Title Image: Mexico City smog, 1981 G Lamson/CC BY-NC-SA2.0
Air Quality and Human Health
On average, every adult human inhales 31 lb (14 kg) of air each day, while drinking just 4.4 lb (2 kg) of water and eating 3.3 lb (1.5 kg) of food. Consequently, the quality of the air we breathe is of paramount importance. Each breath provides us with life-supporting oxygen, but also delivers small amounts of harmful gases and small particles that affect our health. Chronic exposure to these gases and particles is reducing human life expectancy by more than eight months on average and by more than two years in the most polluted areas of the world.
World Health Organization data show that almost all of the global population (99%) breathe air that contains high levels of pollutants exceeding WHO guideline limits, with low- and middle-income countries suffering from the highest exposures.
As a result air pollution is now estimated to be a global health risk on the same level as unhealthy diets and cigarette smoking.
Air pollution is ranked the second-leading risk factor for death, behind high blood pressure, accounting for about 8.1 million premature deaths globally and about 600,000 deaths in the U.S. in 2021 (see Figure 1, below.)
Figure 1 – the global impact of air pollution Credit: State of Global Air 2024
A surprising 38% of deaths are caused by indoor air pollution, primarily in less-developed parts of the world where it is common to burn biofuels indoors for cooking and heating. However, the greatest air pollution threat, responsible for 58% of premature deaths worldwide, comes from particulate matter (PM) smaller than one-hundredth of a millimeter.
The economic impact of air pollution is equally jaw-dropping. According to analysts at Allianz SE, the world’s largest insurance company:
“In 2018, air pollution triggered a staggering cost of USD 2.9 trillion to the global economy, corresponding to 3.3 % of the world’s GDP. In the same year, unhealthy air quality caused 1.8 billion days of work absences worldwide. Other estimates suggest that air pollution accounts for USD 5 trillion, annually, in welfare costs for the global economy” — Allianz
Air quality is closely linked to the earth’s global climate and ecosystems. Many of the drivers of air pollution (combustion of fossil fuels, for example) are also sources of the greenhouse gas emissions driving climate change. In turn, our warming and increasingly chaotic climate also affects air pollution.
In the rest of this post we will review the history of outdoor air pollution that triggered government action; how air pollution is harmful; and how Air Quality is defined, measured and managed. In Part 2 we will discuss the tools and government services that can alert you to air pollution threats; and the air pollution impacts of climate change, including rising heat and wildfires.
How Did We Get Here?
By the mid-twentieth century, it became evident that burning fossil fuels, first coal, and later oil and gas, to power cities and industries had created an air quality crisis. In London, England, the catastrophic Great Smog of 1952 led to the deaths of 12,000 people. Faced with an undeniable cause and effect, the government passed the British Clean Air Act in 1956.
In the U.S., cities like Los Angeles and New York experienced deadly smog episodes caused by coal and oil-fueled power plants, incinerators, transportation and factories (Figure 2.) In 1970, President Nixon presided over the creation of the Environmental Protection Agency (EPA) and the Clean Air Act.
Figure 2 – Dense Los Angeles smog helped spark the Clean Air Act Credit: EPA Archive
Prior to the passage of the Clean Air Act, control of air pollution was the responsibility of state and local authorities. The Clean Air Act put the federal government in charge, establishing a common regulatory framework across the country.
Air Pollution and Human Health
The World Health Organization rates air pollution as a “public health emergency,” with more than 90% of the global population exposed to toxic outdoor air. Figure 3 illustrates the EPA’s assessment of the potential impacts of air pollution on human health in the US. In all cases, children and the elderly are most at risk.
Figure 3 – Impacts of air pollution on human health Credit: EPA
Heart and lung damage is just the tip of the iceberg. Very small “ultra fine” particulate pollutants are able to pass through the lungs, to be carried in the bloodstream throughout the body, reaching every organ, including the brain.
Although smog triggered the formation of the EPA in the 1970’s, at that time smog was thought of as mostly ground-level ozone and particulate pollutants were considered to be a less important nuisance. It was not until the 1990fs that research linked small particles to a range of health effects, including asthma symptoms, emergency room visits and premature mortality.
Defining and Managing Air Quality
The EPA manages air quality in the U.S., setting and adjusting National Ambient Air Quality Standards (NAAQS) for five major air pollutants:
- Carbon Monoxide (CO)
- Nitrogen Dioxide (NO2)
- ground-level Ozone (O3)
- Particulate Matter (PM)
- Sulfur Dioxide (SO2)
Accurately monitoring each of the pollutants is crucial to tracking efforts to reduce pollution at source, with the goal of attaining levels below the current NAAQS for each pollutant. The EPA measures the major pollutants at more than 4,000 state-operated monitoring stations across the U.S.
The U.S. Air Quality Index (AQI) is EPA’s tool for communicating day to day outdoor air quality and health risks to the public. AQI reports focus on data for the two most widely distributed pollutants – particulate matter (PM) and ozone (O3). While monitored, Sulfur dioxide is not included in the AQI because its concentrations tend to be very localized, typically near emissions sources, making it less meaningful at regional scales. Similarly, ground level ozone is strongly correlated with both carbon monoxide and nitrogen dioxide levels.
The AQI includes six color-coded health risk categories, each corresponding to a range of index values (Figure 4.) The higher the AQI value, the greater the level of air pollution and the greater the health concern. For example, an AQI value of 50 or below represents good outdoor air quality, while an AQI value over 300 represents air quality that is hazardous for everyone.
Figure 4 – EPA Air Quality Index indicates overall air pollution health risk Credit: EPA
Particle Pollution
Atmospheric particulate matter (PM) comprises microscopic solids or liquid droplets so small that they can be inhaled. Particles smaller than 10 microns in diameter (roughly 0.0004 inches) can be inhaled deeply into the lungs. The smallest particulates can pass through the lungs into the bloodstream, where they can reach every organ in the body.
Numerous studies have linked particulate matter to heart disease and respiratory diseases such as asthma, bronchitis and emphysema.
Figure 5 below shows the size of PM relative to the width of a human hair or a grain of fine sand.
Figure 5 – Scale of various forms of particulate matter Credit: EPA
The size of PM particles largely determines the potential health risk:
- PM10 – can be inhaled into the body
- PM2.5 – can penetrate deep into the lungs. PM2.5 accounts for more than 90% of the monetized social costs of air pollution
- PM0.1 – ultrafine particles (UFPs), readily pass through the lungs into the bloodstream and into organs, including the brain
Particulate matter (PM), or airborne particles, include dust, dirt, soot, and smoke. Some particles are directly emitted into the air by, for example, cars, trucks, buses, factories, construction sites, and wood burning. Ultrafine particles are most commonly emitted through combustion from natural or human sources. Human activity is believed to be responsible for the largest share of UFPs due to the prevalence of UFPs in cities, where global industrialization and population growth have had the most notable impact on air pollution. UFPs are extremely difficult to measure and levels can vary considerably over small distances. Consequently, an EPA NAAQS has yet to be defined for UFPs.
Exposure to particulate pollution can lead to a variety of health problems, including:
- premature death in people with heart or lung disease
- heart attacks
- irregular heartbeat
- aggravated asthma
- decreased lung function
- magnified respiratory symptoms, such as airway irritation, coughing or difficulty breathing.
Ozone
Ground-level Ozone (O3) is the result of a chemical reaction between nitrogen oxides (NOx) and volatile organic compounds (VOCs), driven by heat and sunlight. Nitrogen oxides are produced by vehicle exhausts and industrial emissions. VOC sources include car exhausts, gasoline vapors, chemical solvents, fuel combustion (wood, garbage, fossil fuels) and a variety of industrial processes.
Figure 6 – Heavy smog in New York City, 1973 Credit: EPA Archive
Figure 6 shows New York City engulfed in absolutely epic smog in May, 1973, the result of stagnant air and unconstrained air pollution from vehicle exhaust emissions, oil and coal fueled power stations, garbage and waste incinerators, and factory and building furnaces fueled by oil and coal.
Ozone pollution is primarily an urban issue, where emissions from transportation and industry is concentrated in a small area. Congestion, the densely built environment and concentrated energy use trap and amplify temperatures—the “urban heat island” effect. Heat and sunlight accelerates the chemical reaction that forms ground-level ozone from other pollutants like transportation emissions and VOCs. When temperatures are high and sunlight is unhindered, ozone levels can rise quickly. Since heat and abundant sunshine is required to power the chemical reaction that produces ozone, peak ozone season in the U.S. runs from June through August.
Although ozone is typically generated in cities, wind can transport ozone long distances, potentially causing high ozone levels even in rural areas.
Depending on the level of exposure, the health impacts of ground level ozone can include airway infections, breathing difficulty, and potentially permanent lung damage. Ozone exposure is dangerous for people with lung cancer, asthma, COPD, and cardiovascular disease. According to the EPA, long-term exposure to ozone is linked to aggravation of asthma and is likely to be one of many causes of asthma development.
What is the State of U.S. Air Quality?
The graphic below is an encouraging summary of the positive changes in key air pollutants over the past 30 years.
Figure 7 – An EPA success story – steady reduction in major pollutants over the last 30 years. Credit: EPA
The graph shows the level of each variable as a percentage over or under that pollutant’s NAAQS. Thus national average levels of ozone in 2022 (light green line) near 0% are roughly equal to the current ozone NAAQS, while the annual average PM2.5 level has tracked within a few percent of the PM2.5 NAAQS since 2012. The bright blue line shows the dramatic reduction in Sulfur Dioxide emissions that followed requirements for the installation of air pollution controls at coal-fired power plants, and power plants’ shift from coal to comparatively clean “natural” gas.
In Part 2 of this post we will discuss the EPA’s tools and services that can alert you to air pollution threats in near-real time. We will also review the growing impact of climate change on air pollution, including rising heat, the increasing frequency, size and intensity of wildfires, and a changing allergy season.