1 Ozone Depletion and Refrigerant Environmental Impacts

The ozone layer

Ozone is a gas made of three oxygen atoms, represented as $\mathrm{O}_3$. Most atmospheric ozone is in the stratosphere, roughly $15$–$35$ km above Earth. This ozone absorbs much of the Sun’s harmful ultraviolet-B (UV-B) radiation. Ozone near the ground is different: it is an air pollutant that can harm people and ecosystems.

Ozone is continually formed and destroyed through natural reactions. Ultraviolet light splits oxygen molecules, $\mathrm{O}_2$, and some resulting oxygen atoms combine with $\mathrm{O}_2$ to form ozone. Other reactions break ozone down again, maintaining a natural balance.

How ozone-depleting substances damage ozone

Many Ozone-depleting substances (ODSs) are stable in the lower atmosphere, allowing them to persist long enough to reach the stratosphere. There, intense ultraviolet light breaks them apart and releases chlorine or bromine. These atoms take part in catalytic reactions: they help destroy ozone and are regenerated, so one atom can participate in many cycles.

A simplified chlorine cycle is:

Cold polar conditions, especially in Antarctic spring, make ozone loss particularly rapid. Reactions on polar stratospheric cloud particles convert chlorine into forms that sunlight can activate. The resulting seasonal region of unusually low ozone is the ozone hole: a thinning of the ozone layer, not a literal hole in the atmosphere.

Important groups of ozone-depleting substances include:

• Chlorofluorocarbons (CFCs): formerly common in refrigeration and air conditioning, foam production, and aerosol applications.

• Hydrochlorofluorocarbons (HCFCs): transitional substitutes for CFCs. They contain chlorine and can still deplete ozone, though generally less per unit mass than CFC-11.

• Halons: bromine-containing chemicals used mainly in fire suppression; bromine is especially effective at destroying ozone.

• Other substances include methyl bromide, carbon tetrachloride, and methyl chloroform.

Ozone-depletion potential (ODP) compares a chemical’s ozone-depleting effect with that of CFC-11, which is assigned an ODP of $1$. ODP is distinct from global-warming potential (GWP), which compares a gas’s heat-trapping effect with carbon dioxide over a specified time period.

Refrigerants and environmental effects

Refrigerants transfer heat in air-conditioning and refrigeration systems. Their environmental effects depend on the chemical and on how much escapes during manufacture, use, servicing, or disposal.

• CFC and HCFC refrigerants can harm the ozone layer and may also be potent greenhouse gases.

• Hydrofluorocarbons (HFCs) contain no chlorine, so they do not deplete stratospheric ozone. However, many HFCs have high GWPs and can contribute substantially to climate change if released.

• Lower-GWP alternatives include some hydrofluoroolefins (HFOs) and natural refrigerants such as carbon dioxide, ammonia, and hydrocarbons. They can reduce climate impacts in suitable applications, but properties such as flammability, toxicity, and operating requirements also matter when comparing environmental and safety risks.

Refrigerants affect climate directly when leaked gas traps heat in the atmosphere, and indirectly through the electricity used to operate equipment. A low-GWP refrigerant can reduce direct emissions, while efficient equipment and leak prevention can reduce indirect and direct impacts. Refrigerant choices should not be judged by ozone impact alone.

Consequences and recovery

When stratospheric ozone declines, more UV-B can reach Earth’s surface. Increased UV-B exposure can raise risks of skin damage, skin cancer, and cataracts. It can also affect immune responses, plants, some marine organisms, and materials such as plastics. The size of the effect at a location depends on ozone levels and other conditions, including cloud cover, elevation, and surface reflection.

International action under the Montreal Protocol has reduced the production and use of many ODSs. Because these substances can have long atmospheric lifetimes, ozone recovery takes decades. Year-to-year conditions can still cause large ozone holes; for example, NOAA and NASA reported that the 2026 Antarctic ozone hole was unusually large in part because of atmospheric conditions, while long-term recovery remained on track.