AP Environmental Science

## Learning Outcomes

- Distinguish between **UV-A**, **UV-B**, and **UV-C** and explain which type is most relevant to ozone depletion
- Explain the cause-and-effect chain from ozone-depleting substances to increased surface **UV-B**
- Identify the main human health effects of increased UV exposure, including **skin cancer**, **cataracts**, and **immune suppression**
- Avoid common APES mistakes, especially confusing stratospheric ozone depletion with ground-level ozone pollution
- Recall key dates linked to ozone science and policy, including **1974**, **1985**, **1987**, and **1989**
- Apply this content to multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand UV increase and health effects from stratospheric ozone depletion, with a focus on the following syllabus points:

- Know that depletion of stratospheric ozone allows more **UV-B** to reach Earth’s surface
- Identify the differences among **UV-A**, **UV-B**, and **UV-C**, especially which are blocked by the atmosphere
- Explain how increased **UV-B** exposure raises risks of **skin cancer**, **cataracts**, and **immune system suppression**
- Understand the causal chain: **CFCs/ODSs → ozone depletion → more surface UV-B → health damage**
- Distinguish **stratospheric ozone** as protective from **tropospheric ozone** as a pollutant
- Recall that **1974** was the year Molina and Rowland warned about CFCs, **1985** marked reporting of the Antarctic ozone hole, and **1987** was the adoption of the Montreal Protocol
- Understand that ozone recovery is gradual because ozone-depleting substances persist for decades

## 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. Which type of ultraviolet radiation is most strongly linked to the health effects of ozone depletion?
2. Why does ozone depletion not greatly increase **UV-A** at Earth’s surface?
3. Name three major human health effects of increased **UV-B** exposure.
4. What is the difference between stratospheric ozone and ground-level ozone?
5. Which international treaty targeted major ozone-depleting substances?

## Introduction

Stratospheric ozone depletion matters because the ozone layer acts like a protective filter high in the atmosphere. When that layer becomes thinner, more harmful ultraviolet radiation reaches Earth’s surface. For AP Environmental Science, the most testable idea is simple: **less ozone means more UV-B, and more UV-B means more biological damage**.

This topic often appears in multiple-choice questions because students mix up the different UV bands or confuse ozone depletion with climate change. It can also appear in free-response answers about atmospheric change, human health, or environmental policy. To score well, you need the exact causal chain and the exact health effects.

> **Key Term: stratospheric ozone depletion**
> The thinning of the ozone layer in the stratosphere, which reduces Earth’s protection from harmful ultraviolet radiation, especially UV-B.

## Introduction to UV radiation and why ozone matters

### The ozone layer as a radiation shield

Most atmospheric ozone is found in the **stratosphere**, roughly 15 to 40 km above Earth’s surface. This matters because ozone absorbs much of the Sun’s ultraviolet radiation before it can reach living organisms.

Without this shielding effect, DNA damage at Earth’s surface would be much greater. The ozone layer does not block all sunlight. It mainly affects specific high-energy wavelengths. That is why APES questions often ask not just whether UV increases, but **which kind** increases.

> **Key Term: ozone layer**
> The region of the stratosphere containing most of Earth’s atmospheric ozone, which absorbs much harmful ultraviolet radiation.

Ozone is constantly being formed and broken down naturally. In a stable system, those processes balance each other. Ozone depletion happens when human-made chemicals, especially **chlorofluorocarbons (CFCs)** and other ozone-depleting substances, disrupt that balance and speed up ozone destruction.

### UV-A, UV-B, and UV-C: the distinction students must know

Ultraviolet radiation is divided into three main bands.

- **UV-A** has the longest wavelength and lower energy than the other two
- **UV-B** has shorter wavelengths and higher energy
- **UV-C** has the shortest wavelengths and the highest energy

The atmosphere treats these three bands differently.

- **UV-A** mostly reaches Earth’s surface
- **UV-B** is partly absorbed by stratospheric ozone, so ozone loss allows more of it through
- **UV-C** is almost completely absorbed by oxygen and ozone, so it normally does not reach the surface

This means ozone depletion mainly changes **UV-B exposure**, not UV-A very much, and not natural solar UV-C at the ground.

> **Key Term: UV-B radiation**
> Ultraviolet radiation with wavelengths about 280–320 nm that is partly absorbed by stratospheric ozone and is the main UV band linked to ozone depletion health effects.
>
> **Key Term: UV-A**
> Longer-wavelength ultraviolet radiation that is less strongly blocked by ozone and reaches Earth’s surface more readily than UV-B.
>
> **Key Term: UV-C**
> The highest-energy ultraviolet radiation, almost completely absorbed by atmospheric oxygen and ozone before reaching Earth’s surface.

A common mistake is to say “ozone depletion lets in more sunlight.” That is too vague. The exam wants **more UV-B**.

> **Test Tip:** If an answer choice says ozone depletion increases “all solar radiation equally,” reject it. APES expects you to know that the main increase is **UV-B**.

## How ozone depletion leads to human health damage

### The full cause-and-effect chain

To explain the health effects properly, start before the UV increase.

Human-made **ozone-depleting substances (ODSs)** such as CFCs are chemically stable in the lower atmosphere. That stability allows them to persist long enough to drift into the stratosphere. There, strong UV radiation breaks them apart and releases chlorine or bromine radicals. These radicals destroy ozone catalytically, meaning they can repeat the process many times.

The important exam chain is:

ODSs/CFCs → stratospheric ozone depletion → more UV-B at surface → more biological damage

This is why the health effects are indirect. CFCs do not usually harm human skin or eyes directly at Earth’s surface. They first thin the ozone layer, and that reduced shielding raises UV-B exposure.

> **Key Term: chlorofluorocarbons (CFCs)**
> Stable human-made chemicals that reach the stratosphere and release chlorine atoms that destroy ozone.
>
> **Key Term: ozone-depleting substances (ODSs)**
> Chemicals such as CFCs and halons that release chlorine or bromine in the stratosphere and break down ozone.

The chemistry is often simplified as:

- Cl· + O₃ → ClO· + O₂
- ClO· + O → Cl· + O₂

Net effect:

- O₃ + O → 2 O₂

The chlorine radical is regenerated, so one atom can destroy many ozone molecules.

### Why UV-B is especially dangerous biologically

UV-B has enough energy to damage molecules in living tissue, especially DNA and proteins, more than UV-A does. It is also the band whose surface level is most altered when ozone thins. So UV-B becomes the main focus in health questions.

That combination matters:

1. **It is energetic enough to do damage**
2. **Its amount at the surface changes when ozone is depleted**

That is why APES teaching and official-style questions stress UV-B rather than simply “UV.”

Lower doses than those causing obvious sunburn can still increase long-term cancer risk. This is another point students miss. Visible damage is not required for biological harm to happen.

> **Test Tip:** In FRQs, write “more **UV-B** causes DNA damage in skin cells and lens damage in eyes.” That is stronger than saying only “more radiation is harmful.”

## Major health effects of increased UV-B exposure

### Skin cancer and skin damage

The best-known health effect is increased risk of **skin cancer**. UV-B damages DNA in skin cells. If that damage is not properly repaired, mutations can accumulate. Over time, repeated exposure raises the chance that cells grow abnormally and form cancer.

APES usually does not require detailed classification of skin cancers, but it is valid to know that UV exposure is linked to both melanoma and non-melanoma skin cancers. The World Health Organization identifies overexposure to UV as a major cause of both.

There are also non-cancer skin effects:

- sunburn
- premature skin aging
- direct skin damage

The key idea is that ozone depletion increases exposure to the UV band most associated with these outcomes.

A useful data point sometimes cited in health discussions is that a **10% decrease in ozone** could lead to about **300,000 additional non-melanoma skin cancer cases** and **4,500 additional melanoma cases**.

Use that carefully as an example, not as a rule to invent new calculations from.

> **Key Term: skin cancer**
> Abnormal growth of skin cells linked to DNA damage, with risk increased by greater exposure to UV-B radiation.

### Cataracts and other eye damage

The eye is another major target of increased UV exposure. UV-B can damage the lens and other eye tissues. The classic APES health effect here is **cataracts**.

A cataract is clouding of the lens of the eye, which reduces vision. Increased UV exposure raises cataract risk because the eye’s tissues absorb some of that radiation and accumulate damage over time.

This matters especially because eyes cannot simply avoid exposure the way some organisms reduce sun exposure behaviorally. In exam questions, cataracts are one of the most common listed consequences of ozone depletion.

> **Key Term: cataract**
> Clouding of the lens of the eye that can impair vision and is made more likely by increased ultraviolet exposure.

Students sometimes confuse this with respiratory effects. Respiratory irritation is more strongly associated with **tropospheric ozone**, not ozone depletion.

### Immune system suppression

The third major health effect you must know is **immune system suppression**. Increased UV-B can weaken or alter immune responses, especially in skin-related immune function.

For APES, you do not need highly detailed immunology. What matters is the outcome: the body may become less effective at defending itself. That can make organisms more vulnerable to some diseases or reduce the efficiency of normal immune responses.

The textbook evidence base and EPA summaries both treat this as a central human health consequence of ozone depletion.

> **Key Term: immune system suppression**
> A weakening or alteration of normal immune responses caused by increased exposure to ultraviolet radiation, especially UV-B.

### Mutations, food supply effects, and wider impacts

Although this subtopic centers on human health, it helps to understand the wider biological context. UV-B can cause mutations because it changes DNA structure. That same mechanism helps explain why skin cancer risk rises.

In addition, increased UV can reduce phytoplankton growth and harm crops. That is not the main health focus here, but it links human health to the environments. Reduced crop yields and weakened marine primary productivity can affect food supply.

The chain can extend like this:

ozone depletion → more UV-B → damage to primary producers → lower productivity → less food availability

That broader pattern matters in APES because human systems depend on ecological systems.

## Distinctions, dates, and policy relevance

### Do not confuse ozone depletion with climate change or ground-level ozone

AP students commonly mix three separate ideas:

1. **Stratospheric ozone depletion**
2. **Ground-level ozone pollution**
3. **Global climate change**

These overlap only partly.

- **Stratospheric ozone** is “good ozone up high” because it blocks harmful UV
- **Tropospheric ozone** is “bad ozone down low” because it is an air pollutant that irritates lungs
- **Climate change** is mainly driven by greenhouse gases warming the lower atmosphere

Ozone depletion can connect to climate issues through some shared chemicals and atmospheric processes, but it is not the same thing as the greenhouse effect.

If a question asks for respiratory irritation, that points more toward **tropospheric ozone**. If it asks for skin cancer, cataracts, or weakened immunity from UV increase, that points toward **stratospheric ozone depletion**.

### Key dates and why they matter

APES often rewards accurate environmental policy chronology.

- **1974** — Mario Molina and F. Sherwood Rowland proposed that CFCs could deplete stratospheric ozone
- **1985** — severe seasonal ozone depletion over Antarctica was reported
- **1987** — the **Montreal Protocol** was adopted
- **1989** — the Montreal Protocol entered into force

These dates matter because they show the sequence:

scientific warning → observed major environmental problem → international treaty response

> **Key Term: Montreal Protocol**
> The 1987 international treaty designed to phase out major ozone-depleting substances such as CFCs and halons.

The Montreal Protocol is a classic APES success story. It is often used as an example of effective international environmental cooperation.

### Why recovery takes decades

Students sometimes assume that once emissions decline, the ozone layer quickly returns to normal. That is not correct.

Recovery is slow because many ODSs are long-lived. Some persist for decades or longer in the atmosphere. So even after production is reduced, previously emitted substances continue affecting stratospheric chemistry.

Approximate recovery projections often cited are:

- around **2040** for global return to about 1980 ozone levels
- around **2045** in the Arctic
- around **2066** over Antarctica

These are projections, not guaranteed dates.

The key reasoning is:

long-lived ODSs → delayed chemical recovery → slow ozone recovery → long-lasting UV-risk concern

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Stratospheric ozone depletion reduces Earth’s protection from harmful ultraviolet radiation
- The main increase caused by ozone depletion is in **UV-B**, not UV-A very much, and not natural UV-C at the surface
- **UV-A** mostly reaches the surface already, while **UV-C** is almost completely blocked by oxygen and ozone
- Ozone-depleting substances such as **CFCs** release chlorine and bromine radicals in the stratosphere
- The exam-ready causal chain is **ODSs/CFCs → ozone depletion → more UV-B → health damage**
- The three major human health effects emphasized in APES are **skin cancer**, **cataracts**, and **immune system suppression**
- UV-B can damage DNA in skin cells, increasing mutation and cancer risk
- UV-B can damage eye tissues, increasing cataract risk
- Stratospheric ozone depletion is different from ground-level ozone pollution
- **1974**, **1985**, **1987**, and **1989** are key dates in ozone science and policy
- The **Montreal Protocol** is the main international treaty addressing ozone depletion
- Ozone recovery is slow because ozone-depleting substances persist for decades

## Key Terms and Concepts

- stratospheric ozone depletion
- ozone layer
- UV-B radiation
- UV-A
- UV-C
- chlorofluorocarbons (CFCs)
- ozone-depleting substances (ODSs)
- skin cancer
- cataract
- immune system suppression
- Montreal Protocol


Stratospheric Ozone Depletion - UV increase and health effects