Common Air Pollutants Explained

The Six Criteria Air Pollutants

The Clean Air Act requires the Environmental Protection Agency (EPA) to set national air quality standards for six common air pollutants, known as "criteria pollutants." These pollutants are found throughout the United States and can harm your health and the environment at high concentrations.

Particulate Matter (PM2.5 and PM10)

Particulate matter is a complex mixture of extremely small particles and liquid droplets suspended in the air. These particles come in many sizes and shapes and can be made up of hundreds of different chemicals.

What Are PM2.5 and PM10?

Particles are defined by their diameter for air quality regulatory purposes:

• PM10: Inhalable particles with diameters 10 micrometers or smaller (about 1/7 the width of a human hair)
• PM2.5: Fine inhalable particles with diameters 2.5 micrometers or smaller (about 1/30 the width of a human hair)

To put this in perspective, a single human hair is about 70 micrometers in diameter. PM2.5 particles are so small that you can only see them with an electron microscope. Thousands of these particles would fit on the period at the end of this sentence.

Sources of Particulate Matter

PM2.5 and PM10 come from numerous sources:

Health Effects of Particulate Matter

PM2.5 is considered especially dangerous because these tiny particles can penetrate deep into your lungs and even enter your bloodstream. Health effects include:

Who Is Most Vulnerable?

• Children (developing lungs are more susceptible)
• Older adults (65+)
• People with heart or lung disease
• People with diabetes
• Pregnant women and their developing fetuses
• People who work or exercise outdoors

Ground-Level Ozone (O₃)

Ground-level ozone is different from the ozone layer high in the atmosphere that protects us from ultraviolet radiation. At ground level, ozone is a harmful pollutant and the main ingredient in smog.

How Ozone Forms

Ozone is not directly emitted into the air. Instead, it forms through chemical reactions between nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) in the presence of sunlight. This is why ozone levels are typically highest on hot, sunny days.

The formation process works like this:

1. Cars, power plants, and industries emit NOₓ and VOCs
2. Sunlight triggers chemical reactions between these pollutants
3. Ozone forms, especially on hot, stagnant summer days
4. Peak ozone levels occur in the afternoon and early evening

Sources of Ozone Precursors

• Motor vehicle exhaust (largest source in urban areas)
• Industrial facility emissions
• Electric utility power plants
• Gasoline vapors from refueling
• Chemical solvents in paints and cleaners
• Natural sources like trees (VOCs)

Health Effects of Ozone

Ozone irritates the respiratory system and can cause health problems even at low levels. Think of it like a sunburn for your lungs—ozone damages the cells lining your airways.

When Ozone Is Highest

Ozone pollution follows predictable patterns:

• Time of day: Peaks in the afternoon (2-6 PM) when sun is strongest
• Season: Worst in summer months (June-August)
• Weather: Highest on hot, sunny, stagnant days
• Location: Higher in urban and suburban areas downwind of cities

Nitrogen Dioxide (NO₂)

Nitrogen dioxide is a reddish-brown gas with a sharp, acrid odor. It's part of a group of gases called nitrogen oxides (NOₓ), which also contribute to ozone formation.

Sources of Nitrogen Dioxide

• Burning fossil fuels (the primary source)
• Motor vehicles, especially diesel engines
• Power plants and industrial boilers
• Off-road equipment (construction equipment, locomotives)
• Residential heating
• Gas stoves and heaters (indoor sources)

Health Effects

NO₂ primarily affects the respiratory system:

• Irritation of airways in the respiratory system
• Exacerbation of asthma and other respiratory diseases
• Increased susceptibility to respiratory infections
• Inflammation of the airways
• Reduced lung function, especially in children
• Increased risk of chronic bronchitis

Short-term exposure (30 minutes to several hours) to elevated NO₂ levels can worsen respiratory diseases like asthma, leading to respiratory symptoms, increased hospital admissions, and emergency room visits.

Indoor NO₂ Exposure

Unlike most outdoor pollutants, NO₂ can also be a significant indoor air quality concern:

• Gas stoves and ovens are major indoor sources
• Unvented gas or kerosene heaters produce high levels
• Indoor levels can exceed outdoor levels in homes with gas appliances
• Using range hoods vented to the outside reduces indoor NO₂

Sulfur Dioxide (SO₂)

Sulfur dioxide is a colorless gas with a strong, pungent odor. It's primarily produced by burning sulfur-containing fuels.

Sources of Sulfur Dioxide

• Coal and oil combustion at power plants (75% of total SO₂ emissions)
• Industrial processes (petroleum refineries, metal processing)
• Locomotives and large ships
• Diesel vehicles
• Volcanic eruptions (natural source)

Health Effects

SO₂ affects the respiratory system, particularly the function of the lungs:

People with asthma are particularly sensitive to SO₂. Brief exposures to SO₂ can cause airways to constrict, making it difficult to breathe. Symptoms include wheezing, chest tightness, and shortness of breath.

Good News About SO₂

SO₂ levels have decreased dramatically in the United States over the past few decades:

• Down 91% since 1980 due to Clean Air Act regulations
• Transition from coal to natural gas for electricity generation
• Installation of scrubbers at power plants
• Low-sulfur fuel requirements

Carbon Monoxide (CO)

Carbon monoxide is a colorless, odorless gas that can be deadly at high concentrations. It forms when carbon in fuel doesn't completely burn.

Sources of Carbon Monoxide

• Motor vehicle exhaust (the primary outdoor source)
• Industrial processes and fuel combustion
• Residential wood burning
• Malfunctioning fuel-burning appliances (furnaces, water heaters)
• Generators and other gasoline-powered equipment
• Cigarette smoke

Health Effects

CO is dangerous because it interferes with oxygen delivery in the body. When you breathe CO, it binds to hemoglobin in your blood, reducing the amount of oxygen that reaches your organs and tissues.

Who Is Most Vulnerable?

• Infants and unborn babies
• Older adults
• People with chronic heart disease
• People with anemia
• People with respiratory problems

CO Levels Have Decreased

Outdoor CO levels have dropped significantly due to:

• Catalytic converters in vehicles (required since 1975)
• Cleaner-burning engines
• Emissions standards for vehicles and equipment
• CO is now rarely a significant outdoor air quality issue in most areas

Indoor CO Dangers

While outdoor CO is less of a concern, indoor CO poisoning remains a serious risk:

• Install CO detectors on every level of your home
• Never use generators, grills, or camp stoves indoors
• Don't run vehicles in attached garages
• Have fuel-burning appliances inspected annually
• Ensure proper ventilation for all fuel-burning devices

Lead

While not typically measured as part of daily AQI, lead is still a criteria pollutant that the EPA monitors. Lead in air has decreased by 99% since 1980, primarily due to the phase-out of leaded gasoline.

Current Sources of Airborne Lead

• Ore and metals processing
• Piston-engine aircraft operating on leaded aviation gasoline
• Waste incinerators
• Battery manufacturing

Health Effects

Lead affects nearly every system in the body. Once absorbed, it accumulates in bones, blood, and soft tissues. There is no safe level of lead exposure, particularly for children:

• Neurological and cognitive impairment in children
• Behavioral problems and reduced IQ
• Cardiovascular effects in adults
• Kidney problems
• Reproductive issues

How Pollutants Interact

Air pollutants don't exist in isolation. They interact with each other and can create secondary pollutants:

• Photochemical smog: Forms when NOₓ and VOCs react in sunlight to create ozone
• Secondary particles: SO₂ and NOₓ react in the atmosphere to form fine particles
• Acid rain: SO₂ and NOₓ combine with water vapor to create sulfuric and nitric acid
• Synergistic effects: Multiple pollutants together may be more harmful than any single pollutant

Protecting Yourself from Air Pollutants

Check Air Quality Daily

• Use AirNow.gov or weather apps to check current AQI
• Pay attention to which pollutant is driving the AQI
• Check forecasts to plan outdoor activities

Limit Exposure on Bad Air Days

• Reduce outdoor exercise when AQI is orange or higher
• Schedule activities for early morning when ozone is lower
• Move strenuous activities indoors
• Keep windows closed when outdoor air quality is poor

Improve Indoor Air Quality

• Use HEPA air purifiers to remove particles
• Ensure proper ventilation for all fuel-burning appliances
• Use kitchen exhaust fans when cooking with gas
• Install and maintain CO detectors
• Avoid indoor smoking

Reduce Your Contribution

• Drive less—carpool, use public transit, bike, or walk
• Keep your vehicle well-maintained
• Avoid idling your car
• Use electric or manual lawn equipment instead of gas-powered
• Choose low-VOC paints and cleaning products

Understanding Your Local Pollutant Mix

The dominant pollutant varies by location and season:

• Urban areas: Often dominated by NOₓ from traffic and ozone in summer
• Near highways: High NO₂ and PM2.5 from vehicle exhaust
• Near power plants: May have elevated SO₂ and PM2.5
• Western U.S. in summer/fall: PM2.5 from wildfire smoke
• Winter in valleys: PM2.5 from wood burning and temperature inversions

Understanding air pollutants empowers you to protect your health and make informed decisions about when and where to spend time outdoors. By checking which pollutants are elevated in your area and adjusting your activities accordingly, you can minimize your exposure and reduce health risks.