Radon is a naturally occurring radioactive gas produced by the decay of uranium in rocks and soil; it seeps into buildings through basements, foundation cracks, and well water, and exposure causes radon-induced lung cancer, the second leading cause of lung cancer in America (AP Enviro Topic 7.5).
Radon is a colorless, odorless radioactive gas that forms when uranium in underground rock and soil decays. Because it's a gas, it doesn't stay put. It moves up through the soil and slips into homes through basements and cracks in walls or foundations. It can also hitch a ride dissolved in groundwater, entering homes that use well water (EK STB-2.F.1).
On the AP exam, radon is one of the three classic natural source indoor air pollutants, alongside mold and dust (EK STB-2.E.4). That "natural source" label matters. Lots of indoor pollutants come from human-made products or combustion, but radon comes straight from geology. The health effect to know is specific: breathing radon over time causes radon-induced lung cancer, which is the second leading cause of lung cancer in America after smoking (EK STB-2.F.2). The danger spots are basements and lower floors, where the heavy gas accumulates, especially in homes built on uranium-rich bedrock.
Radon lives in Unit 7: Atmospheric Pollution, Topic 7.5 (Indoor Air Pollutants) and supports two learning objectives. For 7.5.A (identify indoor air pollutants), you need to classify radon as a natural source pollutant, not human-made and not a combustion product. For 7.5.B (describe the effects of indoor air pollutants), you need both halves of the radon story, which are how it gets in (soil, foundation cracks, well water) and what it does (lung cancer). Radon is one of the cleanest examples of the STB (Sustainability) big idea that pollution exposure depends on where and how people live. Two identical houses can have wildly different radon levels purely because of the rock underneath them.
Keep studying AP® Environmental Science Unit 7
Carbon Monoxide (Unit 7)
CO is radon's partner in the "invisible deadly indoor gas" category, but the mechanisms are totally different. CO is an asphyxiant from incomplete combustion that kills fast by hijacking hemoglobin, while radon is a radioactive gas from geology that kills slowly through lung cancer. MCQs love making you tell them apart.
Mold (Unit 7)
Mold is radon's fellow natural source indoor pollutant in EK STB-2.E.4. If a question asks for a natural indoor pollutant, your options are radon, mold, or dust. Everything else (VOCs, formaldehyde, asbestos insulation) goes in the human-made bucket.
Volatile Organic Compounds (VOCs) (Unit 7)
VOCs off-gas from furniture, carpet, and paneling, so they're human-made. Radon seeps up from bedrock, so it's natural. The pollutant source classification (natural vs. human-made vs. combustion) is the organizing skeleton of Topic 7.5, and radon vs. VOCs is the cleanest contrast.
Radioactive decay and nuclear energy (Unit 6)
Radon exists because uranium-238 in rock undergoes radioactive decay, the same physics behind nuclear power and nuclear waste. Connecting radon to decay chains shows you understand that radioactivity isn't just a power-plant issue; it's happening naturally under your house.
Radon shows up most often in multiple-choice questions that test the EKs almost word for word. A typical stem describes a family in a home built on uranium-rich bedrock worried about a gas seeping through foundation cracks into the basement, and asks you to name the pollutant and its health effect (radon, lung cancer). You may also be asked to classify it as a natural source indoor pollutant or to pick its entry pathways (soil through foundation cracks, dissolved in well water). On FRQs, indoor air pollution appears in real prompts, like the 2018 SAQ on harmful household air pollutants from indoor biomass burning. Radon itself is a combustion-free pollutant, so be careful: if a prompt is about burning fuel indoors, the answer is CO or particulates, not radon. Indoor air quality also pairs with environmental justice framing, since exposure and mitigation access vary across neighborhoods.
Both are invisible, odorless indoor gases that can kill you, which is exactly why the exam tests the difference. Carbon monoxide comes from incomplete combustion (gas stoves, fireplaces, tobacco smoke) and acts as an asphyxiant, binding to hemoglobin about 200 times more strongly than oxygen. Radon comes from natural uranium decay in rock and soil, has nothing to do with combustion, and causes lung cancer over long-term exposure rather than acute poisoning. Quick check: combustion in the stem points to CO; bedrock, basements, or well water points to radon.
Radon is a naturally occurring radioactive gas produced by the decay of uranium in rocks and soils, making it a natural source indoor air pollutant (EK STB-2.E.4).
Radon enters homes by moving up through soil into basements and through cracks in walls or foundations, and it can also arrive dissolved in well water (EK STB-2.F.1).
Long-term radon exposure causes radon-induced lung cancer, the second leading cause of lung cancer in America after smoking (EK STB-2.F.2).
Radon is not a combustion pollutant, so if an exam question describes burning fuel indoors, the answer is carbon monoxide or particulates, not radon.
Radon risk depends on geology, so homes built on uranium-rich bedrock have higher exposure, especially in basements where the gas accumulates.
Radon is a naturally occurring radioactive gas formed by uranium decay in rocks and soil. In APES Topic 7.5, it's a natural source indoor air pollutant that seeps into homes and causes radon-induced lung cancer, the second leading cause of lung cancer in the U.S.
No. Radon has nothing to do with combustion. It comes from uranium decaying in the ground beneath a building. Gas stoves and tobacco smoke produce carbon monoxide and particulates, which are different indoor pollutants in Topic 7.5.
Carbon monoxide is a combustion byproduct and an asphyxiant that binds to hemoglobin roughly 200 times more strongly than oxygen, causing acute poisoning. Radon is radioactive, comes from natural uranium decay in bedrock, and causes lung cancer over long-term exposure.
Radon moves up through the soil and enters through basements and cracks in walls or foundations. It can also enter dissolved in groundwater that reaches homes through wells (EK STB-2.F.1). That's why basements in homes on uranium-rich bedrock have the highest levels.
Radon-induced lung cancer. The CED specifically says it's the second leading cause of lung cancer in America (EK STB-2.F.2), so name lung cancer, not asphyxiation or asthma, when a question asks about radon's effects.
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