AP Environmental Science

## Learning Outcomes

- Understand what indoor particulate pollutants are and how they differ from indoor gases
- Explain the difference between **PM10** and **PM2.5** and why particle size matters
- Identify major indoor particulate sources, including smoke, dust, biological particulates, and asbestos
- Describe short-term and long-term health effects of indoor particulate exposure
- Evaluate the best control methods: source control, ventilation, and filtration
- Apply this topic to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

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

- Know that particulate matter is a mixture of tiny solid particles and liquid droplets suspended in air
- Identify the distinction between **PM10** and **PM2.5** by size and depth of penetration into the respiratory system
- Recognise indoor particulate sources such as dust, cooking smoke, tobacco smoke, fireplaces, candles, and biological particles
- Explain that indoor particulate matter comes from both indoor sources and outdoor particles that enter buildings
- Understand why **secondhand smoke** is a major indoor particulate pollutant with serious health effects
- Explain why **asbestos** is especially dangerous when disturbed and inhaled as airborne fibers
- Distinguish particulate pollutants from gaseous indoor pollutants such as carbon monoxide, radon, and VOCs
- Evaluate source control, ventilation, and air cleaning or filtration as ways to reduce indoor particulate exposure

## 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 the difference between **PM10** and **PM2.5**?
2. Why is **PM2.5** usually more hazardous than larger indoor particles?
3. Name three important indoor particulate sources.
4. Why is asbestos not always equally dangerous in every building where it is present?
5. Why is ventilation not always the best response during a wildfire smoke event?

## Introduction

Indoor air pollution matters because people spend most of their time indoors, yet indoor pollutant concentrations can be higher than outdoor levels. For AP Environmental Science, you need to treat indoor particulate pollutants as a specific part of indoor air pollution: they are physical particles suspended in air, not gases.

This distinction matters on the exam. Students often group all indoor pollutants together, but AP questions may ask you to classify pollutants correctly, explain health effects based on particle size, or choose the best control strategy for a given indoor source.

> **Key Term: particulate matter (PM)**
> A mixture of tiny solid particles and liquid droplets suspended in air.

### What indoor particulate pollutants are and why size matters

Indoor particulate pollutants include particles generated inside buildings and particles that enter from outdoors. Common examples are dust, soot, smoke, pollen, mold spores, pet dander, and asbestos fibers. The key idea is that these materials are suspended in air and can be inhaled.

Particle size is one of the most tested distinctions in this topic.

> **Key Term: PM10**
> Inhalable particulate matter with a diameter of **10 micrometers or smaller**.
>
> **Key Term: PM2.5**
> Fine particulate matter with a diameter of **2.5 micrometers or smaller**.

Why does size matter so much? Larger particles are more likely to be trapped in the nose or upper airways. Smaller particles travel deeper. **PM2.5** is more dangerous because it can penetrate far into the lungs, and some very small particles may enter the bloodstream. That means fine particles are linked not only to respiratory problems but also to cardiovascular effects.

A useful cause-and-effect chain is:

- smaller particle size
- deeper penetration into the respiratory system
- greater exposure of delicate lung tissue
- stronger risk of chronic disease and severe health effects

Students sometimes assume that all particles are equally harmful because they are all “dust.” That is too simple. A coarse dust particle and fine smoke particle are both particulates, but the fine smoke particle is usually the greater health risk because of where it can go in the body.

Indoor particulate matter is usually a **primary pollutant** because it is emitted directly into the air rather than formed later by atmospheric reactions.

> **Test Tip:** If an AP question asks which particulate is more dangerous, choose the smaller size class unless the stem gives a special reason not to. The exam often rewards “smaller particle = deeper lung penetration = greater risk.”

### Major indoor sources of particulate pollutants

Indoor particulate matter comes from both indoor generation and outdoor infiltration. This is important because indoor air quality is not controlled only by what people do inside a building. Outdoor conditions can raise indoor particle levels too.

#### Combustion and smoke sources

Combustion is one of the biggest indoor particle sources. Examples include:

- cooking, especially frying, broiling, and searing
- tobacco smoke
- fireplaces and wood stoves
- candles and incense
- kerosene heaters or other fuel-burning appliances
- indoor fires

These processes release soot and fine particles, often in the **PM2.5** range. That makes them especially hazardous.

A common AP distinction is between smoke particles and combustion gases. Smoke is particulate matter. **Carbon monoxide** from incomplete combustion is a gas, not a particulate. **Radon** is also a gas. **VOCs** are gaseous or vapor-phase pollutants. You must classify these correctly.

#### Dust and resuspended material

Dust seems harmless because it is common, but it is still a pollutant. Indoor dust may contain dead skin cells, soil particles, fibers, pollen, hair, and other fine solids. Walking, vacuuming, and cleaning can resuspend settled dust into the air, increasing inhalation.

Dust matters especially for people with asthma, allergies, or other lung conditions. Even if dust particles are larger than smoke particles, repeated exposure can still worsen respiratory health.

#### Biological particulates

Biological particulates are another major category.

> **Key Term: biological particulates**
> Airborne particles of biological origin, such as mold spores, pollen, dust mites, pet dander, and similar materials.

These are often important in indoor allergy and asthma questions. A key exam point is that mold itself is an organism, but the **spores** released into the air are the particulate pollutant. Pet dander is particulate. Pollen is particulate. Dust mites contribute particulate waste and body fragments.

These sources are especially common in damp or poorly maintained buildings.

#### Outdoor particles entering indoors

Buildings are not sealed perfectly. Outdoor particulate matter can enter through doors, windows, cracks, and ventilation systems. This means indoor particulate levels often reflect both indoor activities and outdoor air quality.

That is why ventilation can help in some situations but worsen conditions in others. If outdoor air is cleaner than indoor air, bringing in more outdoor air can reduce indoor pollutant concentration. But if outdoor air contains wildfire smoke or heavy urban particulates, ventilation may increase indoor PM.

### Secondhand smoke and asbestos: high-yield examples

Two indoor particulate pollutants deserve special attention because they are repeatedly emphasized in AP materials: **secondhand smoke** and **asbestos**.

#### Secondhand smoke

> **Key Term: secondhand smoke**
> Smoke inhaled by nonsmokers from burning tobacco products and from smoke exhaled by smokers; also called environmental tobacco smoke.

Secondhand smoke is particulate-rich indoor air pollution. It contains many harmful chemicals and is associated with cancer, heart disease, respiratory illness, and asthma. Children are especially vulnerable because their lungs are still developing and they breathe more air per unit body mass than adults.

Why is secondhand smoke such a strong AP example?

- it is clearly an indoor source
- it is clearly a particulate source
- it has serious, well-known health effects
- it is preventable through source control

That last point is key. The best response to secondhand smoke is not “clean the air later.” It is to stop smoking indoors in the first place.

#### Asbestos

> **Key Term: asbestos**
> A naturally occurring silicate mineral once widely used in building materials, dangerous when airborne fibers are inhaled.

Asbestos is often misunderstood. Students may think “asbestos in a building always means immediate exposure.” That is not quite right. The main risk comes when asbestos-containing material is **damaged, disturbed, or crumbling**, releasing microscopic fibers into the air.

This is why old insulation, ceiling tiles, pipe wrap, or similar materials can become hazardous during renovation, drilling, sanding, demolition, or water damage.

The health effects are severe and often delayed for decades. Exposure is associated with:

- asbestosis
- lung cancer
- mesothelioma

The long delay between exposure and disease is an important chronic-effect idea. People may not notice illness until many years later.

> **Test Tip:** If an AP question asks about the best response to intact asbestos in an older building, do not assume “rip it out immediately” is always best. If it is undisturbed and in good condition, the hazard is lower than if it is broken apart carelessly.

### Health effects and who is most at risk

Indoor particulate pollutants affect both the respiratory and cardiovascular systems. That broader effect is easy to miss because students often think only about the lungs.

Short-term effects can include:

- eye, nose, and throat irritation
- coughing
- wheezing
- asthma attacks
- shortness of breath

Long-term effects can include:

- reduced lung function
- chronic respiratory disease
- heart and blood vessel problems
- lung cancer in some exposures
- premature death in vulnerable people

Fine particles are especially associated with cardiopulmonary risk. That means PM exposure can worsen both lung disease and heart disease.

Some populations are more vulnerable than others:

- children
- older adults
- people with asthma or COPD
- people with heart disease
- people exposed repeatedly in poorly ventilated homes

There is also a major global household air pollution issue linked to burning solid fuels such as wood, dung, crop waste, charcoal, coal, and kerosene in poorly ventilated homes. In many regions, this is a major indoor **PM2.5** source and contributes to pneumonia, COPD, stroke, heart disease, and lung cancer.

This global perspective matters in AP Environmental Science because the course often compares environmental issues across developed and developing settings. In developed buildings, students are often asked about VOCs, radon, tobacco smoke, or asbestos. In less developed household settings, combustion from polluting fuels is often the main indoor particulate issue.

### Reducing indoor particulate exposure: source control, ventilation, and filtration

The best control approach follows a clear hierarchy.

> **Key Term: source control**
> Reducing or removing pollutants at their origin before they spread through the indoor environment.

#### 1. Source control first

Source control is usually the best strategy. If you stop the particles from being produced, you reduce exposure more effectively than trying to remove them after they enter the air.

Examples include:

- do not smoke indoors
- reduce use of candles and incense
- use vented range hoods during cooking
- maintain fuel-burning appliances
- avoid unvented heaters
- reduce dust buildup
- fix dampness problems to prevent mold
- avoid disturbing asbestos-containing materials

This approach works because it cuts the problem off at the start.

#### 2. Ventilate when outdoor air is cleaner

Ventilation can dilute indoor pollutants by increasing fresh-air exchange. But it is not automatically helpful. It depends on outdoor conditions.

If outside air is relatively clean, ventilation lowers indoor pollutant concentration. If outside air is polluted, ventilation may increase indoor particle levels. Wildfire smoke is the clearest example.

So the logic is:

- indoor source high, outdoor air clean → ventilation helps
- outdoor PM high, such as wildfire smoke → ventilation may harm unless paired with good filtration

Students often miss this conditional thinking. AP questions reward it.

#### 3. Use air cleaning and filtration as support

Filtration and air cleaning can reduce indoor particles, especially after source reduction. They are useful, but they are usually not a full substitute for removing the source.

A common principle is that filtration is best as a supplement:

- reduce the source
- ventilate appropriately
- then filter remaining particles

This is especially relevant if a question asks for the **most effective** strategy. The answer is usually not “buy an air cleaner” if a major indoor source is still operating.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Indoor particulate pollutants are tiny solid particles or droplets suspended in indoor air
- Indoor PM comes from both indoor generation and outdoor infiltration
- **PM10** refers to particles **10 micrometers or smaller**
- **PM2.5** refers to particles **2.5 micrometers or smaller**
- **PM2.5** is usually more hazardous because it penetrates deeper into the lungs
- Important indoor particulate sources include cooking, tobacco smoke, fireplaces, candles, dust, pollen, mold spores, pet dander, and asbestos fibers
- Secondhand smoke is a major indoor particulate pollutant linked to cancer, heart disease, and respiratory illness
- Asbestos is especially dangerous when older asbestos-containing materials are damaged or disturbed
- Indoor particulates can cause both respiratory and cardiovascular health effects
- Children, older adults, and people with heart or lung disease are especially vulnerable
- The best control strategy is usually **source control first**, then ventilation when outdoor air is cleaner, then filtration
- Ventilation is not always helpful during high outdoor particulate events such as wildfire smoke

## Key Terms and Concepts

- particulate matter (PM)
- PM10
- PM2.5
- biological particulates
- secondhand smoke
- asbestos
- source control


Indoor Air Pollutants - Indoor particulate pollutants