AP Environmental Science

## Learning Outcomes

- Explain what natural indoor pollutants are and why they matter for indoor air quality
- Identify the main natural indoor pollutants tested in AP Environmental Science: radon, asbestos, and biological pollutants
- Describe the source, exposure pathway, health effects, and control methods for each pollutant
- Distinguish between radon gas, airborne asbestos fibers, and moisture-linked biological contaminants
- Apply this knowledge to AP-style questions about source, effect, and mitigation

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand natural indoor pollutants, with a focus on the following syllabus points:

- Know that indoor pollutant levels can be higher than outdoor levels because pollutants can accumulate in enclosed spaces
- Identify **radon** as a naturally occurring radioactive gas that can build up indoors and increase lung cancer risk
- Explain how radon enters buildings through cracks, foundations, crawl spaces, sump areas, and sometimes well water
- Know that **asbestos** is a naturally occurring mineral fiber that becomes dangerous mainly when damaged or disturbed
- Identify major asbestos-related diseases, including asbestosis, lung cancer, and mesothelioma
- Recognise **biological pollutants** such as mold, dust mites, pollen, pet dander, and pest allergens as indoor pollutants linked to allergies and asthma
- Understand that moisture control is the main prevention strategy for many biological pollutants
- Distinguish testing and engineering control for radon from non-disturbance or professional removal for asbestos and cleaning or moisture reduction for biological pollutants

## 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. Why can indoor air pollution sometimes be worse than outdoor air pollution?
2. What makes radon especially dangerous in homes, and why can it only be detected by testing?
3. How does asbestos differ from radon in the way people are exposed to it?
4. Name three biological indoor pollutants and explain what condition often allows them to increase.
5. What is the main control strategy for mold problems in buildings?

## Introduction

## Introduction

Indoor air pollution is an important AP Environmental Science topic because it connects human health, pollution sources, and practical solutions. Students often spend a lot of time learning outdoor air pollution, but the exam also expects you to know that harmful pollutants can collect inside homes, schools, and workplaces. In many buildings, especially well-sealed ones, pollutants remain trapped for longer and can reach higher concentrations than outdoors.

Natural indoor pollutants are especially testable because they come from three very different sources. Some come from geology below a building, some from naturally occurring minerals used in building materials, and some from living organisms or their byproducts. On the exam, success often depends on linking each pollutant to its **source**, **health effect**, and **best control method**.

> **Key Term: indoor air pollutant**
> A substance or biological material in indoor air that can harm human health or reduce indoor air quality.

### Why natural indoor pollutants matter

Natural indoor pollutants are not all “natural” in the same way. That is a key distinction students often miss. Radon comes from radioactive decay in soil and rock. Asbestos is a naturally occurring mineral, but it usually becomes an indoor problem because humans used it in building materials. Biological pollutants come from living things such as fungi, dust mites, and bacteria, and they often become a problem when indoor conditions allow them to grow or accumulate.

> **Key Term: natural indoor pollutant**
> An indoor pollutant that originates from natural geological, mineral, or biological sources rather than mainly from synthetic chemicals.

A second point that matters for the exam is exposure pathway. AP questions often ask more than “what is it?” They ask how it gets into the building, how it reaches people, and what type of response makes sense. For example, radon enters as a gas from below the building, so testing and venting are the key responses. Asbestos is dangerous mainly when fibers become airborne, so disturbing the material is the problem. Biological pollutants often increase because of dampness, so moisture control matters more than one-time testing.

Cause-and-effect is central here:

- **Source present**
- pollutant enters or builds up indoors
- people inhale it or react to it
- health effects appear
- specific control method reduces exposure

This sequence is exactly the kind of reasoning used in AP multiple-choice and free-response questions.

> **Test Tip:** For indoor pollutant questions, build your answer in this order: source -> pathway -> health effect -> solution. That structure works well for both MCQs and FRQs.

### Radon: radioactive gas from soil and rock

Radon is the highest-priority natural indoor pollutant for many exam questions. It is a **colorless, odorless, radioactive gas** formed by the decay of uranium in soil and rock. Because it has no smell or visible appearance, people cannot detect it without testing. That makes radon different from many other household hazards.

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

Radon usually enters a building from below. Air pressure inside a house is often lower than the pressure in the soil around the foundation, so the building can draw soil gas inward. Common entry routes include:

- cracks in foundations and basement floors
- construction joints
- gaps around pipes and service lines
- crawl spaces
- sump pits
- sometimes well water

Once radon enters, it can accumulate, especially in basements or lower floors. It is often more common in well-insulated, tightly sealed homes because less outdoor air enters to dilute it.

The main health risk is **lung cancer**. Radon is one of the leading causes of lung cancer, second only to smoking in the United States. The key exam idea is not just “radon is dangerous,” but **why** it is dangerous: radioactive decay releases particles that damage lung tissue when inhaled over long periods.

> **Key Term: radioactive decay**
> The process by which an unstable atom releases energy or particles and changes into a more stable form.

Radon is a good example of chronic rather than immediate risk. A student might think, “If people do not feel sick right away, the danger must be low.” That is wrong. Radon exposure is dangerous because long-term inhalation increases cancer risk even though there may be no short-term warning signs.

Testing is therefore essential. The EPA recommends testing all homes for radon because location, geology, and building design vary too much to guess safely. A commonly cited EPA action level is **4.0 pCi/L**, with lower levels still carrying some risk.

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

The most effective long-term radon solution is usually **sub-slab depressurization**. This system uses pipes and a fan to draw radon-containing soil gas from beneath the building and vent it above the roof before it can enter indoor air.

> **Key Term: sub-slab depressurization**
> A radon mitigation method that uses a pipe-and-fan system to vent radon gas from beneath a building.

Why is this better than simply opening windows? Ventilation may help temporarily, but sub-slab depressurization addresses the source pathway directly. That is why it is more reliable.

> **Test Tip:** If an AP question asks for the best long-term mitigation for radon, choose testing plus a building control such as sub-slab depressurization or sealing entry routes, not just “air fresheners” or “cleaning.”

### Asbestos: mineral fiber hazard in buildings

Asbestos is another major indoor pollutant, but students often confuse it with radon because both are linked to lung disease. The difference is crucial. Radon is a gas that seeps in continuously. Asbestos is a **fibrous mineral** that becomes dangerous mainly when materials containing it are damaged, cut, drilled, or broken.

> **Key Term: asbestos**
> A group of naturally occurring fibrous silicate minerals once widely used in insulation, fireproofing, and building materials.

Asbestos was used in many products because it is durable, heat-resistant, and a good insulator. It appeared in insulation, ceiling and floor tiles, roofing materials, fireproofing products, and other construction materials, especially in older buildings.

The main hazard is inhalation of airborne fibers. If asbestos-containing material remains intact and undisturbed, the immediate risk is much lower. If it becomes **friable**, meaning easily crumbled, fibers can be released into the air and then inhaled.

> **Key Term: asbestos-containing material**
> Building material or product that contains asbestos fibers.

The major health effects are:

- **asbestosis** — scarring of lung tissue
- **lung cancer**
- **mesothelioma** — cancer of the lining around the lungs or abdominal organs
  > **Key Term: mesothelioma**
  > A cancer strongly associated with asbestos exposure, usually affecting the lining of the lungs or abdomen.

One important exam point is the long **latency** of asbestos-related disease. Symptoms may appear decades after exposure, often 20 to 40 years later. That means the risk is easy to underestimate in the short term.

Another key distinction: the solution for asbestos is not usually “remove all of it immediately.” Students often assume removal is always safest. In fact, disturbing intact asbestos can increase the release of fibers. For many situations, the best action is to leave intact material undisturbed and have damaged material assessed or removed by certified professionals.

> **Test Tip:** If a question mentions an older building with intact asbestos tiles, do not assume DIY removal is best. The safer answer is usually monitoring, sealing, or professional abatement if the material is damaged.

### Biological pollutants: mold, mites, dander, and microbes

Biological pollutants include living organisms or their byproducts found indoors. They are common, highly testable, and often linked more to allergies and asthma than to classic poisoning.

> **Key Term: biological pollutant**
> An indoor pollutant consisting of living organisms or their byproducts, such as mold, dust mites, pollen, pet dander, bacteria, or pest waste.

Important examples include:

- mold spores
- dust mites
- pollen
- pet dander
- cockroach or rodent allergens
- bacteria

The key driver for many of these pollutants is **moisture**. This is the main cause-and-effect chain you need:

- leak, flooding, condensation, or poor ventilation
- damp indoor surfaces
- mold growth or increased dust mite habitat
- more allergens or irritants in air
- asthma, allergy, or respiratory symptoms
  > **Key Term: moisture control**
  > Preventing or reducing indoor dampness by fixing leaks, drying wet materials quickly, improving drainage, and managing humidity.

Mold is especially important. It can grow on drywall, wood, carpet, ceiling tiles, fabrics, and even ordinary dust if enough moisture is present. Students sometimes think mold is mainly a “measurement” problem, but the better AP answer is usually that mold is a **building condition** problem. Visible mold often means there is a water issue that must be corrected.

Health effects of biological pollutants include:

- allergic reactions
- asthma attacks
- coughing or wheezing
- respiratory irritation
- worsening symptoms in people with existing lung disease

For sensitive groups such as children, older adults, and people with asthma, these pollutants can be especially harmful.

Dust mites are another common example. They do not usually bite people, but their body parts and waste products can act as allergens. They tend to increase in indoor environments where humidity is high and dust accumulates.

Bacteria can also become indoor pollutants, especially in damp ventilation systems or poorly maintained cooling equipment. A famous example is **Legionella pneumophila**, the bacterium that causes Legionnaires’ disease.

> **Key Term: Legionnaires’ disease**
> A serious respiratory illness caused by Legionella bacteria, often associated with contaminated building water or cooling systems.

The best control strategy for most biological pollutants is not one single air test. It is **source control**, especially moisture control, cleaning, filtration, and ventilation. Fixing leaks, drying wet materials quickly, reducing dust buildup, and maintaining HVAC systems are usually better long-term solutions than routine air sampling alone.

A subtle but important exam distinction: there are no simple universal health standards for indoor mold levels in ordinary building evaluations, so routine mold air sampling is not always the first or best step.

### Comparing radon, asbestos, and biological pollutants

These pollutants are often grouped together on the exam, so compare them directly.

| Pollutant             | Main source                                | Exposure pathway                              | Main health effect                        | Best mitigation                                                 |
| --------------------- | ------------------------------------------ | --------------------------------------------- | ----------------------------------------- | --------------------------------------------------------------- |
| Radon                 | Uranium decay in soil and rock             | Gas enters from below building and is inhaled | Lung cancer                               | Test, seal entry points, sub-slab depressurization              |
| Asbestos              | Mineral fibers in older building materials | Fibers released when disturbed and inhaled    | Asbestosis, lung cancer, mesothelioma     | Leave intact material undisturbed or use professional abatement |
| Biological pollutants | Mold, mites, bacteria, dander, pollen      | Allergens or microbes in indoor air           | Allergies, asthma, respiratory irritation | Moisture control, cleaning, filtration, ventilation             |

This comparison helps with both multiple-choice distractors and short written answers. If the pollutant is a gas from soil, think radon. If the issue is damaged insulation or old tiles releasing fibers, think asbestos. If the scenario involves leaks, dampness, mold, and allergies, think biological pollutants.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Indoor air pollutant levels can be higher than outdoor levels because pollutants may build up in enclosed spaces
- Natural indoor pollutants commonly tested in AP Environmental Science include radon, asbestos, and biological pollutants
- Radon is a colorless, odorless radioactive gas produced by uranium decay in soil and rock
- Radon increases lung cancer risk and must be detected by testing, not by smell or sight
- Common radon entry routes include cracks in foundations, pipe gaps, crawl spaces, and sump areas
- Sub-slab depressurization is a common and effective radon mitigation method
- Asbestos is a naturally occurring fibrous mineral once used in building materials for insulation and fire resistance
- Asbestos is most dangerous when damaged or disturbed material releases airborne fibers
- Major asbestos-related diseases include asbestosis, lung cancer, and mesothelioma
- Biological pollutants include mold, dust mites, pollen, pet dander, bacteria, and pest allergens
- Moisture control is the main prevention method for mold and many other biological indoor pollutants
- Biological pollutants often act as allergens or asthma triggers rather than as classic chemical toxicants

## Key Terms and Concepts

- indoor air pollutant
- natural indoor pollutant
- radon
- radioactive decay
- pCi/L
- sub-slab depressurization
- asbestos
- asbestos-containing material
- mesothelioma
- biological pollutant
- moisture control
- Legionnaires’ disease


Indoor Air Pollutants - Natural indoor pollutants