---
title: "Radon Gas — AP Environmental Science Definition & Review"
description: "Radon is a radioactive gas that seeps from uranium-rich rock into basements and well water. APES Topic 7.5's top natural indoor air pollutant and the #2 cause of lung cancer."
canonical: "https://fiveable.me/ap-enviro/key-terms/radon-gas"
type: "key-term"
subject: "AP Environmental Science"
unit: "Unit 7"
---

# Radon Gas — AP Environmental Science Definition & Review

## Definition

Radon is a naturally occurring radioactive gas that seeps up from uranium-bearing soil and rock into homes through basements, foundation cracks, and well water; in AP Environmental Science it's the go-to example of a natural indoor air pollutant, and exposure is the second leading cause of lung cancer in the U.S.

## What It Is

[Radon](/ap-enviro/key-terms/radon "fv-autolink") is a radioactive gas produced underground when uranium in rock and [soil](/ap-enviro/unit-1/terrestrial-biomes/study-guide/itE0pooQYg0jGiYtQnws "fv-autolink") decays. You can't see it, smell it, or taste it, which is exactly why it's dangerous indoors. The CED (STB-2.F.1) names three entry routes you should know cold: it moves up through soil and enters through the basement, it slips in through cracks in walls or the foundation, and it can arrive dissolved in groundwater pumped into the house through a well.

Once inside, radon and its [decay](/ap-enviro/unit-6/nuclear-power/study-guide/6cp8hJAGRndDsFGLiCIq "fv-autolink") products get inhaled and irradiate lung tissue, which is why STB-2.F.2 flags radon-induced lung cancer as the second leading cause of lung cancer in America (smoking is first). On the source side, the CED classifies radon as a *natural* indoor air pollutant alongside mold and dust (STB-2.E.4). That natural-vs-human-made distinction is one of the most common ways Topic 7.5 gets tested.

## Why It Matters

Radon lives in **[Unit 7](/ap-enviro/unit-7 "fv-autolink"): Atmospheric Pollution, Topic 7.5 (Indoor Air Pollutants)** and supports two learning objectives. For **[AP Enviro](/ap-enviro "fv-autolink") 7.5.A** (identify indoor air pollutants), radon is your textbook natural-source example, grouped with mold and dust against human-made pollutants like VOCs and formaldehyde. For **AP Enviro 7.5.B** (describe the effects of indoor air pollutants), radon is the cancer example, while carbon monoxide is the asphyxiant example. It also matters because indoor air pollution is often worse than outdoor air pollution, and radon shows why. The hazard depends on geology under the house, not on anything anyone emitted. For the full pollutant lineup, head to the 7.5 Indoor Air Pollutants study guide.

## Connections

### [Carbon Monoxide (Unit 7)](/ap-enviro/key-terms/carbon-monoxide)

Radon and CO are the two invisible, odorless killer gases of [Topic 7.5](/ap-enviro/unit-7/indoor-air-pollutants/study-guide/y1B4lwSL0xpAcpSCHRjj "fv-autolink"), but they harm you in completely different ways. CO is an asphyxiant that blocks oxygen transport right now; radon is a radioactive carcinogen that damages lung cells over years.

### [Radon Mitigation (Unit 7)](/ap-enviro/key-terms/radon-mitigation)

Because radon enters from below, fixes target the entry pathway. Sealing foundation cracks and venting soil gas away from the house cuts exposure, which is why elevated basement readings are the trigger for [mitigation](/ap-enviro/key-terms/mitigation "fv-autolink").

### [Volatile Organic Compounds (VOCs) (Unit 7)](/ap-enviro/key-terms/volatile-organic-compounds-vocs)

VOCs are radon's human-made counterpart. They off-gas from furniture, carpets, and paneling, while radon seeps up from rock. Sorting [pollutants](/ap-enviro/unit-5/clearcutting/study-guide/z93clkKj7xsyG7zZPAtG "fv-autolink") into natural vs. human-made bins is exactly what LO 7.5.A asks you to do.

### Earth's Geology and Soil (Unit 4)

Radon risk is a geology story. Homes built on uranium-rich bedrock like granite sit on a continuous radon source, so the rock beneath a region predicts indoor exposure before a single building goes up.

## On the AP Exam

Radon shows up almost entirely in multiple-choice land, usually in two flavors. Scenario stems give you a setting (a family in a home built on granite bedrock with high uranium content, or a homeowner finding elevated basement readings) and ask you to identify the health risk or explain the mechanism. The mechanism answer is that inhaled radioactive gas damages lung tissue and causes lung cancer. Classification stems ask you to sort radon as a natural-source pollutant versus human-made ones like VOCs or asbestos insulation. No released FRQ has used radon verbatim, but it fits FRQ prompts about identifying an indoor pollutant, describing its health effect, and proposing a mitigation strategy, so be ready to write all three steps in two sentences.

## radon gas vs Carbon monoxide

Both are colorless, odorless indoor gases, so they blur together fast. The split is source and mechanism. Carbon monoxide comes from combustion (furnaces, stoves) and is classified as an asphyxiant that prevents your blood from carrying oxygen, causing acute symptoms or death. Radon comes from natural uranium decay in rock and soil and is a radioactive carcinogen, so its damage is chronic, showing up as lung cancer after long-term exposure. If the question says 'combustion' or 'asphyxiant,' think CO; if it says 'soil,' 'bedrock,' 'well water,' or 'lung cancer,' think radon.

## Key Takeaways

- Radon is a naturally occurring radioactive gas produced by the decay of uranium in rock and soil, making it a natural-source indoor air pollutant alongside mold and dust (STB-2.E.4).
- Radon enters homes three ways: rising up through soil into basements, slipping through cracks in walls or foundations, and dissolving in groundwater that enters through wells (STB-2.F.1).
- Exposure to radon causes radon-induced lung cancer, the second leading cause of lung cancer in America after smoking (STB-2.F.2).
- Radon is a carcinogen with chronic effects, while carbon monoxide is an asphyxiant with acute effects; the exam loves making you tell these two invisible gases apart.
- Radon risk depends on local geology, so homes built on uranium-rich bedrock like granite face higher exposure regardless of how the house was built.

## FAQs

### What is radon gas in AP Environmental Science?

Radon is a radioactive gas from the natural decay of uranium in soil and rock. In APES Topic 7.5 it's the prime example of a natural indoor air pollutant, entering homes through basements, foundation cracks, and well water.

### Is radon a human-made pollutant?

No. The CED explicitly classifies radon as a natural-source indoor air pollutant (STB-2.E.4), grouped with mold and dust. Human-made indoor pollutants include VOCs from furniture and carpets and formaldehyde from building materials.

### Does radon cause the most lung cancer in the US?

Not quite. Radon is the second leading cause of lung cancer in America; smoking is first. That 'second leading cause' phrasing comes straight from the CED (STB-2.F.2) and is a favorite MCQ detail.

### How is radon different from carbon monoxide?

Radon is a natural radioactive gas from uranium decay that causes lung cancer over long-term exposure. Carbon monoxide comes from combustion and is an asphyxiant that blocks oxygen transport, causing acute harm. Different source, different mechanism, different timescale.

### How does radon get into a house?

Three routes, all worth memorizing: it moves up through soil into the basement, it enters through cracks in walls or the foundation, and it arrives dissolved in groundwater pumped in through a well. That's why elevated readings almost always show up in the basement first.

## Related Study Guides

- [7.5 Indoor Air Pollutants](/ap-enviro/unit-7/indoor-air-pollutants/study-guide/y1B4lwSL0xpAcpSCHRjj)

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