AP Environmental Science

## Learning Outcomes

- Understand why **radon** is a major indoor air pollutant in developed countries
- Explain how radon forms and why it enters homes from the ground
- Identify the main entry pathways, including cracks, joints, sump pits, and pipe openings
- Understand how pressure differences and the **stack effect** pull radon indoors
- Distinguish between the main radon source, the entry pathway, and the health effect
- Recall the **EPA action level of 4 pCi/L** and the main mitigation methods
- Apply this knowledge to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand radon entry into homes, with a focus on the following syllabus points:

- Know that radon is a naturally occurring, colorless, odorless **radioactive gas** formed from the decay of uranium in rocks and soil
- Identify **soil gas** beneath homes as the main source of indoor radon in most buildings
- Explain how radon enters through **foundation cracks and openings**, including joints, sump holes, crawl spaces, and gaps around pipes
- Understand how **pressure differences** between indoor air and soil gas drive radon into homes
- Know that the **stack effect** helps create lower pressure in basements and lower floors
- Distinguish the main source of radon from the less common contribution from **private well water**
- Explain why inhaled radon raises **lung cancer** risk through exposure to **alpha radiation**
- Recall that the **EPA recommends action at 4.0 pCi/L or higher** and that **sub-slab depressurization** is the main mitigation method

## 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 the soil beneath a house usually the main source of indoor radon?
3. How does the stack effect help radon enter a home?
4. Name four common radon entry pathways into a building.
5. What is the EPA action level for indoor radon, and what is the main mitigation system used in homes?

## Introduction

Radon is one of the most tested indoor air pollutants in AP Environmental Science because it links geology, atmospheric movement, building design, and human health. Unlike photochemical smog or acid rain, radon is not mainly an outdoor pollution problem created by human industry. It is a **naturally occurring indoor pollutant** that can accumulate inside homes, especially in lower levels.

This matters for the exam because students often remember only that “radon comes through cracks.” That is incomplete. The stronger explanation is a full cause-and-effect chain: uranium in rocks and soil decays, producing radon gas; radon moves through the ground as soil gas; pressure differences pull that gas into buildings through openings; radon builds up indoors; long-term inhalation increases lung cancer risk.

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

### How radon forms and why it is an indoor pollutant

Radon begins with radioactive decay in the geosphere. Uranium occurs naturally in some rocks and soils, especially in areas with granite and other uranium-bearing materials. As uranium decays, it eventually produces radium, and radium decays into radon gas. Because radon is a gas, it can move out of rock and soil pore spaces and into the surrounding air or groundwater.

This is why radon is different from many air pollutants students study elsewhere in Unit 7. It is not mainly emitted from tailpipes, smokestacks, or combustion. It is produced naturally underground. The AP distinction is important: radon is a **primary indoor pollutant** from a natural geologic source, not a secondary smog product.

Why does this become an indoor problem rather than simply dispersing outdoors? Outdoors, radon is usually diluted quickly into the atmosphere. Inside buildings, air exchange is slower. Well-insulated homes can trap the gas, allowing concentrations to rise. Because people spend much of their time indoors, chronic exposure matters even when the pollutant is invisible and odorless.

> **Key Term: soil gas**
> Gas present in the pore spaces between soil particles and rock fragments; radon commonly moves into homes as part of this soil gas.

The textbook emphasis that indoor air pollution can be **2 to 5 times worse than outdoor levels** helps explain why radon matters so much. A pollutant that is harmlessly diluted outside may become dangerous when enclosed indoors for long periods.

Students also need to separate three ideas clearly:

- **Source:** uranium and radium in rocks and soil
- **Pathway:** openings and pressure-driven movement into the building
- **Effect:** increased lung cancer risk after inhalation

Missing this distinction leads to weak FRQ answers.

> **Test Tip:** If an exam question asks where indoor radon mainly comes from, the best answer is usually **soil beneath the home**, not “air pollution,” “household cleaners,” or “outdoor smog.”

### How radon enters homes: pathways and pressure-driven flow

The most important idea here is that radon usually does not just “leak randomly” into a house. It is often **drawn** in. In most homes, the lower part of the building can have slightly lower air pressure than the surrounding soil. When that happens, the house acts like a weak vacuum, pulling radon-bearing soil gas through any available opening.

Common entry pathways include:

- cracks in concrete slab floors
- cracks in foundation walls
- construction joints
- gaps around service pipes
- sump pits or sump holes
- floor drains
- crawl spaces
- wall cavities

Basements are especially vulnerable because they are below grade and directly in contact with the surrounding soil. Slab-on-grade homes can also have radon entry, but the lower structure still matters because it is closest to the source.

> **Key Term: radon entry pathways**
> The physical openings through which radon-bearing soil gas enters a building, such as foundation cracks, joints, gaps around pipes, sump pits, and crawl spaces.

A useful way to think this through is with a simple chain:

1. Radon is generated in soil and rock.
2. The gas moves through pore spaces underground.
3. The house has a slightly lower pressure than the soil nearby.
4. Soil gas moves toward lower pressure.
5. Radon enters through cracks and openings.
6. It accumulates in enclosed indoor air.

Notice that the opening alone is not the full explanation. A crack is only the route. The pressure difference is what drives the movement.

Students sometimes assume that visible cracks are the whole problem and that sealing them must fully solve it. In reality, openings can be tiny, hidden, or spread across many locations. More importantly, the pressure gradient can keep drawing gas inward unless that pressure problem is addressed directly.

Another point that appears in exam material is that radon can also enter from **groundwater**. This is most relevant in homes using private wells. Radon dissolved in groundwater can be released into indoor air during showering, washing, and other water use. However, this is usually a **secondary** source compared with soil gas.

> **Key Term: private well radon source**
> Radon dissolved in groundwater from a private well that can be released into indoor air during household water use.

In most homes, water contributes only a small fraction of indoor radon compared with soil gas. That means if a multiple-choice question asks for the major source, choose the ground beneath the house unless the question specifically centers on private wells.

### Why stack effect and basements matter

The **stack effect** is one of the highest-value ideas in this subtopic because it connects building physics to pollution movement. Warm indoor air rises. As it rises and leaves the upper parts of a building through small leaks and openings, the lower part of the house can be left at slightly lower pressure. That lower pressure then helps pull soil gas inward from below.

> **Key Term: stack effect**
> The movement of warm indoor air upward and out of a building, which lowers pressure in the lower part of the structure and can pull radon-bearing soil gas indoors.

This explains why basements and lower floors are often the first places where radon accumulates. There are two linked reasons:

- they are closest to the source in the soil
- they are where the pressure-driven entry often occurs

This also explains why well-sealed or well-insulated homes may have higher radon levels. Better insulation reduces air exchange with the outdoors, so once radon enters, it can remain trapped more easily.

The pollutant’s health effect is tied to inhalation, not skin contact. When radon is breathed in, its radioactive decay products can lodge in lung tissue. These decay products emit **alpha radiation**, which damages cells and increases the chance of lung cancer over time. In the United States, radon is the **second leading cause of lung cancer after smoking**.

> **Key Term: alpha radiation**
> A type of radioactive emission from radon decay products that can damage lung tissue when inhaled.

Students should be precise here: radon exposure raises cancer risk mainly through **inhalation of radon and its decay products**, not through simple external exposure in air around the body.

There is also an important interaction with smoking. Smoking already damages the lungs and greatly raises lung cancer risk. Radon exposure adds to that risk, which is why the combination is especially dangerous.

> **Test Tip:** On FRQs, explain radon health effects as a chain: **inhalation → radioactive decay products in lungs → alpha radiation damages tissue → higher lung cancer risk**. That scores better than just writing “radon is bad for health.”

### Testing, action levels, and mitigation

Because radon is invisible and odorless, you cannot detect it by smell or sight. A home must be **tested**. This is a frequent exam point because students may confuse radon with pollutants that can be directly noticed, like smoke or mold.

The key measurement unit is **pCi/L**, which stands for picocuries per liter of air. The **EPA action level is 4.0 pCi/L**. If indoor radon is confirmed at or above this level, mitigation is recommended.

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

The most important mitigation method to know is **sub-slab depressurization**. This system usually uses a pipe and fan to pull radon-bearing soil gas from beneath the house and vent it safely above the roofline before it can enter the living space.

> **Key Term: sub-slab depressurization**
> A radon mitigation system that uses a pipe-and-fan setup to draw soil gas from beneath a building and vent it outdoors above the roof.

This is more effective than simply sealing cracks alone. Sealing helps, but mainly as a supporting strategy. It may improve the efficiency of the mitigation system and reduce air leakage, yet by itself it often does not stop the pressure-driven flow that brings radon in.

Other mitigation approaches include:

- increasing indoor ventilation with outdoor air
- sealing basement foundations and slab cracks
- using submembrane depressurization in crawl spaces
- addressing radon in private well water if that source is relevant

A good AP comparison is:

- **Sealing** = reduces some entry routes, but may not solve the driving force
- **Sub-slab depressurization** = addresses the driving force by changing pressure and redirecting soil gas

That distinction is a common place where students lose marks.

Here is a model FRQ-style response idea:

> A homeowner should first test the house for radon because the gas is colorless and odorless. If levels are at or above **4 pCi/L**, the best action is usually **sub-slab depressurization**, which vents radon-bearing soil gas from beneath the house before it enters indoor air. Sealing foundation cracks can support the system, but sealing alone is often not enough. If the house uses a private well, groundwater should also be considered as a secondary radon source.

One more misconception to avoid: radon-resistant features in newer homes can reduce risk, but they do not remove the need for testing. Geology and building conditions still vary.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Radon is a naturally occurring radioactive gas formed during the decay of uranium and radium in rocks and soil
- Outdoor radon is usually diluted, but indoor radon can accumulate because air exchange is limited
- The main source of indoor radon in most homes is **soil gas beneath the building**
- Common radon entry pathways include foundation cracks, construction joints, gaps around pipes, sump pits, drains, crawl spaces, and wall cavities
- Pressure differences between indoor air and surrounding soil help draw radon into homes
- The **stack effect** helps create lower pressure in the lower part of a building, especially basements
- Basements and other ground-contact spaces are at greatest risk because they are nearest the source and common entry zones
- Radon from **private well water** can contribute to indoor air, but it is usually a smaller source than soil gas
- Radon increases lung cancer risk because inhaled radon decay products emit **alpha radiation** in lung tissue
- The **EPA action level is 4.0 pCi/L**
- The main mitigation method is **sub-slab depressurization**
- Sealing cracks helps but is usually a supporting measure rather than a complete solution by itself

## Key Terms and Concepts

- radon
- soil gas
- radon entry pathways
- private well radon source
- stack effect
- alpha radiation
- pCi/L
- sub-slab depressurization


Indoor Air Pollutants - Radon entry into homes