AP Environmental Science

## Learning Outcomes

- Explain why photochemical smog harms human health mainly through **ground-level ozone**
- Identify key short-term health effects such as coughing, chest tightness, and eye irritation
- Distinguish between **tropospheric ozone** and **stratospheric ozone**
- Describe why certain groups are more vulnerable on high-smog days
- Apply **AQI** guidance to real exam scenarios about exposure and risk reduction
- Use accurate AP Environmental Science language to explain smog health effects in FRQs

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand smog health effects, with a focus on the following syllabus points:

- Know that **photochemical smog** forms when **NOₓ** and **VOCs** react in sunlight, producing **ground-level ozone** and other oxidants
- Identify **ground-level ozone** as the main harmful component of photochemical smog for human health
- Explain how ozone causes **respiratory irritation**, **reduced lung function**, and **asthma aggravation**
- Understand that **PANs** and related oxidants are linked to **eye irritation** and some respiratory effects
- Distinguish between **short-term effects** such as coughing and chest tightness and longer-term concerns such as repeated airway damage
- Identify **sensitive groups**, including children, older adults, people with asthma, and people active outdoors
- Understand how **AQI** warnings are used to reduce exposure on high-ozone days
- Explain why outdoor exercise on sunny polluted afternoons can worsen smog-related symptoms

## 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 main pollutant in photochemical smog that harms human health?
2. How does ground-level ozone affect the respiratory system?
3. Why are children and people exercising outdoors more at risk on smoggy days?
4. What role do PANs play in smog health effects?
5. How does the **AQI** help reduce harm from photochemical smog?

## Introduction

Photochemical smog is one of the most tested forms of air pollution in AP Environmental Science because it links chemistry, weather, human activity, and public health. Students often remember how smog forms but forget what examiners most want in health questions: **ground-level ozone irritates and inflames the respiratory system, reduces lung function, and worsens asthma**.

This matters because AP questions often ask you to move from cause to effect. You may be given a city with heavy traffic, hot sunny weather, or thermal inversion conditions and then asked to explain the consequences for human health. To do that well, you need to know not only what smog is, but why its components damage the body.

> **Key Term: photochemical smog**
> A type of air pollution formed when **nitrogen oxides (NOₓ)** and **volatile organic compounds (VOCs)** react in sunlight, producing **ground-level ozone** and other oxidants.

### Why photochemical smog is harmful

Photochemical smog is harmful mainly because it contains **ground-level ozone**, also called **tropospheric ozone**. This is a **secondary pollutant**, meaning it is not emitted directly from a tailpipe or smokestack. Instead, it forms in the atmosphere after primary pollutants react.

That distinction matters. Many students wrongly say that cars “release ozone.” They do not. Vehicles release **NOₓ** and **VOCs**. Sunlight then drives reactions that produce ozone. On an FRQ, that extra step often earns the point.

> **Key Term: secondary pollutant**
> A pollutant formed in the atmosphere by reactions between primary pollutants, rather than emitted directly.

The basic APES relationship is:

- **NOₓ + VOCs + sunlight → O₃ + other photochemical oxidants**

Ground-level ozone is chemically reactive. That is why it is dangerous in the troposphere. It interacts with tissues in the respiratory tract, especially when inhaled deeply. The damage is not just “discomfort.” Ozone can inflame airway linings and make breathing less efficient.

A common misconception is to confuse good ozone with bad ozone.

> **Key Term: tropospheric ozone**
> Ozone in the lower atmosphere; a harmful air pollutant and major component of photochemical smog.

Tropospheric ozone is harmful because people and animals breathe it. **Stratospheric ozone**, by contrast, is beneficial because it absorbs harmful ultraviolet radiation high above Earth’s surface. Same molecule, different location, different effect.

A good exam sentence is: **stratospheric ozone protects life, but tropospheric ozone harms respiratory health**.

Smog also includes other oxidants, especially **PANs**.

> **Key Term: PANs**
> Peroxyacyl nitrates; secondary pollutants in photochemical smog that commonly irritate eyes and can also irritate the respiratory system.

In APES answers, ozone should be your main health focus, but PANs matter because they help explain the classic “burning eyes” description of smog.

> **Test Tip:** If a question asks for a health effect of photochemical smog, lead with **respiratory irritation from ground-level ozone**. Eye irritation is valid, but it is usually a weaker main answer than ozone-linked lung effects.

### Respiratory effects: how ozone damages the body

The clearest health effects of photochemical smog are respiratory. Ozone can cause:

- coughing
- throat irritation
- chest tightness
- wheezing
- shortness of breath
- pain or discomfort during deep breathing

These symptoms happen because ozone irritates the lining of the respiratory tract. Think through the process:

1. A person inhales polluted air
2. Ozone enters the airways
3. The airway lining becomes irritated and inflamed
4. Breathing becomes more difficult and less efficient
5. Symptoms such as coughing and chest tightness appear

That chain is exactly the sort of reasoning APES rewards.

Repeated or stronger exposure can reduce lung function. This means the lungs do a worse job moving air in and out. In exam language, **reduced lung function** is stronger than saying “breathing problems” because it explains the mechanism more precisely.

Ozone also worsens existing respiratory diseases. The most important one to mention is **asthma**. If someone already has inflamed or sensitive airways, ozone exposure makes attacks more likely and symptoms more severe.

You may also see links to bronchitis or emphysema in textbook summaries. These are acceptable, but asthma is the highest-yield condition for APES.

Here is the reasoning students should be able to write:

- Ozone inflames airway tissue
- Inflamed airways narrow more easily
- Narrower airways make airflow harder
- This can trigger or worsen asthma attacks

The health effect is not limited to visibly sick people. Even healthy people can experience irritation and temporarily reduced lung function, especially during exercise. That is a point students often miss.

At the population level, higher ozone exposure is associated with:

- more doctor visits for breathing problems
- more emergency room visits for asthma
- more hospital admissions for respiratory illness

Some broader studies also connect ozone exposure with premature death risk, but APES answers should not make that the main point unless the question specifically invites broader public-health impacts. The core idea is still respiratory harm.

### Eye irritation, exposure patterns, and sensitive groups

Smog is often described as making eyes sting or water. That effect is real, but it is usually tied more strongly to **PANs** and related oxidants than to ozone itself.

A strong APES summary would be:

- **ozone** mainly harms the lungs and airways
- **PANs** are especially associated with eye irritation

This helps you avoid vague “smog is bad” answers.

Common eye symptoms include:

- stinging
- burning
- watering
- irritation

Because PANs break down slowly, they can also travel away from where they formed. That means photochemical smog can affect areas beyond the exact site of the original emissions.

Who is most at risk? APES expects you to know the main **sensitive groups**:

- children and teens
- older adults
- people with asthma or other lung disease
- people active outdoors, especially during exercise
- outdoor workers

Children are vulnerable for two reasons students should know. First, their lungs are still developing. Second, they often spend more time outdoors being physically active. That raises exposure.

People who exercise outdoors are also at extra risk because exercise changes breathing. During strenuous activity, people breathe faster and deeper. That pulls more polluted air into the lungs and sends pollutants farther down the respiratory tract.

This is why a sunny afternoon football practice or cross-country session can be riskier than the same activity on a clean-air day.

Time of day matters too. Ozone often peaks in the **afternoon and early evening**, after sunlight has driven the photochemical reactions for several hours. Morning conditions are often safer on high-ozone days.

> **Key Term: sensitive groups**
> People who are more likely to be harmed by air pollution, including children, older adults, people with asthma, and people active outdoors.

A good applied example: if a city has high traffic in the morning and strong sun through midday, ozone levels may climb later in the day. Children playing outside in the afternoon may then face greater risk of coughing, wheezing, or asthma symptoms.

> **Test Tip:** On FRQs, connect exposure to activity. Do not just say “children are vulnerable.” Add why: **they spend more time outdoors and breathe more deeply during activity**.

### Short-term and longer-term health effects

Students often lump all health effects together, but APES questions can reward a short-term versus repeated-exposure distinction.

Short-term effects include:

- coughing
- throat irritation
- chest tightness
- watery or burning eyes
- shortness of breath
- temporary reduction in lung function

These effects may appear on a single high-smog day, especially during outdoor activity.

Longer-term or repeated exposure concerns include:

- repeated airway inflammation
- worsening of asthma over time
- impaired lung function
- possible effects on lung development in children

The important point is not that one afternoon of smog instantly causes permanent damage in every person. The point is that **repeated exposure increases the chance of lasting harm**, especially in vulnerable groups.

Improved air quality is linked with better lung function and fewer asthma symptoms, especially in children. That matters because it shows the health damage is not just theoretical. Reducing pollution leads to measurable public-health gains.

### AQI, public health, and exam application

The **Air Quality Index (AQI)** is a tool used to report how polluted the air is and what health risk that pollution may pose. For APES, you do not need every number memorized, but you should know how AQI guides behavior.

> **Key Term: Air Quality Index (AQI)**
> A system used to report air quality and related health risk, including warnings for sensitive groups on high-pollution days.

For ozone, the major guidance points are:

- **AQI 101–150: Unhealthy for Sensitive Groups**
  - sensitive groups should limit prolonged or heavy outdoor exertion
- **AQI 151–200: Unhealthy**
  - sensitive groups should avoid prolonged outdoor exertion
  - others should limit it
- **AQI 201+: Very Unhealthy**
  - stronger warnings apply more broadly

Why this matters for APES: many questions ask for a management or exposure-reduction strategy rather than just a health effect.

A complete answer might say:

- Ground-level ozone in photochemical smog irritates airways and worsens asthma.
- Schools or cities can reduce health impacts by checking AQI alerts.
- On high-ozone days, sensitive groups should reduce outdoor exertion, especially in the afternoon.

That links pollution science to public policy and health management.

Here is a worked exam-style reasoning chain:

- Hot sunny conditions plus vehicle emissions increase photochemical smog formation
- Ground-level ozone concentration rises during the afternoon
- A child with asthma playing outdoor sports breathes faster and deeper
- More ozone reaches the lungs
- Airway inflammation worsens
- The child is more likely to wheeze or have an asthma attack
- AQI alerts can reduce harm by moving strenuous activity indoors or to earlier in the day

That kind of chain is strong APES writing because it shows process, not just memorized facts.

One final distinction matters: photochemical smog is not the same as industrial or sulfurous smog. If the question is about **sunlight, urban traffic, ozone, and summer conditions**, think photochemical smog. If it focuses on **coal burning, sulfur dioxide, and gray smog**, think industrial smog.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Photochemical smog forms when **NOₓ** and **VOCs** react in sunlight
- **Ground-level ozone** is the main harmful health component of photochemical smog
- Ground-level ozone is a **secondary pollutant**, not a direct vehicle emission
- **Tropospheric ozone** harms health, while **stratospheric ozone** protects Earth from UV radiation
- Major short-term effects include coughing, throat irritation, chest tightness, wheezing, and shortness of breath
- Ozone harms health by irritating and inflaming the respiratory system and reducing lung function
- Ozone commonly worsens **asthma** and other respiratory problems
- **PANs** are secondary pollutants in smog associated especially with eye irritation
- Sensitive groups include children, older adults, people with asthma, and people active outdoors
- Smog effects are often worse during exercise because pollutants are inhaled more deeply
- Ozone is often highest in the afternoon and early evening on sunny polluted days
- **AQI** guidance helps reduce exposure by warning people to limit outdoor exertion on high-ozone days

## Key Terms and Concepts

- photochemical smog
- secondary pollutant
- tropospheric ozone
- PANs
- sensitive groups
- Air Quality Index (AQI)


Photochemical Smog - Smog health effects