AP Environmental Science

## Learning Outcomes

- Understand what **indoor combustion pollutants** are and why they build up indoors
- Explain how **complete combustion** differs from **incomplete combustion**
- Identify the main indoor combustion pollutants: **carbon monoxide**, **nitrogen dioxide**, and **particulate matter**
- Describe common indoor sources such as gas stoves, furnaces, woodstoves, fireplaces, tobacco smoke, and generators
- Explain how these pollutants harm human health, including respiratory and cardiovascular effects
- Propose practical methods to reduce indoor exposure, especially through ventilation, maintenance, and safer appliance use
- Distinguish combustion indoor pollutants from non-combustion indoor pollutants such as radon and VOCs

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand indoor air pollution caused by fuel burning indoors, with a focus on the following syllabus points:

- Know that combustion indoors can produce **carbon monoxide (CO)**, **nitrogen dioxide (NO2)**, and **particulate matter (PM)**
- Identify indoor combustion sources such as gas stoves, furnaces, fireplaces, woodstoves, kerosene heaters, tobacco smoke, generators, and vehicle exhaust from attached garages
- Explain that **incomplete combustion** produces **CO** and soot or particulate matter, while high-temperature combustion also forms **NO2**
- Understand why **poor ventilation** increases indoor pollutant concentrations
- Compare the health effects of CO, NO2, PM2.5, and, more cautiously, SO2
- Describe prevention methods including venting appliances outdoors, maintaining flues and chimneys, using range hoods, avoiding indoor smoking, and installing CO alarms
- Distinguish indoor combustion pollutants from pollutants such as radon, asbestos, and formaldehyde, which are indoor pollutants but not combustion 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. What is the difference between complete combustion and incomplete combustion?
2. Why is **carbon monoxide** especially dangerous in homes and enclosed spaces?
3. Name three common indoor sources of combustion pollutants.
4. How does **PM2.5** differ from larger particles in terms of health risk?
5. Why can a gas stove still be a pollution source even if it appears to be working normally?

## Introduction

Indoor air pollution is a major AP Environmental Science topic because people spend much of their time indoors, and pollutant concentrations inside buildings can be higher than those outdoors. One important subgroup is **combustion indoor pollutants**: gases and particles released when fuels burn inside enclosed spaces.

This topic links Unit 6 and Unit 7 very neatly. Unit 6 explains fuel use and combustion, while Unit 7 focuses on the pollutants produced. For AP exam questions, you need to think in a chain: **fuel-burning source -> pollutant formed -> health effect -> mitigation strategy**. If you can follow that chain clearly, you can answer both multiple-choice and free-response questions well.

> **Key Term: indoor combustion pollutants**
> Pollutants released when fuels are burned indoors, especially carbon monoxide, nitrogen dioxide, and particulate matter.

## Introduction to combustion pollutants and how they form

### Complete combustion, incomplete combustion, and why indoor spaces are risky

Combustion means burning a fuel in oxygen. In an ideal case, a fuel burns completely and produces mostly carbon dioxide and water. For example, methane in natural gas can undergo complete combustion:

$$
\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}
$$

But indoor combustion is often not ideal. If there is too little oxygen, poor burner adjustment, blocked vents, or poor mixing of fuel and air, **incomplete combustion** occurs. That produces **carbon monoxide** and soot instead of only carbon dioxide and water. A simplified example is:

$$
\mathrm{2CH_4 + 3O_2 \rightarrow 2CO + 4H_2O}
$$

That difference matters hugely. Complete combustion mainly raises indoor CO2 and humidity. Incomplete combustion can create a direct poisoning hazard.

> **Key Term: incomplete combustion**
> Burning in which there is not enough oxygen for fuel to oxidize fully, producing carbon monoxide and often soot or particulate matter.

Indoor spaces make this more dangerous because pollutants do not disperse easily. Outdoors, wind and atmospheric mixing dilute emissions. Indoors, walls and ceilings trap them, especially if windows are closed and ventilation is weak. This is why the same fuel-burning activity can be much riskier inside than outside.

A common exam trap is assuming that only obviously smoky fires produce combustion pollution. In reality, even "clean-looking" gas appliances can emit harmful gases, especially **NO2** and some **CO**.

> **Test Tip:** If an APES question mentions an enclosed space, attached garage, blocked flue, or unvented heater, think immediately about **indoor combustion pollutants**, especially **CO**.

### The main combustion pollutants: CO, NO2, PM, and the smaller role of SO2

For AP Environmental Science, the three central indoor combustion pollutants are **carbon monoxide**, **nitrogen dioxide**, and **particulate matter**.

**Carbon monoxide (CO)** forms when carbon-containing fuels burn without enough oxygen. It is colorless and odorless, so people often do not realize it is present until symptoms start.

**Nitrogen dioxide (NO2)** forms during high-temperature combustion. Nitrogen and oxygen in air react first to form nitric oxide, then that oxidizes further:

$$
\mathrm{N_2 + O_2 \rightarrow 2NO}
$$

$$
\mathrm{2NO + O_2 \rightarrow 2NO_2}
$$

So even if a stove flame looks normal, the high temperature itself can generate NO2.

**Particulate matter (PM)** is a mixture of tiny solid and liquid particles released by smoke and incomplete combustion. The most important fine fraction is **PM2.5**, particles 2.5 micrometers or smaller. These are especially harmful because they penetrate deeply into the lungs.

> **Key Term: particulate matter (PM)**
> Tiny solid or liquid particles suspended in air; fine particles from combustion can damage the lungs and heart.

**Sulfur dioxide (SO2)** can also be produced when sulfur-containing fuels are burned. However, for ordinary home settings, APES usually puts less emphasis on SO2 than on CO, NO2, and PM. It becomes more relevant with certain fuels, tobacco smoke, or poorly vented heaters.

Why do students mix these up? Because all four are linked to combustion, but their mechanisms differ:

- **CO** -> incomplete combustion -> reduced oxygen delivery in the body
- **NO2** -> high-temperature combustion -> airway irritation
- **PM** -> smoke and fine particles -> lung and cardiovascular damage
- **SO2** -> sulfur in fuel -> respiratory irritation

That source-effect match is very testable.

## Sources and health effects of indoor combustion pollutants

### Common household sources and the pollutant patterns they produce

AP questions often ask you to match a source with likely pollutants. That means you should know not just a list of sources, but what each source tends to produce.

**Gas stoves and gas ovens** are major indoor sources of **NO2** and can also emit **CO**, especially if poorly adjusted. They are often treated by students as just heat sources, but they are combustion devices.

**Woodstoves and fireplaces** are major indoor sources of **PM** and also produce **CO**. Wood smoke contains many fine particles, so a smoky fireplace is not just an odor issue.

**Unvented kerosene heaters** and **unvented gas space heaters** can produce **CO**, **NO2**, and particles. Because their emissions stay indoors, the risk rises quickly.

**Furnaces** and **water heaters** may be safer when vented properly, but they become dangerous if flues are blocked, disconnected, leaking, or back-drafting.

**Tobacco smoke** is one of the most important indoor combustion sources. It contains particulate matter, CO, and many toxic chemicals.

**Generators**, **charcoal grills**, and **vehicles in attached garages** are especially dangerous CO sources. A key safety point is that a partially open garage door is not enough to make idling safe.

This is where exam reasoning matters. The same source can produce more than one pollutant. A gas stove does not produce "just CO" or "just NO2." A fireplace does not produce "just PM." Combustion creates a pollutant mixture.

A very useful APES memory line is:

- **Gas stove** -> CO + NO2
- **Wood smoke** -> PM + CO
- **Tobacco smoke** -> PM + CO + toxic chemicals
- **Generator/garage exhaust** -> CO
  > **Key Term: fuel-burning appliance**
  > Any indoor device that burns fuel for cooking, heating, or power, such as a stove, furnace, water heater, fireplace, or heater.

### Why these pollutants harm people in different ways

The health effects differ because each pollutant acts on the body differently.

**Carbon monoxide** is dangerous because it binds strongly to hemoglobin in blood. Hemoglobin normally carries oxygen, but CO competes for those binding sites and prevents oxygen transport. So even if a person is breathing air, their tissues may still be starved of oxygen.

Symptoms of CO exposure include headache, dizziness, nausea, confusion, and weakness. At higher concentrations, unconsciousness and death can occur quickly.

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

**Nitrogen dioxide** mainly damages the respiratory system. It irritates the airways, worsens asthma, can decrease lung function, and raises the risk of respiratory infections, especially in children.

> **Key Term: nitrogen dioxide (NO2)**
> A reactive gas produced during high-temperature combustion that irritates the respiratory system and can worsen asthma.

**Particulate matter**, especially **PM2.5**, can travel past the upper respiratory defenses and reach deep into the lungs. Fine particles can enter the alveoli and, in some cases, the bloodstream. That is why PM is linked not only to asthma and bronchitis, but also to cardiovascular disease.

> **Key Term: PM2.5**
> Fine particulate matter with particles 2.5 micrometers or smaller that can penetrate deep into the lungs and pose serious health risks.

A subtle but important distinction: students often think PM is only a lung issue, while CO is only an immediate poisoning issue. But PM can contribute to long-term heart and lung disease, while CO can also contribute to severe acute outcomes very rapidly.

Another major reason these pollutants matter is unequal exposure. In many lower-income or developing settings, people may rely on biomass such as wood, dung, or crop residues for cooking and heating. These fuels often burn in simple stoves with incomplete combustion. That means repeated daily exposure to high PM and CO, especially for women and children who spend more time near cooking areas.

> **Test Tip:** In FRQs, do not just say "combustion pollutants are harmful." Name the pollutant and the mechanism: **CO reduces oxygen transport**, **NO2 irritates airways**, **PM2.5 penetrates deep into lungs**.

## Prevention, mitigation, and AP exam distinctions

### How to reduce indoor combustion pollution

The best prevention methods target either the **source** or the **build-up** of pollutants.

First, improve **ventilation**. Range hoods vented outdoors can remove combustion gases released while cooking. This matters especially for gas stoves. A recirculating fan is less effective because it may move air without removing the gases outdoors.

Second, make sure appliances are **properly vented**. Chimneys, flues, and exhaust systems should not be blocked, leaking, disconnected, or poorly installed.

Third, **maintain and inspect** fuel-burning appliances regularly. A furnace or water heater that is poorly adjusted may produce much higher emissions.

Fourth, avoid especially risky practices:

- never use a gas oven or range to heat the home
- never run a generator indoors or in a garage
- never use charcoal grills indoors
- never idle a vehicle in an attached garage
- avoid indoor smoking

Fifth, install **carbon monoxide alarms**.

> **Key Term: carbon monoxide alarm**
> A detector that warns building occupants when carbon monoxide is present at dangerous levels.

A useful AP-style way to think about mitigation is:

- **source reduction**: stop or reduce combustion indoors
- **engineering control**: vent emissions outdoors
- **maintenance**: keep systems operating properly
- **monitoring**: use alarms and testing

There is no single fix that works in every case. A CO alarm helps with CO, but it does not solve NO2 or PM exposure by itself. Ventilation helps, but if the source is very strong or the appliance is malfunctioning, ventilation alone may be inadequate.

### Common misconceptions and how AP questions test them

One common mistake is confusing combustion pollutants with all indoor pollutants. **Radon**, **asbestos**, **formaldehyde**, and many **VOCs** are indoor pollutants, but they are not combustion pollutants in the same sense. Radon comes from radioactive decay in rock and soil, not burning. Asbestos comes from building materials. VOCs can come from paints, plastics, and cleaning products.

Another misconception is thinking only unvented appliances are dangerous. They are certainly risky, but even vented appliances can pollute indoors if vents leak or back-draft.

A third misconception is assuming wood smoke is mainly an odor nuisance. In APES, wood combustion should make you think of **fine particulate pollution** and **CO**.

A fourth is assuming that opening a garage door makes exhaust safe. It does not reliably prevent CO build-up or entry into the house.

A final distinction that APES likes is between **primary** and **secondary** pollutants. Indoor combustion pollutants such as CO, NO2, and PM are mainly treated as **primary pollutants** because they are emitted directly by the source. Tropospheric ozone, by contrast, is a **secondary pollutant** formed through atmospheric reactions.

If you get an FRQ asking for a solution, structure it this way:

1. identify the combustion source
2. identify the main pollutant(s)
3. explain the health effect
4. propose a realistic mitigation method
5. justify why it works

For example, if the source is a gas stove in a small apartment:

- source: gas combustion indoors
- pollutants: NO2 and some CO
- effect: airway irritation, asthma risk
- mitigation: use an outdoor-vented range hood and maintain the stove
- justification: removes combustion gases and lowers indoor concentration

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Indoor combustion pollutants are gases and particles produced by burning fuels inside enclosed spaces
- The most important APES combustion pollutants indoors are carbon monoxide, nitrogen dioxide, and particulate matter
- Complete combustion mainly produces carbon dioxide and water, while incomplete combustion produces carbon monoxide and soot
- Carbon monoxide is colorless and odorless and reduces oxygen transport by binding to hemoglobin
- Nitrogen dioxide forms during high-temperature combustion and irritates the respiratory system
- Particulate matter, especially PM2.5, can penetrate deep into the lungs and increase respiratory and cardiovascular risk
- Major indoor sources include gas stoves, furnaces, fireplaces, woodstoves, kerosene heaters, tobacco smoke, generators, and vehicle exhaust from attached garages
- Poor ventilation increases indoor pollutant concentrations because emissions remain trapped indoors
- Proper venting, appliance maintenance, outdoor-vented range hoods, and avoiding indoor smoking reduce exposure
- Carbon monoxide alarms are essential because CO cannot be detected by smell or sight
- Radon, asbestos, and many VOCs are indoor pollutants but are not combustion pollutants
- AP exam questions often test source-pollutant-effect-mitigation chains

## Key Terms and Concepts

- indoor combustion pollutants
- incomplete combustion
- particulate matter (PM)
- fuel-burning appliance
- carbon monoxide (CO)
- nitrogen dioxide (NO2)
- PM2.5
- carbon monoxide alarm


Indoor Air Pollutants - Combustion indoor pollutants