AP Environmental Science

## Learning Outcomes

- Understand how **radon-222** forms in the **uranium-238 decay series**
- Identify **radium-226** as the immediate source of radon-222
- Explain why radon can move from soil and rock into buildings
- Distinguish between the **source** of radon and its **pathway** into indoor air
- Describe why radon accumulates most in basements and crawl spaces
- Recall the main health risk from inhaled radon and radon progeny
- Interpret the common units **pCi/L** and **Bq/m³**, including the EPA action level

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand indoor radon as a naturally occurring indoor air pollutant, with a focus on the following syllabus points:

- Know that **radon-222** is a naturally occurring radioactive gas in the **uranium-238 decay series**
- Identify **radium-226** as the immediate parent isotope that decays to radon-222
- Understand that radon forms in **soil, rock, and sometimes groundwater**, then enters buildings mainly through **soil gas infiltration**
- Explain why radon is considered an indoor pollutant even though its origin is natural
- Distinguish between **radon gas** and **radon progeny**, which are the main inhalation hazard
- Recall that radon commonly accumulates in **basements, crawl spaces, and lower floors**
- Know that **radon-222 has a half-life of about 3.8 days**
- Recall the **EPA action level of 4.0 pCi/L**, about **148 Bq/m³**

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What radioactive parent isotope produces radon-222 directly?
2. Why can radon enter houses more easily than uranium or radium?
3. What is the main source of residential indoor radon: soil gas, furniture, or outdoor smog?
4. Why are basements and crawl spaces often the parts of a house with the highest radon concentration?
5. What indoor radon level does the EPA recommend fixing?

## Introduction

Radon is one of the most testable indoor air pollutants in AP Environmental Science because it links geology, radioactivity, indoor air quality, and human health. Unlike pollutants such as carbon monoxide or VOCs, radon is not mainly produced by appliances, vehicles, or industry. Its origin is natural. That is exactly why students sometimes miss the key point: **natural** does not mean **safe**.

For the exam, you need more than the statement that radon is a radioactive gas. You need the full cause-and-effect chain: uranium in rocks and soil decays over time, one step in that decay series produces radium-226, radium-226 decays into radon-222, and radon as a gas can move through pore spaces in soil and enter buildings. Once indoors, especially in enclosed lower levels, it can build up to harmful concentrations.

> **Key Term: radon-222**
> A naturally occurring radioactive gas in the uranium-238 decay series, formed directly from radium-226 and important as an indoor air pollutant.

### How radon-222 forms: the uranium-238 decay series

To understand radon origin properly, start with the long radioactive decay chain. **Uranium-238** occurs naturally in many rocks and soils. It does not become radon in one single step. Instead, it decays through a sequence of daughter isotopes over time. One of the most important later stages in that chain is **radium-226**, and that is the immediate parent of radon-222.

The key relationship you need is:

- uranium-238 → ... → radium-226 → radon-222

The AP exam may ask you to identify the origin of radon, not just describe radon itself. A weak answer would say “radon comes from uranium.” A stronger answer would say “radon-222 forms in the uranium-238 decay series, with radium-226 as its immediate source.” That distinction matters because it shows you know the chain, not just the starting material.

> **Key Term: uranium-238 decay series**
> A sequence of radioactive decay steps in which uranium-238 changes through multiple daughter isotopes, eventually leading to stable lead.
>
> **Key Term: radium-226**
> A radioactive isotope in the uranium-238 decay series that decays directly to radon-222.

Radium-226 decays by **alpha decay**. In simplified form:

- 226Ra → 222Rn + alpha

This means the radium nucleus emits an alpha particle and changes into radon-222. Students do not need advanced nuclear chemistry here, but they do need to know that alpha decay is the mechanism that produces radon from radium.

> **Key Term: alpha decay**
> A type of radioactive decay in which an unstable nucleus emits an alpha particle, reducing its mass and forming a different element.

This process happens naturally in geologic materials. Granite is often used as a textbook example because it may contain uranium-bearing minerals, but radon is not limited to granite alone. The deeper principle is that many soils and rocks contain trace amounts of uranium and radium, so radon can be produced in many locations.

A common student mistake is to confuse radon with pollutants formed by chemical reactions in air, such as ozone or photochemical smog. Radon is not a secondary pollutant produced by sunlight acting on exhaust gases. It is a **radioactive geologic pollutant** produced in the ground.

> **Test Tip:** If an FRQ asks you to explain the origin of radon-222, make sure your answer includes both the **uranium-238 decay series** and **radium-226**. Naming only “uranium” is often too vague.

### Why radon becomes an indoor pollutant: gas movement and entry pathways

The most important reason radon is a residential pollutant is simple: it is a **gas**. Uranium and radium are solids in rock and soil. They stay where the minerals are. Radon-222, by contrast, is a noble gas, so once it forms, it can escape from mineral grains and move through spaces between soil particles.

> **Key Term: soil gas infiltration**
> The movement of gases from soil into a building through cracks, openings, or other entry points in the foundation.

This is the source-pathway distinction you must know:

- **Source:** radioactive decay in soil and rock
- **Pathway:** radon gas moving through soil and entering buildings

Many exam answers lose marks because they blur these together. The source is not “the basement.” The basement is where radon may accumulate, not where it is produced. Radon is mainly produced in the **ground beneath and around the building**.

Once formed, radon can move through:

- pore spaces in soil
- fractures in rock
- cracks in foundations
- construction joints
- gaps around pipes
- sump openings
- crawl spaces

Buildings often have slightly lower air pressure than the surrounding soil, especially when warm indoor air rises and escapes from upper parts of a house. This can help draw soil gas inward. You do not need a deep physics explanation for APES, but you should understand that houses can pull in gases from the ground.

The **most important source of indoor radon is soil gas infiltration**. Building materials and well water can also contribute, but they are usually secondary.

That ranking matters. If a multiple-choice option says radon in homes comes mainly from furniture, carpets, or car exhaust, that is wrong. Those are more relevant to VOCs or other indoor pollutants. Radon is mainly geologic.

Well water can matter because groundwater moving through uranium-bearing rock may dissolve radon. When that water enters a home and is used in showers, washing, or cooking, some radon can be released into indoor air. Still, on most AP-style questions, **soil gas** is the main answer.

> **Key Term: pathway**
> The route by which a pollutant moves from its source to a place where exposure can occur.
>
> **Test Tip:** In short-answer and FRQ responses, use the phrase “**mainly through cracks and openings in the foundation**.” That wording is specific and usually scores better than simply writing “it gets into the house.”

### Why radon accumulates indoors and why the health risk is serious

Radon exists outdoors too, but the reason it becomes more dangerous indoors is **accumulation**. Outdoors, the atmosphere is huge and air mixes continuously. Indoors, air exchange is much more limited. This is especially true in modern, well-insulated buildings.

The textbook point that radon is more prevalent in well-insulated homes is not saying insulation creates radon. It means less fresh-air exchange can let radon build up once it enters. Lower levels of buildings are most affected because they are closest to the source and nearest to common entry routes.

> **Key Term: half-life**
> The time required for half of the atoms in a radioactive sample to decay.

Radon-222 has a half-life of about **3.8 days**. That is long enough for it to move through soil and enter a house before decaying. If its half-life were extremely short, it would decay before traveling far. If it were extremely long, it could spread even more widely. The 3.8-day half-life is one reason it is such an effective indoor pollutant.

Students sometimes ask: if radon decays, why is that bad? The answer is that the health risk is tied strongly to the decay products, often called **radon progeny**. After radon-222 decays, it forms short-lived radioactive daughters such as polonium-218, lead-214, bismuth-214, and polonium-214. These particles can attach to dust or aerosols, be inhaled, and lodge in the lungs.

> **Key Term: radon progeny**
> The short-lived radioactive decay products formed after radon-222 decays; these are the main source of radiation dose to the lungs.

The key health idea is this:

- radon gas is inhaled
- radon decays into progeny
- progeny emit alpha radiation
- alpha radiation damages lung tissue
- repeated exposure raises lung cancer risk

This is why radon is the **second leading cause of lung cancer in the United States after smoking**. It also explains why smokers face much higher risk from radon exposure: the effects of tobacco smoke and alpha-radiation damage combine.

A common exam pitfall is to write “radon causes skin cancer because it is radioactive.” That is not the main issue in indoor air questions. The major risk is **lung cancer from inhalation**.

The concentration of radon is commonly reported in **picocuries per liter (pCi/L)** in the United States and **becquerels per cubic meter (Bq/m³)** internationally. The EPA action level is **4.0 pCi/L**, which is about **148 Bq/m³**.

> **Key Term: pCi/L**
> Picocuries per liter, a common unit used in the United States to measure radon concentration in indoor air.
>
> **Key Term: Bq/m³**
> Becquerels per cubic meter, an international unit used to measure radon concentration in air.

You do not usually need to convert between these on the AP exam unless the value is already given, but you should recognize both units and know the EPA benchmark.

Risk exists below 4.0 pCi/L as well, but the EPA recommends mitigation at or above that level. The practical testing rule is to test the **lowest level suitable for occupancy**, often the basement if it is used or could be used as living space.

### How to explain radon-222 origin in exam answers

This topic often appears in small but easy-to-miss ways. You might get a multiple-choice question asking where radon comes from, or an FRQ asking you to explain why a naturally occurring pollutant can become dangerous indoors. Strong answers are clear, sequenced, and specific.

A strong one-sentence answer would be:

Radon-222 is a radioactive gas formed when radium-226 in the uranium-238 decay series undergoes alpha decay in soil and rock, after which the gas moves through soil and enters homes through cracks and other foundation openings.

Notice what makes that answer strong:

- names **radon-222**
- names **radium-226**
- places it in the **uranium-238 decay series**
- explains **how** it forms
- explains **how** it gets indoors

A weaker answer would be:

Radon is a gas that comes from the ground and can cause cancer.

That is partly true, but it misses the chain and pathway details that APES often rewards.

Here is a worked exam-style example.

> **Worked Example: FRQ-style prompt**
>
> Explain the origin of indoor radon and why it is a human health concern.
>
> **Answer:**
> Radon-222 forms naturally in the uranium-238 decay series when radium-226 undergoes alpha decay in soil and rock. Because radon is a gas, it can move through pore spaces in soil and enter buildings mainly through cracks and openings in foundations, especially in basements and crawl spaces. Indoors it can accumulate when ventilation is limited. Its decay products, or radon progeny, can be inhaled and emit alpha radiation in the lungs, increasing lung cancer risk.

Common student errors on this topic include:

- saying radon is mainly produced by smoking, vehicles, or combustion
- forgetting that **radium-226** is the immediate parent
- confusing the **source** with the **entry pathway**
- describing the health effect only as “radioactivity is dangerous” without linking it to lungs and cancer
- assuming only old homes have radon

Any home can have elevated radon if the geology and entry routes allow it.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Radon-222 is a naturally occurring **radioactive gas**
- It forms in the **uranium-238 decay series**
- **Radium-226** is the immediate parent isotope of radon-222
- Radium-226 produces radon-222 by **alpha decay**
- Radon differs from uranium and radium because it is a **gas** and can move through soil
- The main indoor source of residential radon is **soil gas infiltration**
- Radon commonly enters buildings through **cracks, joints, and gaps around pipes**
- Building materials and well water can contribute radon, but they are usually secondary sources
- Radon often accumulates in **basements, crawl spaces, and lower floors**
- Radon-222 has a **half-life of about 3.8 days**
- The main health risk comes from inhaled **radon progeny** emitting alpha radiation in the lungs
- The EPA action level is **4.0 pCi/L**, about **148 Bq/m³**

## Key Terms and Concepts

- radon-222
- uranium-238 decay series
- radium-226
- alpha decay
- soil gas infiltration
- pathway
- half-life
- radon progeny
- pCi/L
- Bq/m³


Indoor Air Pollutants - Radon-222 origin