The UN has many world days, but this one is special: the International Day for the Preservation of the Ozone Layer. It's 30 years since the world realized it had to plug the hole in the Earth's UV shield.
It's not much - but the ozone layer is all there is between you and UV.
As the United Nations puts it, "If we could bring the entire ozone layer to sea level, it would be only about 3mm thick. That's what protects us from harmful ultraviolet radiation."
That really isn't very much, is it - just 3mm of protection? You can't even see it.
And in some regions of the stratosphere, the ozone is so depleted that we refer to an "ozone hole." In fact, we have known for decades that the ozone layer is itself under threat and needs our protection.
So it's perhaps with a touch of optimism that the United Nations should choose to mark this year's International Day for the Preservation of the Ozone Layer (September 16) by celebrating "30 years of healing the ozone together."
We still have a way to go before the ozone layer has healed.
But the optimist always wins. So in a similar spirit of optimism, here's our brief guide to what you need to know about our very precious ozone.
What is the ozone layer?
Ozone is a molecule, consisting of three oxygen atoms (O3). Ozone molecules occur naturally in the upper atmosphere - known as the stratosphere - and form a layer of gas. This layer of gas protects life on Earth by filtering some (but not all) of the sun's ultraviolet radiation.
Ozone is also created by chemical reactions between air pollutants and other emissions in the lower atmosphere - the troposphere.
While ozone provides us with a protective shield in the stratosphere, direct contact in the troposphere can be harmful to plants, animals and humans.
So what's the problem with ultraviolet radiation (UV)?
It is often said we need the ozone layer to stop UV radiation from "sterilizing" the Earth's surface. We know that sun rays do have the power to sterilize things, and that can be good - think about your washing drying on the line on a hot summer's day. But what we really mean to say is UV radiation can kill.
There are three types of UV radiation: A, B and C.
The ozone layer and atmosphere absorb all of the UVC - the most energetic form of UV radiation - and some UVB. UVA is not absorbed by the ozone layer, and reaches the Earth's surface in its entirety.
Humans need UVB radiation to generate vitamin D, which is good for people in appropriate doses. But too much UVB and UVA can cause serious illnesses, such as skin cancer, cataracts, suppression of the immune system, and premature aging of the skin. An excess of UV is also linked with reduced crop yields, and adverse effects in the marine food chain.
Aside from that, why should I care?
Both human-made and naturally occurring gases have been depleting the ozone layer, exposing us to life-threatening UV. But it's not just a threat to our own lives - it's also a threat to our environments. Humans are a part of the natural world and when ecosystems fail, we fail too.
What exactly causes ozone depletion?
To a large extent, humans cause ozone depletion - through our use of "ozone depleting substances" (ODSs). Gases such as chlorofluorocarbons (CFCs), halons, CH3CCl3 (Methyl chloroform), CCI4 (Carbon tetrachloride), hydrochlorofluorocarbons (HCFCs) and methyl bromide destroy the ozone layer.
These substances can be found in refrigerators, air conditioners, aerosols, solvents and pesticides, to name a few.
They deplete the ozone layer by releasing chlorine and bromine atoms, which degrade ozone molecules, into the stratosphere.
Earlier this year, scientists warned that "very short-lived substances" (VSLSs) were also threatening the ozone layer. These are naturally-occurring in marine life, among seaweed and phytoplankton. But our production of human-made VSLSs, such as dichloromethane, is increasing.
So what about the ozone hole?
Australia has been heavily affected by the ozone hole - perhaps more than other nations. Its environment department says the largest ozone holes on record appeared in 2000 and 2006. They measured 29.8 and 29.6 million square kilometers, respectively. That's over three and a half times the size of the Australian continent - and "at times extended over populated areas."
NASA's Ozone Hole Watch provides daily updates, including images and animations from 1979 to 2015.
But it's not really a hole, is it?
No, strictly speaking, there is no ozone "hole" - rather, there is a region of "exceptionally depleted ozone over the Antarctic." It is also referred to as a "dramatic thinning" of ozone, which show up during spring in the southern hemisphere (August-October).
It is thought the ozone hole first appeared over Antarctica due to meteorological conditions unique to the region "facilitating" ozone destruction.
There are "mini-holes" and other aberrations in the northern hemisphere, too.
What's being done to stop ozone depletion?
The Montreal Protocol on Substances that Deplete the Ozone Layer is considered one of the most successful United Nations treaties. It includes a mandatory timetable for the phase-out or reduction of ODSs.
It is reviewed regularly, and adjusted to reflect developments in science and technology. Developed countries are required to phase out ODSs earlier than developing countries - but by 2030, most should have achieved their goals.
The Vienna Convention and the Montreal Protocol became the first treaties in the history of the UN to achieve universal ratification on September 16, 2009.
Will the ozone layer ever heal completely?
Scientists say the future looks good for the ozone layer. It may recover to pre-1980 levels over the next 50 years - sometime between 2050 and 2065 - if we stick to the Montreal Protocol.
And remind me, what's the deal with the International Day for the Preservation of the Ozone Layer?
The International Day for the Preservation of the Ozone Layer (September 16) is an annual event to commemorate the signing of the Montreal Protocol. The commemorative day was agreed to by the United Nations General Assembly in 1994.
The UN says programs to phase out or reduce the use of ozone depleting substances have "not only helped protect the ozone layer for this and future generations, but have also contributed significantly to global efforts to address climate change."
This year also marks the 30th anniversary of the Vienna Convention for the Protection of the Ozone Layer - hence the slogan: "30 Years of Healing the Ozone Together."