Last updated: 03 Sep 2023

The United Nations proclaimed 16 September as the International Day for the Preservation of the Ozone Layer. The date commemorates the adoption of the Montreal Protocol in 1985 when governments, scientists and industry worked together to cut 99% of all ozone-depleting substances. Thanks to the Montreal Protocol, the ozone layer is healing and expected to return to pre-1980 values by mid-century.

World Ozone Day

Life on Earth would not be possible without sunlight. But the energy emanating from the sun would be too much for life on Earth to thrive were it not for the ozone layer. This stratospheric layer shields us from most of the sun’s harmful ultraviolet radiation. Sunlight makes life possible, but the ozone layer makes life as we know it possible.

When scientists in the late 1970s discovered that humanity was creating a hole in this protective shield, they raised the alarm. The hole – caused by ozone-depleting gases (ODSs) used in aerosols and cooling such as refrigerators and air-conditioners – was threatening to increase cases of skin cancer and cataracts, and damage plants, crops, and ecosystems.

The global response was decisive and World Ozone Day, held each year on 16 September, celebrates this achievement. Adopted by the UN General Assembly in 1994, the International Day for the Preservation of the Ozone Layer, commemorates the date of the signing, in 1987, of the Montreal Protocol on Substances that Deplete the Ozone Layer (resolution 49/114). It shows that collective decisions and action, guided by science, are the only way to solve major global crises. It reminds us that not only is ozone crucial for life on Earth, but that we must continue to protect the ozone layer for future generations.

Vienna Convention for the Protection of the Ozone Layer

In 1985, the world’s governments adopted the Vienna Convention for the Protection of the Ozone Layer and entered into force in 1988. The signatory countries  agreed to research and monitor the effects of human activities on the ozone layer and to take concrete action against activities that are likely to have adverse effects on the ozone layer. The Convention did not require countries to take specific actions to control ozone-depleting substances.

Montreal Protocol

Under the Vienna Convention, the Montreal Protocol on Substances that Deplete the Ozone Layer is the landmark multilateral environmental agreement that regulates the production and consumption of nearly 100 man-made chemicals referred to as ozone depleting substances (ODS). When released to the atmosphere, those chemicals damage the stratospheric ozone layer, Earth’s protective shield that protects humans and the environment from harmful levels of ultraviolet radiation from the sun. Adopted on 15 September 1987, the Protocol is to date the only UN treaty ever that has been ratified every country on Earth – all 198 UN Member States.

Thanks to the Montreal Protocol, the ozone layer is healing and expected to return to pre-1980 values by mid-century.

Kigali Amendment

Although the Montreal Protocol was designed to phase out the production and consumption of ODSs, some replacements of these substances, known as hydrofluorocarbons (HFCs), have proven to be powerful greenhouse gases. In fact, some HFCs are more than a thousand times more potent than carbon dioxide in contributing to climate change.

In support of the Protocol, the Kigali Amendment, which came into force in 2019, will work towards HFCs. A successful HFC phasedown is expected to avoid up to 0.4 degree Celsius of global temperature rise by 2100, while continuing to protect the ozone layer.

2023 Theme

Montreal Protocol: fixing the ozone layer and reducing climate change

World Ozone Day 2023 celebrates the achievements of the Montreal Protocol on Substances that Deplete the Ozone Layer in fixing the ozone layer and reducing climate change.

The latest update from the Scientific Assessment Panel to the Montreal Protocol confirmed that ozone layer recovery is on track and ozone levels are expected to return to 1980 levels by around 2066 over the Antarctic. By banning ozone-depleting substances and allowing the ozone layer to slowly recover, the treaty protecting millions of people from skin cancer and eye cataracts, safeguarding ecosystems and slowing down climate change – as many ozone-depleting substances were also climate warming gases.

However, the work and the benefits of the Montreal Protocol is far from over. The Kigali Amendment to the Montreal Protocol aims to phase down the production and consumption of hydrofluorocarbons (HFCs) – powerful climate-warming gases that replaced ozone-depleting substances in the cooling industry. Universal ratification of the Amendment and full implementation are crucial, for multiple reasons.
The planet is heating up, which increases the need for air-conditioning in homes, schools and workplaces. At the same time, expanding access to sustainable cold chain – to keep food fresh and vaccines viable – is essential to meet sustainable development aspirations. This growth in cooling must be sustainable, which means both finding safe and environmentally friendly alternatives to HFCs and increasing the energy efficiency of cooling equipment. By phasing down HFCs, the Kigali Amendment may result in avoidance of up to 0.5°C of warming by 2100. Implementing energy efficiency measures could potentially double this figure.

The conclusion is clear: for nearly 4 decades, the Montreal Protocol has been instrumental – and will continue to be instrumental – in protecting human health, nature and the climate.

Benefits of the Ozone Layer

Ozone and Climate

According to the Ozone Secretariat of the United Nations Environment Programme (UNEP), for the last two decades, some signs of recovery of the ozone layer have been observed. For example, in the mid-latitudes, upper stratospheric ozone has increased by 1–3 per cent per decade since 2000. Assuming continued compliance with the Montreal Protocol, the expected recovery of the ozone layer is as follows:

  • In the Antarctic, springtime column ozone returns to 1980 values in the 2060s;
  • In the Northern Hemisphere, mid-latitude ozone returns to 1980 values by the 2030s;
  • In the Southern Hemisphere, mid-latitude ozone returns to 1980 values around mid-century.

Without the Montreal Protocol, the Antarctic ozone hole would have been about 40 per cent larger by 2013 and deeper Arctic ozone depletion would have occurred in the very cold winter of 2011.

Ozone protection efforts have contributed significantly to slowing climate change by avoiding an estimated 135 billion tons of carbon dioxide (CO2) equivalent emissions from 1990 to 2010. This is five times larger than the annual emissions reduction target for the first commitment period (2008-2012) of the Kyoto Protocol, the predecessor to the Paris Agreement on Climate Change.

Models indicate that in the absence of the Montreal Protocol, global mean temperatures would have risen over 2°C by 2070, due to warming from ozone-depleting substances alone, and that tropical cyclones would likely have been three times as intense in 2065.

Ozone and Health

NASA model simulations have shown that without the Montreal Protocol, global ozone would have fallen so low by 2065 that light-skinned people in northern mid-latitude locations would have perceptible sunburn in about 5-10 minutes during summer outdoors.

With the Montreal Protocol, up to 2 million cases of skin cancer may be prevented globally each year by 2030.

In the United States, full implementation of the Montreal Protocol is expected to prevent approximately 443 million cases of skin cancer, 2.3 million skin cancer deaths, and 63 million cases of cataracts for people born in the years 1890–2100, according to the US Environmental Protection Agency.

Ozone and Economy

The Montreal Protocol established the Multilateral Fund for the Implementation of the Montreal Protocol to cover work in developing countries. As of December 2019, contributions had reached US$4.07 billion. The Fund has also received additional voluntary contributions of US$25.5 million from a group of donor countries to finance fast-start activities for the implementation of the HFC phase-down.

The Montreal Protocol will result in an estimated USD 1.8 trillion in global health benefits (USD 1.109 trillion for skin cancer alone) and almost USD 460 billion in avoided damages to agriculture, fisheries, and materials (both cumulative estimates from 1987 to 2060).

Consequences of Ozone Depletion

Because of the Montreal Protocol, we have avoided a world where severe ozone holes would have occurred every year over the Arctic and Antarctic. By the mid-21st century, severe ozone depletion would have spread across the planet, including the tropics. But how large an increase in UV-B would have resulted from uncontrolled ozone depletion? And how would increased UV-B have affected people, food production, ecosystems and even construction materials? Let’s explore some of the consequences of failing to control ozone depletion.

Skin Cancer

There are strong links between over-exposure to UV radiation and the development of the three most common forms of skin cancer (malignant melanoma, basal cell carcinoma and squamous cell carcinoma). Even now, with the successful implementation of the Montreal Protocol, skin cancers are amongst the most common forms of cancer, especially in pale-skinned populations, according to the World Health Organization (WHO).

Understanding how the prevalence of skin cancers would have increased with uncontrolled ozone depletion comes from computer models of the world avoided. These models combine our understanding of how ozone-depleting substances affect the ozone layer, of how changes in ozone affect UV radiation and of how UV radiation affects the incidence of skin cancers.

For example, one global model suggests that by 2030 the successful implementation of the Montreal Protocol will be preventing about two million skin cancers every year. A longer-term model focused on health effects in people born in the USA between 1890 and 2100. This model estimates that protecting the ozone layer will have prevented a total of approximately 443 million cases of skin cancers and 2.3 million skin cancer deaths in the USA alone. This includes 8-10 million cases of malignant melanoma. As yet there are no long-term world-avoided models for skin cancers globally. However, all the existing models lead to the same conclusion. Uncontrolled ozone depletion would have substantially increased the risk of skin cancers worldwide.

Eye Disease

Exposure to high levels of UV radiation leads to an increased risk of cataracts. The WHO already considers cataracts as a priority eye disease. Cataracts are responsible for around half of blindness world-wide, equivalent to about 20 million people in 2010. Currently, a world-avoided model for cataracts is only available for the USA, which indicates that failure to control ozone depletion effectively would have led to almost 63 million additional cataract cases in people born in the USA between 1890 and 2100.

Other Health Matters

As well as skin cancers and cataracts, UV radiation can have other health effects. These effects include the production of vitamin D in the skin that is beneficial to health. In the world we live in now, with effective protection of the ozone layer, there is a balance between the positive and negative effects of UV-B, according to the WHO. Had we failed to protect the ozone layer that balance would have swung dramatically towards the negatives, above all the increased risks of skin cancer and cataracts. By avoiding these negative consequences, the Montreal Protocol has made a major contribution to good health and well-being, one of the sustainable development goals adopted by all UN member states in 2015.

Food Security

Over the course of evolution, animals, plants and microbes have developed mechanisms that allow them to cope with the variation in UV-B radiation that they experience in their normal environments, protected by the intact ozone layer. This includes the plants and animals that we all rely on for our food.

Crops need sunlight for photosynthesis, so cannot avoid exposure to UV-B. They have evolved systems that reduce or repair damage, including pigments that act as “sun-screens.” As with human health, there is a balance between the positive and negative effects of UV-B on plants. Uncontrolled ozone depletion would have shifted this balance very much towards the negative.

Increased exposure to UV radiation can damage aquatic food chains and cause direct damage to crustaceans and fish eggs. As a result, uncontrolled ozone depletion would have threatened fisheries and other aquatic resources that make a significant contribution to global food supply.

As yet there are no ‘world avoided’ models for food production. There are “ball-park” figures of the relationship between ozone depletion and plant growth. These suggest that a 10% reduction in stratospheric ozone might reduce plant production by about 6 per cent. If this relationship holds for the very severe ozone depletions expected in the world avoided then uncontrolled ozone depletion would have substantially reduced crop production globally.

Overall, while we cannot yet quantify the loss in food production, it is clear that without the Montreal Protocol ozone depletion would have made it progressively harder to deliver the sustainable development goal of zero hunger.


Just as uncontrolled ozone depletion threatens food production, it also threatens plants, animals and microbes in natural ecosystems. Those ecosystems provide the “ecosystem services” that we all rely on for clean air and clean water, and to absorb carbon dioxide for the atmosphere.

As with crops, wild plants are able to cope with current levels of UV-B radiation, but their growth can be reduced by large increases in UV-B. Most animals also seem to be able to avoid the damaging effects of current levels of UV-B radiation. We do not know at what point animals’ protection mechanisms would have been overwhelmed by the unprecedented increases in UV-B in the world avoided. Even so, damage to plants would reduce the food available for herbivores, with consequences for the whole food web.

Oceans are the Earth’s biggest ecosystems. They contain micro-organisms, animals and plants that provide us with half of the oxygen we breathe and much of the food we eat. A healthy ocean is vital to our survival.

In the oceans, lakes and rivers UV-B has adverse effects on many different aspects of the biology of organisms across the food web. While there are no ‘world avoided’ models for ecosystem responses, large increases in UV-B then whole food webs would have been disrupted, threatening biodiversity and ecosystem services.

Through these effects on ecosystems, large-scale increases in UV-B could alter the exchange of carbon dioxide between the atmosphere and the biosphere. Increased UV radiation also stimulates the breakdown of decaying leaves and other organic matter. Together, the effects of increased UV-B would reduce the ability of ecosystems to trap carbon dioxide, including carbon dioxide produced by human activities. In this way, large-scale ozone depletion would have worsened the build-up of carbon dioxide in the atmosphere that is causing climate change. Changing UV-B also alters the cycling of nitrogen and other chemicals in the environment, which may worsen air pollution.

The Ozone Layer Today

While we still have work to do there is good news. As of now, 99 per cent of the ozone-depleting substances that are controlled under the Montreal Protocol have been phased out. Scientists and researchers around the world continually monitor the ozone layer’s progress. They also measure ozone-depleting substances, including some that are not controlled under the Montreal Protocol. These substances have low concentrations in the atmosphere and don’t pose an immediate threat to the ozone layer.

The Montreal Protocol is widely lauded as a huge environmental success. Whilst the damage we have done to the ozone layer has not yet been undone, thanks to this agreement and the collaborative effort of nations around the world, there is scientific evidence that the ozone layer is healing itself and is expected to recover by the middle of this century.

The Montreal Protocol has also considerably reduced climate warming because many ozone-depleting substances are also potent greenhouse gases that contribute to climate forcing when they accumulate in the atmosphere. Montreal Protocol controls have led to a substantial reduction in the emissions of ozone-depleting substances over the last two decades. In June 2023, the first WMO Ozone and UV Bulletin showed that the production and consumption of controlled ozone-depleting substances have been phased out up to 99%, thanks to the Montreal Protocol and its amendments. These reductions, while protecting the ozone layer, have the additional benefit of reducing the human contribution to climate change.

The Montreal Protocol can be further strengthened to control ozone-depleting substances and hydrofluorocarbons used as feedstocks to provide additional protection of the stratospheric ozone layer and the climate system while also mitigating plastics pollution. The Montreal Protocol can narrow the scope of feedstock exemptions to reduce inadvertent and unauthorized emissions while continuing to exempt production of feedstocks for time-limited, essential uses. This upstream approach can be an effective and efficient complement to other efforts to reduce plastic pollution by up to 20% by reducing the production of some feedstock-derived plastics and controlling vinyl chloride and ethylene dichloride, made into PVC.

Existing mechanisms in the Montreal Protocol such as the Assessment Panels and national implementation strategies can guide the choice of environmentally superior substitutes for feedstock-derived plastics. Without Montreal Protocol controls, the climate forcing due to ozone-depleting substances could now be nearly two and a half times the present value.

The Role of Geneva

Organizations are listed in alphabetical order

Group on Earth Observations (GEO)

There are several activities in the GEO Work Programme that are monitoring stratospheric ozone such as the Copernicus Atmosphere Monitoring Service (CAMS), Earth Observations for Health (EO4Health) and GEO Global Urban Observation and Information (GUOI).


The United Nations Information Portal on Multilateral Environmental Agreements (InforMEA) provides easy access to information on Multilateral Environmental Agreements (MEAs). You can consult treaty texts and provisions of decisions and resolutions adopted by the Governing Bodies of MEAs, including on the Montreal Protocol and how ozone is addressed in other MEAs. You may browse Party information including contacts, national reports and national plans submitted under MEAs and access numerous free courses.

Intergovernmental Panel on Climate Change (IPCC)

The IPCC is the United Nations body for assessing the science related to climate change. Through its assessments, the IPCC determines the state of knowledge on climate change. It identifies where there is agreement in the scientific community on topics related to climate change, and where further research is needed. The reports are drafted and reviewed in several stages, thus guaranteeing objectivity and transparency.

United Nations Environment Programme (UNEP)

UNEP became an implementing agency of the Montreal Protocol’s Multilateral Fund on 19 June 1991, and it was given this clearinghouse mandate. Since that date, UNEP OzonAction has been strengthening the capacity of governments – particularly the operational focal points for the Montreal Protocol, known as National Ozone Units (NOUs) – and industry in developing countries to elaborate and enforce the policies required to implement the Protocol and to make informed decisions about alternative technologies.

The Chemicals and Health Branch of UNEP also promotes chemical safety and provides countries with access to information on toxic chemicals,  policy advice, technical guidance and capacity building to developing countries and those with economies in transition.

World Health Organization (WHO)

The WHO has published several resources on UV radiation. The WHO indicates that the ozone is a particularly effective absorber of UV radiation. As the ozone layer gets thinner, the protective filter activity of the atmosphere is progressively reduced. Consequently, the people and the environment are exposed to higher levels of UV radiation, especially UVB. With excessive UV exposure, raised awareness and changes in lifestyle are needed to alter ongoing trends of skin cancer.

World Meteorological Organization (WMO)

The WMO provides atmospheric composition forecast and assessments, including ozone forecasts. The Global Atmosphere Watch (GAW) provides data essential for understanding the state of, and changes to, the ozone layer. The WMO also recently published the “Scientific Assessment of Ozone Depletion: 2018” report documenting the advances in scientific understanding of ozone depletion. These advances add to the scientific basis for decisions made by the Parties to the Montreal Protocol and is based on longer observational records, new chemistry-climate model simulations, and new analyses. In addition, it regularly publishes reports for the meetings of Ozone Research Managers of the Parties to the Vienna Convention for the Protection of the Ozone Layer.

The WMO also compiles the United in Science report, a multi-organization high-level compilation of the latest climate science information.