pose higher health risks because they can be breathed deeply into the lungs and may cross into the bloodstream.
Air pollutants cause less-direct health effects when they contribute to climate change. Heat waves, extreme weather, food supply disruptions, and other effects related to increased greenhouse gases can have negative impacts on human health. The U.S. Fourth National Climate Assessment released in 2018 noted, for example, that a changing climate "could expose more people in North America to ticks that carry Lyme disease and mosquitoes that transmit viruses such as West Nile, chikungunya, dengue, and Zika."
Environmental impacts
Though many living things emit carbon dioxide when they breathe, the gas is widely considered to be a pollutant when associated with cars, planes, power plants, and other human activities that involve the burning of fossil fuels such as gasoline and natural gas. That's because carbon dioxide is the most common of the greenhouse gases, which trap heat in the atmosphere and contribute to climate change. Humans have pumped enough carbon dioxide into the atmosphere over the past 150 years to raise its levels higher than they have been for hundreds of thousands of years.
Other greenhouse gases include methane —which comes from such sources as landfills, the natural gas industry, and gas emitted by livestock—and chlorofluorocarbons (CFCs), which were used in refrigerants and aerosol propellants until they were banned in the late 1980s because of their deteriorating effect on Earth's ozone layer.
Another pollutant associated with climate change is sulfur dioxide, a component of smog. Sulfur dioxide and closely related chemicals are known primarily as a cause of acid rain. But they also reflect light when released in the atmosphere, which keeps sunlight out and creates a cooling effect. Volcanic eruptions can spew massive amounts of sulfur dioxide into the atmosphere, sometimes causing cooling that lasts for years. In fact, volcanoes used to be the main source of atmospheric sulfur dioxide; today, people are.
Airborne particles, depending on their chemical makeup, can also have direct effects separate from climate change. They can change or deplete nutrients in soil and waterways, harm forests and crops, and damage cultural icons such as monuments and statues.
What can be done?
Countries around the world are tackling various forms of air pollution. China, for example, is making strides in cleaning up smog-choked skies from years of rapid industrial expansion, partly by closing or canceling coal-fired power plants. In the U.S., California has been a leader in setting emissions standards aimed at improving air quality, especially in places like famously hazy Los Angeles. And a variety of efforts aim to bring cleaner cooking options to places where hazardous cookstoves are prevalent.
In any home, people can safeguard against indoor air pollution by increasing ventilation, testing for radon gas, using air purifiers, running kitchen and bathroom exhaust fans, and avoiding smoking. When working on home projects, look for paint and other products low in volatile organic compounds: organizations such as Green Seal, UL (GREENGUARD), and the U.S. Green Building Council can help.
To curb global warming, a variety of measures need to be taken, such as adding more renewable energy and replacing gasoline-fueled cars with zero-emissions vehicles such as electric ones. On a larger scale, governments at all levels are making commitments to limit emissions of carbon dioxide and other greenhouse gases. The Paris Agreement, ratified on November 4, 2016, is one effort to combat climate change on a global scale. And the Kigali Amendment seeks to further the progress made by the Montreal Protocol, banning heat-trapping hydrofluorocarbons (HFCs) in addition to CFCs.
