7.1 - Introduction to Air Pollution

Learning Objectives

I. Defining Air Pollutants and the Clean Air Act (CAA)

• Air pollutants defined: Specific chemicals, compounds, or particles that have definite environmental and human health effects. Using the general term “pollution” is considered too vague in an FRQ setting.

• The Clean Air Act (CAA): Passed in 1970, this act gives the EPA (Environmental Protection Agency) the authority to regulate six criteria air pollutants.
• The EPA can set acceptable air quality standard levels for these pollutants and punish companies or entities that exceed the allowed emissions.
• Air Quality Index (AQI): the EPA's daily scale for how polluted the air is, running from 0 to 500. It's based on five pollutants: ground-level ozone, particulate matter, CO, $\text{SO}_2$, and $\text{NO}_2$. An AQI of 100 lines up with the national health standard for that pollutant, so anything above 100 is unhealthy, first for sensitive groups (kids, older adults, people with asthma) and then for everyone. The categories: 0 to 50 good, 51 to 100 moderate, 101 to 150 unhealthy for sensitive groups, 151 to 200 unhealthy, 201 to 300 very unhealthy, and 301 and up hazardous.

II. The Six Criteria Air Pollutants

III. Carbon Dioxide: The APES Distinction

• Not a criteria pollutant: $\text{CO}_2$ was not one of the six criteria air pollutants initially laid out in the 1970 Clean Air Act.

• EPA regulation: In Massachusetts v. EPA (2007), the Supreme Court ruled that $\text{CO}_2$ and other greenhouse gases count as air pollutants under the Clean Air Act, so the EPA had to decide whether they endanger public health. In 2009 the EPA found that they do (the "endangerment finding"), and its first greenhouse gas limits for cars and light trucks followed in 2010.
• As of 2026: in February 2026 the EPA rescinded the 2009 endangerment finding and repealed all federal greenhouse gas standards for cars and trucks. Health and environmental groups and a coalition of 24 states sued in federal court (the D.C. Circuit) to challenge the repeal.

• APES scoring guidance: In AP Environmental Science, $\text{CO}_2$ has not typically been included on FRQ scoring guides as an air pollutant.
• That’s because $\text{CO}_2$ does not directly lower air quality from a human health standpoint. It isn’t toxic to organisms, doesn’t damage eyes or lungs, and doesn’t lead to smog or decrease visibility. It’s primarily a greenhouse gas that leads to Earth warming.

IV. Primary vs. Secondary Pollutants

• Primary air pollutants: Pollutants that come directly from a source, like straight out of a smokestack, exhaust pipe, volcano, or forest fire.
• Examples include most of the criteria pollutants: $\text{SO}_2$, NOx, CO, Pb, and particulate matter.

• Secondary air pollutants: Primary pollutants that have undergone some transformation catalyzed by sunlight, water, or oxygen. They’re more prevalent during the daytime.
• Examples: Tropospheric ozone ($\text{O}_3$) is the only criteria pollutant that is secondary. Others include sulfuric acid and nitric acid, which form acid rain.

V. Coal Combustion Impacts

• Most polluting fuel: Coal releases more air pollutants per unit of mass burned than any other fossil fuel.

• Emissions from coal: Burning coal releases a real laundry list of pollutants, including CO, $\text{CO}_2$, $\text{SO}_2$, and NOx. It also releases toxic metals like mercury, arsenic, and lead, which bind to particulate matter and can travel far from the source.
• Why so much $\text{SO}_2$: coal naturally contains sulfur. When it burns, the sulfur combines with oxygen: $\text{S} + \text{O}_2 \rightarrow \text{SO}_2$.
• Toxic metals: mercury escapes up the smokestack as a gas, settles into lakes and oceans, and gets turned into methylmercury by bacteria, which then biomagnifies up food chains into fish (8.8). Other metals, like arsenic and lead, leave the stack stuck to fly ash (particulates).

VI. Other Fossil Fuel Pollutants: NOx, Diesel, and More

• Where NOx comes from: burning any fossil fuel (gasoline, diesel, natural gas, or coal) gets hot enough to make the nitrogen ($\text{N}_2$) and oxygen ($\text{O}_2$) that are already in the air react. Most of the nitrogen in NOx comes from the air, not from the fuel, which is why even natural gas makes NOx.
• In the engine or furnace: $\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$
• In the air soon after: $2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2$

• What NOx turns into: NOx is a primary pollutant, but it leads to secondary pollutants.
• Ozone and photochemical smog: sunlight splits $\text{NO}_2$, and the free oxygen atom joins an $\text{O}_2$ to make ozone ($\text{O}_3$), the main part of photochemical smog (7.2).
• Nitric acid and acid rain: $\text{NO}_2$ reacts with oxygen and water in the air to form nitric acid ($\text{HNO}_3$), which falls as acid rain (7.7).

• Other pollutants from burning fossil fuels:
• Carbon monoxide (CO): comes from incomplete combustion, when there isn't enough oxygen to make $\text{CO}_2$. Vehicle exhaust is a major source.
• Hydrocarbons: unburned or partly burned fuel that leaves the tailpipe, plus fuel that evaporates. They're VOCs, the other ingredient in photochemical smog (7.2).
• Particulate matter: soot from diesel engines and ash from burning coal.

• Sulfur dioxide from diesel: like coal, crude oil contains some sulfur, and diesel fuel used to hold a lot of it. When diesel burns, that sulfur becomes $\text{SO}_2$ ($\text{S} + \text{O}_2 \rightarrow \text{SO}_2$), which irritates the lungs and forms sulfuric acid (acid rain) and tiny sulfate particles.
• Example: starting in 2006, U.S. highway diesel was limited to 15 ppm sulfur (ultra-low sulfur diesel), down from 500 ppm, a 97% cut. Ships burn much dirtier fuel oil: a global rule from the International Maritime Organization cut the sulfur limit for ships' fuel from 3.5% to 0.5% in 2020, but 0.5% (5,000 ppm) is still over 300 times the limit for road diesel.

VII. Lead: A Clean Air Act Success Story

• Leaded gasoline: starting in the 1920s, a lead compound (tetraethyl lead) was added to gasoline to keep engines from "knocking." Burning it sent tiny lead particles out of every tailpipe, and by the 1970s gasoline was the biggest source of lead in the air.

• What the EPA did: using its authority under the Clean Air Act, the EPA began phasing lead out of gasoline in the 1970s. New cars starting in 1975 had catalytic converters, which lead ruins, so they needed unleaded gas (7.6). Leaded gasoline was banned for cars and trucks on January 1, 1996.

• Result: average lead levels in U.S. air dropped about 98% between 1980 and 2019, and lead levels in children's blood fell sharply too. It's the classic example of an air regulation that worked.

• What's left (as of 2026): small piston-engine airplanes still burn leaded aviation gasoline, which is now the largest source of lead in U.S. air. The EPA ruled in 2023 that these emissions endanger public health, and the FAA and the aviation industry are aiming to end leaded aviation fuel by the end of 2030.

Big picture: The Clean Air Act regulates six specific criteria air pollutants, meaning $\text{SO}_2$, NOx, CO, lead, ozone, and particulate matter. All of them pose direct threats as respiratory irritants or neurotoxicants, or they contribute to acid rain and smog. Notably, $\text{CO}_2$ isn’t typically classified as an air pollutant in APES, because it doesn’t directly lower air quality or cause immediate toxicity. It functions instead as a greenhouse gas responsible for long term climate change. Fossil fuel combustion, especially coal burning, is the primary source of most of these pollutants, and it leads to the creation of secondary pollutants like ozone and sulfuric and nitric acid.