AP Environmental Science

## Learning Outcomes

- Explain why substitutes for ozone-depleting substances are needed
- Distinguish among **CFCs**, **HCFCs**, **HFCs**, **HFOs**, and natural refrigerants
- Compare substitute chemicals using **ozone depletion potential (ODP)** and **global warming potential (GWP)**
- Understand why **HCFCs** are transitional substitutes rather than final solutions
- Explain why **HFCs** protect ozone but can still worsen climate change
- Identify the roles of the **Montreal Protocol**, **Kigali Amendment**, and EPA **SNAP Program**
- Evaluate tradeoffs such as flammability, toxicity, pressure, and end-use suitability

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand substitutes for ozone-depleting chemicals, with a focus on the following syllabus points:

- Know that ozone-depleting substances include **CFCs**, **halons**, **HCFCs**, and **methyl bromide**
- Identify **ODP** as a measure of ozone harm relative to **CFC-11 = 1.0**
- Identify **GWP** as a measure of heat-trapping ability relative to **CO2 = 1**
- Explain why **HCFCs** have lower ozone impact than CFCs but still deplete ozone
- Understand that **HFCs** generally have **ODP = 0** but may have high **GWP**
- Explain why **HFOs** and natural refrigerants are used as lower-GWP alternatives in some applications
- Know that the **Montreal Protocol** was agreed in **1987** and that the **Kigali Amendment** was adopted on **15 October 2016** and entered into force on **1 January 2019**
- Understand that EPA’s **SNAP Program** evaluates substitutes by specific end use, not as a one-size-fits-all list

## 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. Why were CFCs replaced in the first place?
2. What is the difference between **ODP** and **GWP**?
3. Why are **HCFCs** called transitional substitutes?
4. Why did **HFCs** solve one environmental problem but create another?
5. Name two natural refrigerants and one tradeoff linked to their use.

## Introduction

Substitutes for ozone-depleting chemicals are a classic AP Environmental Science topic because they show how solving one environmental problem can create another. Scientists and policymakers first worked to replace chemicals that damaged the **stratospheric ozone layer**, especially **chlorofluorocarbons (CFCs)**. Later, they realized that some replacement chemicals, while safer for ozone, still had a serious role in climate change.

This topic matters because exam questions often ask students to compare chemical groups, explain policy changes over time, and evaluate tradeoffs. You need to know not just which chemicals replaced older ones, but why those substitutes were chosen, why some later became a problem, and why no single substitute works best in every use.

> **Key Term: ozone-depleting substances (ODS)**
> Chemicals that damage stratospheric ozone, including major examples such as CFCs, halons, HCFCs, and methyl bromide.

### Why substitutes were needed in the first place

The ozone layer in the stratosphere protects life by absorbing much of the Sun’s harmful ultraviolet radiation. CFCs became a major problem because they are very stable. That stability made them useful in refrigerators, air conditioners, aerosols, and foams, but it also meant they survived long enough to reach the stratosphere.

Once there, ultraviolet radiation broke CFC molecules apart and released chlorine atoms. Those chlorine atoms acted as agentss in ozone destruction. A a agent is especially dangerous here because it is not used up in the reaction, so one chlorine atom can help destroy many ozone molecules.

> **Key Term: chlorofluorocarbons (CFCs)**
> Human-made chemicals once widely used as refrigerants, propellants, and foam-blowing agents that strongly deplete stratospheric ozone.

The key idea is cause and effect:

- CFCs are released at Earth’s surface
- Their stability lets them persist in the atmosphere
- They rise into the stratosphere
- UV light breaks them apart
- Chlorine is released
- Chlorine destroys ozone
- More UV reaches Earth’s surface

That final step matters for both the environments and people. More UV can damage phytoplankton, lower crop productivity, increase skin cancer risk, and cause cataracts.

This is why simple substitution mattered so much. If society still needed refrigeration, foam insulation, aerosols, and fire suppression systems, it had to replace the chemicals causing ozone loss.

> **Key Term: ozone depletion potential (ODP)**
> A measure of a substance’s ability to damage stratospheric ozone relative to CFC-11, which is assigned a value of 1.0.

A high-yield exam distinction is this: **ODP** measures ozone damage, not climate impact. Students often confuse that with **GWP**, which measures heat-trapping effect.

> **Test Tip:** If a question asks why a chemical was phased out under ozone policy, focus on chlorine or bromine release and **ODP**, not greenhouse warming.

### The replacement pathway: CFCs to HCFCs to HFCs

Replacement did not happen in one perfect jump. It happened in stages.

The first major substitute group was **hydrochlorofluorocarbons (HCFCs)**. These were considered better than CFCs because they had lower ozone depletion potential. However, they still contained chlorine, so they still damaged ozone. That is why HCFCs are best understood as an interim step, not a final answer.

> **Key Term: hydrochlorofluorocarbons (HCFCs)**
> Replacement chemicals for CFCs that have lower ODP than CFCs but still deplete ozone because they contain chlorine.

Why use a transitional substitute at all? Because industries needed time. Refrigeration and air-conditioning systems cannot always switch instantly to a totally different chemical without redesigning equipment, retraining technicians, and updating safety rules. HCFCs gave manufacturers a bridge between highly damaging CFCs and newer alternatives.

This is an important APES point: environmental policy often works through partial improvements first, then stricter improvements later.

The next major substitute group was **hydrofluorocarbons (HFCs)**. These became popular because they do not contain chlorine or bromine, so they generally have **ODP = 0**. In other words, they are ozone-safe substitutes.

> **Key Term: hydrofluorocarbons (HFCs)**
> Chemicals used to replace CFCs and HCFCs that generally have zero ozone depletion potential because they contain no chlorine.

At first, HFCs looked like a major success. They protected the ozone layer and worked well in refrigeration, air conditioning, foams, and aerosols. But then a second problem became clear: many HFCs are powerful greenhouse gases.

> **Key Term: global warming potential (GWP)**
> A measure of how strongly a gas traps heat in the atmosphere relative to carbon dioxide, with CO2 assigned a value of 1.

This creates the central tradeoff in the topic:

- **CFCs**: high ODP, often high GWP
- **HCFCs**: lower ODP than CFCs, but still above zero; often significant GWP
- **HFCs**: zero ODP, but often high GWP

So HFCs solved the ozone problem but could worsen climate change.

That is exactly the kind of two-sided reasoning AP Environmental Science rewards.

> **Test Tip:** Do not write that HFCs are ozone-depleting. They are controlled for climate reasons, not because they directly destroy ozone.

### Newer substitutes: HFOs and natural refrigerants

Because many HFCs have high GWP, newer substitutes have been developed that aim to reduce both ozone damage and climate impact. One important synthetic group is **hydrofluoroolefins (HFOs)**.

> **Key Term: hydrofluoroolefins (HFOs)**
> Newer synthetic refrigerants that generally have zero ODP and lower GWP than many HFCs.

For APES purposes, the headline is simple: HFOs are usually an improvement over many HFCs because they keep **ODP at zero** while lowering **GWP**. But they are not automatically perfect. Some are mildly flammable, so their use depends on equipment design and safety standards.

Another major category is **natural refrigerants**. These include **ammonia**, **carbon dioxide**, and hydrocarbons such as **propane** and **isobutane**.

> **Key Term: natural refrigerants**
> Refrigerants such as ammonia, carbon dioxide, propane, and isobutane that are used as alternatives to ozone-depleting substances.

These are often attractive because they generally have **zero ODP** and lower climate impact than many older synthetic refrigerants. But they come with engineering and safety tradeoffs.

A clear comparison helps:

| Substitute type            |             ODP |      Climate concern | Common tradeoff                |
| -------------------------- | --------------: | -------------------: | ------------------------------ |
| HCFCs                      | low but above 0 |          can be high | still damages ozone            |
| HFCs                       |               0 |       often high GWP | climate impact                 |
| HFOs                       |               0 | lower than many HFCs | mild flammability in some uses |
| Ammonia                    |               0 |                  low | toxicity concerns              |
| Carbon dioxide refrigerant |               0 |  low in this context | very high operating pressure   |
| Propane/isobutane          |               0 |                  low | flammability                   |

Now the reasoning matters. A student might think natural refrigerants are always the best option because they sound “natural.” That is too simple.

For example:

- **Ammonia** works well in some industrial systems, especially large cold-storage uses, but toxicity is a concern.
- **Carbon dioxide** can be nonflammable and ozone-safe, but systems often run at high pressure.
- **Propane** and **isobutane** can have low climate impact, but flammability means stricter controls are needed.

So the “best” substitute depends on the application.

### Why policy kept changing: Montreal, Kigali, and SNAP

The policy history matters because the science kept developing.

The **Montreal Protocol** was agreed in **1987** and entered into force in **1989**. Its main purpose was to phase out substances that deplete the ozone layer, especially CFCs and later other ozone-depleting substances.

> **Key Term: Montreal Protocol**
> The 1987 international treaty designed to phase out substances that deplete the ozone layer.

This treaty is widely seen as a major environmental success because it sharply reduced releases of ozone-depleting chemicals and supported ozone recovery.

But the success of the Montreal Protocol also led to widespread use of HFCs. Since HFCs do not deplete ozone, they fit the original ozone goal. Yet many turned out to be strong greenhouse gases.

That led to the **Kigali Amendment**.

> **Key Term: Kigali Amendment**
> An amendment adopted on 15 October 2016 to phase down HFCs for climate reasons; it entered into force on 1 January 2019.

The Kigali Amendment does **not** classify HFCs as ozone-depleting substances. Instead, it adds them to the Montreal system because of their climate impact. This distinction is very commonly tested.

The policy sequence is worth memorising:

- CFCs phased out because they strongly deplete ozone
- HCFCs used as transitional substitutes
- HFCs adopted because they have zero ODP
- HFCs later phased down because of high GWP
- HFOs and natural refrigerants promoted in some uses as lower-GWP options

In the United States, EPA’s **Significant New Alternatives Policy (SNAP) Program** evaluates substitutes for ozone-depleting substances.

> **Key Term: SNAP Program**
> The EPA program that evaluates substitutes for ozone-depleting substances by specific end use.

The phrase **by specific end use** is crucial. A chemical may be acceptable in one kind of equipment but not in another. Why? Because different systems have different design limits, leak risks, pressures, flammability concerns, or toxicity concerns.

For instance, a substitute may be acceptable in household refrigerators but not motor vehicle air conditioning, or acceptable only with conditions such as charge-size limits or safety compliance.

That means APES students should avoid sweeping claims such as “HFOs are always best” or “natural refrigerants are always safest.”

### How to evaluate substitutes in exam questions

When an FRQ or MCQ asks you to evaluate a substitute, use a structured approach.

Start with ozone protection:

- Does it have **zero or low ODP**?
- Does it contain chlorine or bromine?

Then consider climate:

- Does it have **low or high GWP**?

Then consider use and risk:

- Is it flammable?
- Is it toxic?
- Does it require high-pressure equipment?
- Is it suitable for that specific end use?

A strong APES answer often looks like this in logic form:

1. Identify the environmental problem being solved
2. Compare alternatives using ODP
3. Add the climate tradeoff using GWP
4. Add safety or engineering tradeoffs
5. Reach a balanced judgement

Example reasoning:

A company replacing a CFC refrigerant should avoid another ozone-depleting substance. An HFC would reduce ozone damage because its ODP is zero, but many HFCs still have high GWP, so they can worsen climate change. An HFO or natural refrigerant may reduce both ozone and climate impact, but the final choice depends on equipment design and safety constraints such as flammability, toxicity, or pressure.

That answer is stronger than just saying “use HFCs” or “use natural refrigerants.”

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Ozone-depleting substances include **CFCs**, **HCFCs**, halons, and methyl bromide
- **CFCs** were phased out because they release chlorine in the stratosphere and strongly damage ozone
- **ODP** measures ozone damage relative to **CFC-11 = 1.0**
- **HCFCs** have lower ODP than CFCs but still deplete ozone and are transitional substitutes
- **HFCs** generally have **ODP = 0** because they contain no chlorine
- Many **HFCs** are still major greenhouse gases with high **GWP**
- **GWP** measures heat-trapping ability relative to **CO2 = 1**
- **HFOs** generally have zero ODP and lower GWP than many HFCs
- Natural refrigerants include **ammonia**, **carbon dioxide**, **propane**, and **isobutane**
- The **Montreal Protocol (1987)** phased out ozone-depleting substances
- The **Kigali Amendment** phases down HFCs for climate reasons, not ozone reasons
- EPA’s **SNAP Program** evaluates substitutes by specific end use, so no single substitute is ideal for every application

## Key Terms and Concepts

- ozone-depleting substances (ODS)
- chlorofluorocarbons (CFCs)
- ozone depletion potential (ODP)
- hydrochlorofluorocarbons (HCFCs)
- hydrofluorocarbons (HFCs)
- global warming potential (GWP)
- hydrofluoroolefins (HFOs)
- natural refrigerants
- Montreal Protocol
- Kigali Amendment
- SNAP Program


Reducing Ozone Depletion - Substitutes for ozone-depleting chemicals