AP Environmental Science

## Learning Outcomes

- Explain what **radon** is and why it is a major indoor air pollutant
- Describe how radon moves from rocks and soil into buildings
- Understand how inhaled radon increases lung cancer risk
- Identify why smoking greatly increases radon-related harm
- Recall the **EPA action level of 4 pCi/L** and convert it to **Bq/m³**
- Distinguish between short-term and long-term radon tests
- Evaluate practical radon mitigation methods that reduce indoor exposure

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand indoor radon exposure and its health effects, with a focus on the following syllabus points:

- Know that **radon** is a colorless, odorless, radioactive gas produced naturally from the decay of uranium in rocks and soil
- Identify how radon enters homes through foundation cracks, gaps around pipes, crawl spaces, basements, and sometimes groundwater
- Understand that radon is a major indoor air pollutant and the **second leading cause of lung cancer** in the United States after smoking
- Explain how inhaled radon and its decay products expose lung tissue to **alpha radiation**
- Know that smoking and radon exposure together create a much higher lung cancer risk than either factor alone
- Recall that the **EPA recommends action at 4 pCi/L**, which is about **148 Bq/m³**
- Distinguish between **short-term** radon tests and **long-term** radon tests
- Identify mitigation strategies such as ventilation, sealing entry points, and active radon reduction systems

## 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 is radon, and where does it come from?
2. Why is radon considered an indoor air pollutant even though it is naturally occurring?
3. How does radon increase the risk of lung cancer?
4. What is the EPA action level for radon, and what does it mean in Bq/m³?
5. Why are smokers at much greater risk from radon exposure than nonsmokers?

## Introduction

Indoor air pollution matters in AP Environmental Science because it shows that environmental hazards are not limited to outdoor smog or factory emissions. A pollutant can come from the ground beneath a building, build up silently indoors, and cause severe health effects over many years. Radon is one of the clearest examples of that process.

Radon is especially important because it is both **natural** and **dangerous**. That combination often appears in APES questions. Students sometimes assume that “natural” means harmless. Radon proves the opposite. It forms naturally in rocks and soil, but once it accumulates in homes, schools, or workplaces, it becomes a major health risk linked mainly to lung cancer.

> **Key Term: radon**
> A colorless, odorless, radioactive gas produced naturally during the decay of uranium in rocks and soil.

### Why radon becomes an indoor pollution hazard

Radon begins as part of a geologic process. Uranium in rocks and soil decays over time. One step in that decay chain produces radon gas, commonly **radon-222**. Outdoors, radon is usually diluted in the atmosphere, so concentrations are much lower. Indoors, the story changes.

A building can trap radon. Soil gas is drawn upward from the ground and into the structure because indoor air pressure is often slightly lower than the pressure in the soil beneath the building. That pressure difference acts like a weak vacuum. Radon then enters through small openings: foundation cracks, gaps around pipes, floor drains, crawl spaces, and basement walls. In some homes, radon can also enter through well water.

> **Key Term: indoor air pollutant**
> A harmful substance present inside buildings at concentrations high enough to affect health.

This is why radon is such a strong APES example of source-pathway-effect thinking:

- **Source:** uranium-bearing rock and soil
- **Pathway:** movement as soil gas through cracks and openings
- **Exposure:** inhalation in indoor spaces
- **Effect:** lung tissue damage and increased lung cancer risk

Students often confuse radon with pollutants produced by combustion, such as carbon monoxide or particulates. Radon is different. It does not come from burning fuel. It comes from **radioactive decay in the ground**. That makes it a geologic indoor pollutant, not a combustion pollutant.

Radon also tends to accumulate most in the lowest part of a building. Textbook material notes that it can collect at floor level and is often more common in well-insulated homes with limited air exchange. That does not mean radon is “only a basement problem.” If the lowest lived-in level is the first floor, that is the correct place to test.

> **Key Term: radioactive decay**
> The natural process by which an unstable atom loses energy and changes into another form, often releasing radiation.
>
> **Test Tip:** If an APES question asks how a naturally occurring substance becomes an indoor hazard, explain the full chain: uranium in rock or soil decays to radon, radon enters through foundations, accumulates indoors, and is inhaled.

### How radon causes lung cancer

The health danger from radon is not that it smells bad or irritates the throat. In fact, it has no smell and gives no immediate warning. The main danger is radiation exposure after inhalation.

When radon is breathed in, it and its decay products can lodge in the lungs. These materials emit **alpha particle radiation**, which can damage lung cells and DNA. Over time, repeated damage raises the chance that cells will become cancerous. This is why lung cancer is the main cancer clearly associated with radon exposure.

> **Key Term: alpha radiation**
> A form of ionizing radiation made of alpha particles that can damage living tissue when radioactive substances are inhaled or ingested.

This process is a good example of **chronic exposure**. Radon usually does not make someone feel ill right away. Instead, long-term exposure raises risk over years or decades. That is why radon can be especially dangerous: people may live with elevated levels for a long time without knowing it.

Several major health agencies identify radon as the **second leading cause of lung cancer** in the United States, after smoking. EPA estimates link radon to about **21,000 lung cancer deaths each year** in the U.S. Textbook summaries also describe tens of thousands of deaths from lung cancer each year in the United States and Europe.

A common student error is to describe radon as causing “respiratory illness” in a broad way, as if it acts like soot or ozone. On APES, be specific. Radon is associated primarily with **lung cancer risk**, not with smog-type short-term breathing irritation.

### Why smoking makes radon much more dangerous

Radon risk is high on its own, but smoking raises that risk sharply. This is one of the most testable facts in the subtopic.

Cigarette smoke already damages lung tissue and is itself a leading cause of lung cancer. When smoking is combined with radon exposure, the effects are not merely added together in a simple way. The combination greatly increases the chance of lung cancer. CDC guidance states that a person who smokes and is exposed to radon has about a **tenfold greater radon-related lung cancer risk** than a nonsmoker exposed to the same radon level.

The logic matters here. Radon exposes lung tissue to alpha radiation. Smoking adds carcinogens and damages lung defenses. Together, they create a much more dangerous environment for lung cells. That is why public health messages often treat smoking cessation as one of the most effective ways to reduce radon-related harm.

This also helps with comparison questions. Tobacco smoke and radon are both major indoor pollutants in developed countries and both are linked to lung cancer, but they differ in source:

- **Tobacco smoke:** combustion product, human behavior source
- **Radon:** radioactive gas, geologic source
  > **Test Tip:** If a question asks which population faces the highest radon risk, the best answer is usually people who both **smoke** and live in homes with elevated radon levels.

### Testing, action levels, and mitigation

Radon cannot be detected by sight or smell, so **testing is the only way to know** whether a building has a problem. This is one of the most important applied points for the exam.

There are two main types of tests:

- **Short-term tests** measure radon over about **2 to 90 days**
- **Long-term tests** measure radon for **more than 90 days**

Long-term tests are better for estimating year-round average exposure because radon levels can vary with season, weather, and ventilation. Short-term tests are useful for quick screening, but they are less reliable as a picture of annual exposure.

Students also need to know **where** to test. The correct location is the **lowest lived-in level** of the building. That means not always the basement. If the basement is unfinished and rarely occupied, the first floor may be more appropriate.

> **Key Term: radon testing**
> The measurement of indoor radon concentration using a short-term or long-term test device.

The EPA recommends fixing a home if radon levels are **4 pCi/L or higher**.

> **Key Term: pCi/L**
> Picocuries per liter, a unit commonly used in the United States to measure radon concentration in air.

The metric unit often used internationally is **becquerels per cubic meter**.

> **Key Term: Bq/m³**
> Becquerels per cubic meter, an SI unit used to measure radon concentration in air.

The unit conversion is:

- **1 pCi/L = 37 Bq/m³**

Worked example:

If a home tests at **4 pCi/L**, convert to Bq/m³.

Step 1: Write the conversion  
4 pCi/L × 37 Bq/m³ per pCi/L

Step 2: Calculate  
4 × 37 = 148

Step 3: State the answer  
4 pCi/L = 148 Bq/m³

This is often rounded to **150 Bq/m³**.

The World Health Organization recommends a reference level of **100 Bq/m³** where possible and says it should not exceed **300 Bq/m³**. That does **not** replace the EPA figure in U.S.-focused questions. Use the unit system and guidance asked for in the question.

One subtle point students miss: **4 pCi/L is an action level, not a safe threshold**. Risk exists below that level too. EPA also suggests considering action between **2 and 4 pCi/L** because there is no known completely safe level of radon exposure.

Mitigation can involve simple textbook methods such as:

- increasing ventilation with outdoor air
- sealing basement foundations
- sealing concrete slab floors

In practice, the most effective common method is often **sub-slab depressurization**, which uses a vent pipe and usually a fan to draw radon-laden soil gas from beneath the building and vent it above the roofline before it enters indoor air.

> **Key Term: radon mitigation**
> Actions taken to lower indoor radon levels, such as sealing entry routes, improving ventilation, or installing active radon reduction systems.

After mitigation, the building should be retested. This checks whether radon levels have actually been reduced.

A final high-yield distinction: homes built with **radon-resistant features** may still need testing. Reduced risk is not the same as zero risk.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Radon is a colorless, odorless, radioactive indoor air pollutant
- Radon forms naturally from the decay of uranium in rocks and soil
- Radon enters buildings through cracks, gaps around pipes, crawl spaces, floor drains, and basements
- Outdoor radon is usually diluted, but indoor radon can accumulate, especially in lower building levels
- Radon is the second leading cause of lung cancer in the United States after smoking
- Inhaled radon and its decay products damage lung tissue through alpha radiation
- Smoking greatly increases radon-related lung cancer risk
- Testing is the only reliable way to detect radon exposure
- Short-term tests give quick results, but long-term tests better estimate annual exposure
- The EPA action level is 4 pCi/L, equal to about 148 Bq/m³
- Levels below 4 pCi/L can still carry risk
- Mitigation methods include ventilation, sealing entry points, and active radon reduction systems

## Key Terms and Concepts

- radon
- indoor air pollutant
- radioactive decay
- alpha radiation
- radon testing
- pCi/L
- Bq/m³
- radon mitigation


Indoor Air Pollutants - Radon and lung cancer