Often confused · Ozone depletion
ODP vs GWP: two numbers the EPA 608 likes to swap
Ozone depletion potential (ODP) scores how much stratospheric ozone a refrigerant can destroy, while global warming potential (GWP) scores how much heat it traps against carbon dioxide; the EPA 608 grades the two as separate facts, and a refrigerant can be low on one and high on the other.
Part of Ozone depletion in the Study guide
Two scales that answer different questions
- ODP
- Ozone depletion potential. Ozone damage per unit of mass, scaled so that CFC-11 (R-11) is 1.0. Chlorine in the molecule is what drives it.
- GWP
- Global warming potential. Heat trapped over a set time span, compared with the same mass of carbon dioxide, which is fixed at 1.
- Zero ODP
- The score of any refrigerant without chlorine, which covers HFCs and HFOs. It says nothing about the same gas's GWP.
- Class I / Class II
- Regulatory labels for ozone-depleting substances: CFCs (and halons) are Class I, HCFCs are Class II. HFCs belong to neither; the rules call them substitutes.
How the Core section ranks refrigerants
The Core section treats ozone as a ladder. The test topics list it plainly: CFCs have the highest ODP, HCFCs come next, and HFCs sit at zero (EPA test topics, checked October 7, 2026). Learn a refrigerant's class and you have its rank. The ladder is what the topics list; a table of decimals for single gases isn't.
Reading ODP as a warming score is the classic slip. An HFC can score zero for ozone and still carry a large warming number, because the two scales use different reference gases and answer different questions. It's a bit like reading a tire gauge as a thermometer: the needle moves, it just measures something else.
Whether a zero-ODP gas may be released is a separate rule, worked through on the venting prohibition and its narrow exceptions page.
The ladder, class by class
Refrigerant classes by chlorine, ODP and warming
| Class | Chlorine | ODP | Example | Warming note |
|---|---|---|---|---|
| CFC | Yes | Highest; CFC-11 sets the 1.0 reference | R-11 | Ozone made it the first class regulated |
| HCFC | Yes, less | About 0.01–0.1 | R-123 | Small ODP still counts as depleting |
| HFC | No | 0 | R-410A, R-32 | R-410A sits above the GWP 700 line the AIM Act uses for new home AC |
| HFO | No | 0 | R-1234yf | Built for low GWP; still a substitute under Section 608 |
Ranking and the HCFC range: EPA test topics, checked October 7, 2026. GWP 700 line: 40 CFR 84.54(c), eCFR as of October 5, 2026, an AIM Act rule outside the 608 exam.
Sort the scales
Before you read the options, decide whether the stem is asking about ozone or about warming.
0 right · 0 of 7 answered
HFCs are being considered in HVAC systems for their ability to:
Why each option is right or wrong
Answer: C. replace CFCs due to no ozone depletion potential but having a high Global Warming Potential (GWP).
- ALow-pressure operation is a feature of certain chillers, not the reason HFCs replaced CFCs.
- BRefrigerants do not add humidity to aid cooling; AC removes moisture.
- CCorrect: HFCs replaced CFCs because they contain no chlorine and have zero ozone depletion potential, though many have a high global warming potential.
- DHFCs do not replace water as a heat-transfer medium; water is still used in chillers and cooling towers.
Hydrofluorocarbons (HFCs) like R-134a are used to replace CFCs because they do not deplete the ozone layer. However, they have a high Global Warming Potential (GWP), contributing significantly to climate change.
What is the definition of 'ozone depletion potential' (ODP)?
Why each option is right or wrong
Answer: A. A relative measure of a substance's ability to destroy stratospheric ozone compared to CFC-11
- ACorrect: ODP compares how much ozone a substance destroys with CFC-11, which is set at 1.0.
- BODP is not a temperature; it is a ratio compared with CFC-11.
- CODP does not measure time; atmospheric lifetime affects it but is a separate quantity.
- DODP is not an amount of refrigerant; it is a relative index with CFC-11 as 1.0.
Ozone Depletion Potential (ODP) is a relative measure of a substance's ability to destroy stratospheric ozone compared to CFC-11, which has an ODP of 1.0. Higher ODP values indicate greater potential to damage the ozone layer.
A technician is substituting an HCFC refrigerant with an HFC refrigerant in a small appliance. How does this substitution primarily alter the environmental impact profile of the system's charge?
Why each option is right or wrong
Answer: B. It eliminates the ozone depletion potential entirely
- AODP and GWP are separate measures, and switching refrigerants does not make them equal.
- BCorrect: HFCs contain no chlorine, so replacing an HCFC with an HFC removes the ozone depletion potential of the charge.
- CHFCs have zero ODP, so the switch lowers ozone depletion potential rather than raising it.
- DHFCs still have a global warming potential, often a high one, so GWP does not drop to zero.
HFC refrigerants contain no chlorine, meaning they have zero ozone depletion potential. However, HFCs still possess a significant global warming potential.
R-410A has an ozone depletion potential of zero. Why is knowingly venting it during service still prohibited?
Why each option is right or wrong
Answer: A. They are non-exempt substitutes covered by the venting prohibition.
- ACorrect: Section 608's venting prohibition covers substitute refrigerants such as HFCs unless EPA lists them as exempt for that use; zero ODP doesn't take R-410A off the list.
- BHFCs do not break down into heavy metals; they contain only hydrogen, fluorine and carbon.
- CWrong: smog isn't the reason; the ban follows the regulation's list of covered refrigerants.
- DHFCs have zero ozone depletion potential; high ODP describes CFCs.
The venting prohibition covers ozone-depleting refrigerants and substitute refrigerants such as HFCs, unless EPA lists a substitute as exempt for a specific end-use. Whether a refrigerant may be released depends on that list, not on its ODP or GWP.
A system leaks 10 kg of a blend with a GWP of 2,000. What is the leak's CO₂ equivalent?
Why each option is right or wrong
Answer: D. Twenty metric tons
- A200 metric tons is ten times too high; 10 kg × 2,000 is 20,000 kg.
- B2 metric tons drops a factor of ten; 10 × 2,000 = 20,000 kg, not 2,000 kg.
- C0.2 tons is a hundred times too small.
- DCorrect: CO₂ equivalent = mass × GWP = 10 kg × 2,000 = 20,000 kg, or 20 metric tons.
CO₂ equivalent is mass times GWP: 10 kg × 2,000 = 20,000 kg, or 20 metric tons.
What determines a refrigerant's ozone depletion potential (ODP)?
Why each option is right or wrong
Answer: D. Chlorine/bromine content and atmospheric lifetime
- AColor has no relation to ozone depletion; most refrigerants are colorless.
- BTemperature affects operation, not a refrigerant's ozone depletion potential.
- COperating pressure is used to classify equipment, not to rate ozone depletion.
- DCorrect: ODP depends on the chlorine or bromine in the molecule and how long it lasts in the atmosphere to reach the stratosphere.
A refrigerant's ozone depletion potential is determined by its chlorine or bromine content and atmospheric lifetime.
Which list orders these refrigerants from highest to lowest ozone depletion potential: R-22, R-134a, and R-12?
Why each option is right or wrong
Answer: D. R-12, then R-22, then R-134a
- AR-12 is a CFC and has a higher ODP than the HCFC R-22.
- BR-134a is an HFC with zero ODP, so it cannot rank above R-22.
- CThis reverses the ladder; R-134a has no chlorine and an ODP of zero.
- DCorrect: the CFC R-12 is highest, the HCFC R-22 is lower, and the HFC R-134a has zero ODP.
Ozone depletion potential follows chlorine content and stability: CFCs are highest, HCFCs are much lower, and HFCs are zero. CFC-11 is the reference with an ODP of 1.0.
More practice across all four sections: Practice test · Timed mock
Ranking refrigerants by ODP
Which refrigerant class has the highest ozone depletion potential?
CFCs. CFC-11 (R-11) defines the scale at 1.0, HCFCs fall roughly between 0.01 and 0.1, and HFCs score zero (EPA test topics, checked October 7, 2026).
Do HFOs like R-1234yf have any ozone impact?
Their ODP is zero because the molecule has no chlorine. They are still substitute refrigerants, so the sales restriction that has covered substitutes since January 1, 2018 applies to them (40 CFR 82.154(c), eCFR as of October 5, 2026). The wider class picture is on the ozone depletion study page.