AP Environmental Science

## Learning Outcomes

- Identify the main source categories of indoor air pollutants
- Distinguish clearly between a **pollutant** and its **source**
- Explain how combustion sources, household products, building materials, biological contaminants, and soil gases affect indoor air quality
- Recognise major pollutant-source pairs such as **CO**, **VOCs**, **formaldehyde**, and **radon**
- Apply source knowledge to AP-style questions about homes, buildings, and sick building syndrome
- Avoid common exam confusions, including radon versus combustion pollutants and biological versus chemical pollutants

## AP Environmental Science Syllabus

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

- Know that indoor pollutant concentrations are often higher than outdoor concentrations because pollutants can enter buildings and become trapped
- Identify the major indoor source categories: combustion, building materials and furnishings, household chemical products, biological sources, and soil or foundation entry
- Explain that **carbon monoxide (CO)** comes from incomplete combustion, especially from faulty or unvented fuel-burning appliances
- Understand that **VOCs** are released from paints, solvents, cleaners, adhesives, pesticides, air fresheners, and furnishings
- Recognise that **formaldehyde** is a specific VOC strongly linked to pressed-wood products, glues, coatings, and some combustion sources
- Know that **radon** is a radioactive gas produced by uranium decay in rock and soil and enters buildings through cracks and openings
- Identify biological pollutant sources such as mold, dust mites, pet dander, pest debris, and contaminated HVAC systems
- Understand that older buildings may contain indoor pollutant sources such as asbestos and lead-based paint dust

## 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 an indoor pollutant and the source that releases it?
2. Why can indoor air pollution be worse than outdoor air pollution?
3. Name two indoor sources of carbon monoxide.
4. How does radon usually enter a house?
5. Why might new furniture, carpeting, or flooring increase indoor air pollution?

## Introduction

Indoor air pollution matters in AP Environmental Science because the exam often asks you to connect a pollutant to its source, then explain the likely environmental or health effect. Students often memorise a list of pollutants but miss the more testable skill: identifying where those pollutants come from in a home, school, or workplace.

Indoor air is often more polluted than outdoor air because pollutants from outside can enter buildings, and indoor pollutants may then remain trapped if ventilation is poor. Modern buildings are often well sealed, which can improve energy efficiency but also reduce air exchange. That is why indoor pollutant sources are high yield for both multiple-choice and free-response questions.

> **Key Term: indoor air pollution**
> Contamination of air inside buildings by harmful gases, particles, or biological materials at levels that can affect health.

## Introduction to source categories

APES questions on this topic are easiest when you organise sources into categories instead of treating them as one long list. The five most useful categories are:

![Major indoor air pollutant source categories, including combustion, household chemicals, building materials, biological contaminants, and soil gas entry.](https://static.pastpaperhero.com/images/ap-environmental-science-indoor-air-pollutants-sources-of-indoor-pollutants_00.png)

_Indoor pollutant sources are grouped into combustion, consumer products, furnishings, biological agents, and radon entry through foundations._

- combustion sources
- building materials and furnishings
- household chemical products
- biological sources
- soil or foundation entry sources

This structure helps because exam questions often give a household example and ask you to infer the pollutant. For example, “attached garage” should make you think of vehicle exhaust and **CO**, while “pressed-wood cabinets” should make you think of **formaldehyde** and other **VOCs**.

> **Key Term: source**
> The material, product, activity, or location that releases a pollutant into indoor air.
>
> **Key Term: pollutant**
> A harmful substance or agent in the environment, such as a gas, particulate, or biological contaminant.

A common mistake is mixing up source and pollutant. **Formaldehyde** is a pollutant. Pressed-wood furniture is a source. **Radon** is a pollutant. Cracks in a basement floor are an entry route from the source below the house.

> **Test Tip:** On APES questions, underline the object or activity named in the prompt, then ask: “Is this the pollutant itself, or the thing releasing it?” That step avoids many easy mistakes.

### Combustion sources: why fuel burning is such a major indoor risk

Combustion sources are among the most important indoor pollutant sources because many homes contain fuel-burning appliances. These include gas stoves, furnaces, fireplaces, wood stoves, kerosene heaters, gas space heaters, generators, and tobacco smoke. The danger comes from the fact that combustion does not always happen completely.

When combustion is incomplete, one major product is **carbon monoxide**. CO is especially dangerous because it is colorless and odorless, so people may not notice exposure until symptoms appear. In homes, common CO source situations include malfunctioning furnaces, poorly vented heaters, gas stoves, fireplaces, and vehicle exhaust entering from attached garages.

> **Key Term: carbon monoxide (CO)**
> A colorless, odorless gas produced by incomplete combustion that reduces the blood’s ability to carry oxygen.

The important reasoning chain is:

fuel burns incompletely -> CO is produced -> indoor air concentration rises in enclosed spaces -> humans inhale it -> oxygen transport in the body is reduced

That chain explains why combustion sources are more dangerous indoors than outdoors. Outdoors, pollutants disperse more easily. Indoors, especially in poorly ventilated rooms, they can accumulate.

Combustion also produces **particulate matter** and often **nitrogen dioxide**. A smoky fireplace, wood stove, or even frequent high-heat cooking can add particles to indoor air. Gas stoves are especially worth remembering because they are linked not only to CO but also to nitrogen dioxide and particles.

Secondhand smoke belongs in this source category too. Students sometimes treat it only as a health issue, but APES may frame it as an indoor pollution source. It is both a pollutant category and a source of multiple pollutants, including particulates and CO.

This “both/and” nature is easy to miss. If a question asks for a source of particulate matter indoors, tobacco smoke works. If it asks for an indoor pollutant itself, secondhand smoke also works.

> **Key Term: secondhand smoke**
> Indoor tobacco smoke inhaled by nonsmokers; a major source of toxic gases and particulates.

In developing countries, combustion sources can be even more severe because households may burn biomass such as wood, dung, or crop waste indoors for cooking and heating. These fuels often burn inefficiently and produce very high levels of smoke and particulates.

> **Test Tip:** If the question mentions gas appliances, think beyond one pollutant. The strongest answers often link the source to multiple likely pollutants: CO, particulates, and sometimes nitrogen dioxide.

### Building materials, furnishings, and household products: off-gassing indoors

Some of the most testable indoor pollutants do not come from flames or smoke. They come from ordinary products inside the building. Carpets, flooring, furniture, insulation, plastics, paints, adhesives, and cleaning products can all release chemicals into the air.

> **Key Term: volatile organic compounds (VOCs)**
> Carbon-containing chemicals that evaporate easily at room temperature and can build up in indoor air.

The key word here is evaporation. VOCs have relatively high vapor pressure, so they enter the air at room temperature. That is why a newly painted room, new carpet, or recently installed furniture may smell “chemical.” That smell is evidence of compounds entering the air.

This process is called off-gassing.

> **Key Term: off-gassing**
> The release of gases, especially VOCs, from materials such as new carpets, furniture, paints, or plastics.

Students should understand why newer materials are often worse at first. Freshly manufactured or newly installed products have more volatile chemicals available to escape. Over time, the release rate usually falls, though not always to zero.

One especially important VOC is **formaldehyde**. APES questions often single it out because it is strongly linked to building materials and furnishings.

> **Key Term: formaldehyde**
> A VOC commonly released from pressed-wood products, glues, coatings, insulation, and some combustion sources.

The source-pollutant pair you must know is:

pressed-wood products -> formaldehyde

This includes particleboard, plywood, cabinets, decorative paneling, and some furniture made with urea-formaldehyde resins. If an exam question asks why a remodeled room might have worse air quality, the best answer is often that new pressed-wood materials, carpeting, adhesives, or finishes are off-gassing VOCs such as formaldehyde.

Household chemical products form a closely related source category. Paints, solvents, cleaners, air fresheners, pesticides, and adhesives all release VOCs. Pesticides are sometimes listed separately because they are a distinct indoor pollutant group, even though they fit the broader category of household chemicals.

The distinction students often miss is this:

- building materials and furnishings are long-term built-in or semi-permanent indoor sources
- household products are consumer-use chemical sources that may be stored or applied indoors

That difference matters in FRQs. A strong answer can say that indoor pollutants may come from both the building itself and from products used inside it.

Older buildings introduce another issue: some harmful materials may already be present but become dangerous mainly when they deteriorate or are disturbed.

**Asbestos** is the clearest example.

> **Key Term: asbestos**
> A fibrous mineral once used in insulation and building products that is hazardous when inhaled.

Asbestos in old tiles, insulation, pipe wrapping, or ceiling materials is most dangerous when fibers become airborne during aging, damage, renovation, or demolition. Students sometimes think asbestos is only a construction-site problem, but exposure can happen during normal occupancy if old materials are breaking down.

Lead is similar. The indoor source is often not fresh paint, but old lead-based paint that peels or creates contaminated dust.

### Biological sources and soil entry: living pollutants and radon pathways

Not all indoor pollutants are chemicals. Some are biological contaminants, and APES often expects you to separate these from chemical pollutants.

> **Key Term: biological pollutants**
> Living organisms or biological materials in indoor air, such as mold, dust mites, pet dander, bacteria, and pest debris.

The major indoor biological sources include:

- mold from damp walls, bathrooms, basements, or water-damaged materials
- dust mites in bedding, carpets, and upholstery
- pet dander from animals
- cockroach or rodent debris from pest infestations
- bacteria growing in standing water or poorly maintained HVAC systems

The unifying factor is usually **moisture**. Mold needs damp conditions. Dust mites do better in humid indoor spaces. Bacteria can grow in water reservoirs or wet ventilation components.

That means many AP questions on biological pollutants are really testing whether you see the environmental condition behind the source:

moisture problem -> biological growth -> indoor air contamination

Poor ventilation can worsen this because spores, allergens, and microbes are less effectively removed.

A second major non-combustion source category is soil or foundation entry, especially **radon**.

> **Key Term: radon**
> A radioactive gas produced by uranium decay in rock and soil that can enter buildings through cracks and openings in foundations.

Radon is often confused with combustion pollutants, but its source is completely different. It is not produced by stoves, heaters, or smoke. It forms naturally in underlying rock and soil, especially where uranium-containing materials occur. From there, it moves upward and can enter homes through:

- cracks in basement floors
- openings around pipes
- gaps in foundations
- crawl spaces
- sometimes groundwater from wells

The main exam distinction is:

CO -> incomplete combustion  
radon -> radioactive decay in soil and rock

Why does radon often accumulate indoors? Because it enters from below and can become trapped, especially in basements and lower floors with limited ventilation. Well-sealed houses may reduce heat loss, but they can also allow radon to build up more easily if entry pathways are present.

This also connects to sick building syndrome and indoor air quality more generally. When pollutants from multiple indoor sources build up in poorly ventilated spaces, occupants may experience shared symptoms such as headaches, irritation, dizziness, or breathing problems.

> **Key Term: sick building syndrome**
> A set of symptoms linked to time spent in a building, often associated with poor indoor air quality and inadequate ventilation.

A common APES logic pattern is:
source present -> pollutant released or enters -> poor ventilation traps pollutant -> concentration increases -> symptoms or health risks rise

Use that chain when explaining indoor pollution problems in free-response answers.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Indoor pollutant levels are often higher than outdoor levels because pollutants can enter buildings and become trapped
- APES source groupings are easiest to revise as combustion, building materials and furnishings, household chemical products, biological sources, and soil or foundation entry
- A pollutant is the harmful substance itself, while a source is the object, material, or activity releasing it
- Combustion sources include gas stoves, furnaces, fireplaces, wood stoves, heaters, generators, and tobacco smoke
- **Carbon monoxide** comes from incomplete combustion, especially from faulty, unvented, or poorly vented fuel-burning devices
- **VOCs** are commonly released from paints, solvents, cleaners, adhesives, air fresheners, pesticides, and furnishings
- **Formaldehyde** is a VOC strongly linked to pressed-wood products such as particleboard, plywood, cabinets, and some furniture
- Off-gassing explains why new carpets, flooring, furniture, and finishes may worsen indoor air quality
- Biological pollutants include mold, dust mites, pet dander, bacteria, and pest debris, and they are strongly linked to moisture
- **Radon** is a radioactive soil gas that enters buildings through foundation cracks and openings, not through combustion
- Older buildings may contain indoor pollutant sources such as asbestos and lead-based paint dust
- Poor ventilation increases indoor pollutant concentrations and can contribute to sick building syndrome

## Key Terms and Concepts

- indoor air pollution
- source
- pollutant
- carbon monoxide (CO)
- secondhand smoke
- volatile organic compounds (VOCs)
- off-gassing
- formaldehyde
- asbestos
- biological pollutants
- radon
- sick building syndrome


Indoor Air Pollutants - Sources of indoor pollutants