Air Pollutants & the Clean Air Act

You’ll be able to

Distinguish primary from secondary air pollutants
Identify the criteria pollutants and their major sources
Express and interpret pollutant concentrations in parts per million

Primary vs. secondary pollutants

Air pollutants come in two forms. A primary pollutant is emitted directly into the air from a source — carbon monoxide from a tailpipe, sulfur dioxide from a coal plant, soot from a fire. A secondary pollutant is not emitted directly but forms in the atmosphere when primary pollutants react with sunlight, water, or each other. The most important example is ground-level ozone (O₃), which forms when nitrogen oxides and volatile organic compounds react in sunlight. Recognizing which pollutants are primary and which are secondary is a frequent exam skill.

The criteria pollutants

The U.S. Clean Air Act directs the EPA to set standards for six criteria pollutants. Carbon monoxide (CO) — a colorless, odorless gas from incomplete combustion that binds hemoglobin. Sulfur dioxide (SO₂) — from burning coal, a driver of acid rain. Nitrogen oxides (NOₓ) — from high-temperature combustion in engines, contributing to smog and acid rain. Particulate matter (PM) — tiny solid/liquid particles that lodge deep in the lungs. Ground-level ozone (O₃) — the main ingredient of photochemical smog (secondary). Lead (Pb) — a neurotoxin, sharply reduced after leaded gasoline was phased out.

Measuring air pollution

Pollutant concentrations are usually tiny, so they are expressed in parts per million (ppm) or parts per billion (ppb) — the number of pollutant molecules per million or billion molecules of air. The EPA sets a National Ambient Air Quality Standard for each criteria pollutant and reports daily conditions with the Air Quality Index (AQI), a 0–500 scale where higher numbers mean more hazardous air. Because ppm is a ratio, 1 ppm equals 0.0001% — a reminder that even small-sounding concentrations can harm health.

Parts per million

1 ppm = 1 part per 1,000,000 = 0.0001% ; ppm = (parts of pollutant / total parts) × 1,000,000

ppm is a concentration ratio, not a mass. To convert ppm to a percent, divide by 10,000 (35 ppm ÷ 10,000 = 0.0035%).

Worked example

An air sample contains carbon monoxide at 35 ppm. Express this concentration as a percentage, and as a simple fraction.

1.Recall the definition: 35 ppm means 35 molecules of CO per 1,000,000 molecules of air.
2.Write it as a fraction: 35 / 1,000,000.
3.Convert to a percent by multiplying the fraction by 100: (35 / 1,000,000) × 100 = 35 / 10,000 = 0.0035%.
4.So 35 ppm = 0.0035%, or the fraction 35 per million.

Answer: 35 ppm = 0.0035% = 35/1,000,000. Even this small percentage of CO is dangerous because CO binds hemoglobin far more strongly than oxygen does.

Checkpoint

Ground-level ozone (O₃) is best classified as which type of pollutant, and why?

Watch out

Keep the "two ozones" straight. Stratospheric ozone (high up) is the protective layer that blocks UV — "good up high." Ground-level ozone (down low) is a harmful secondary pollutant and the key ingredient of smog — "bad nearby." Same molecule, opposite effect depending on altitude.

Checkpoint

Which criteria pollutant is a colorless, odorless gas produced by the incomplete combustion of fuel that is dangerous because it binds to hemoglobin in the blood?

On the exam

Memorize the six criteria pollutants (CO, SO₂, NOₓ, PM, ozone, lead) and which is the odd one out as a secondary pollutant — ground-level ozone. To convert ppm to percent, divide by 10,000; the AQI scale runs 0–500 with higher = worse.

Answer the 2 checkpoints as you read.