While scientists had expected a global decrease in ozone, they were surprised when, in 1985, a gradual decline in springtime stratospheric ozone levels over Halley Bay, Antarctica, was reported. It was speculated at the time that ozone loss was an Antarctic phenomenon only and that it might be linked to increasing atmospheric levels of CFCs. NASA scientists soon confirmed the Antarctic-wide nature of the observed ozone loss, as shown in the satellite measurements of total atmospheric ozone. The CFC link was more difficult to show, but extensive observations in 1987, including information collected by stratospheric research aircraft, found convincing links between rising stratospheric chlorine levels, a reactive chemical (chlorine monoxide) and the ozone decrease.

The Antarctic ozone hole has become a regular feature of each southern hemisphere spring. The return of spring sunshine leads to rapid ozone destruction with total loss of 60-70 % overall. The stratosphere returns to normal at the end of spring when rising temperatures force the large parcel of ozone-depleting air to disperse, being replaced by the ozone-rich air from the lower latitudes.

While the Antarctic ozone hole is the most dramatic example of damage to the ozone layer, world wide losses are becoming well-documented. Significant physical differences between the northern and southern hemisphere have stopped an Arctic hole from occurring, although it may occur over the next decade. Annual ozone losses of 2-4% over the last decade have been reported for the mid-latitudes. Over the past 20-30 years, sufficient ODS have been released into the atmosphere to cause serious damage to the ozone layer; peak ozone depletion is expected over the next few years. Over the northern mid-latitudes covering most of Europe and North America, cumulative ozone losses of 12-13 % are predicted in winter and spring, while 6-7 % losses are expected in the summer and autumn. Over the southern mid-latitudes, cumulative ozone losses are predicted to be around 11 % all year round.

Chlorofluorocarbons (CFCs), the most widely-known ozone-depleting chemicals were first synthesised in 1928. Because of their inflammability and low toxicity, they were used in applications as diverse as refrigerants in refrigerators and air-conditioners, propellants in aerosol spray cans, blowing agents in the manufacture of foams, and cleaning agents for electronic equipment.

Hydrochlorofluorocarbons (HCFCs), were developed as substitutes for CFC refrigerants and blowing agents. Though less destructive than CFCs, the ozone-depleting potential (ODP) of these chemicals are too high to allow long-term use. (ODP is a measure of a substance ability to destroy stratospheric ozone. It depends on the substance's atmospheric lifetime, stability, reactivity and the ozone-depleting elements it contains such as chlorine and bromine. All ODP values are expressed in relation to the baseline value of 1 for CFC-11.)

Two other chlorine containing chemicals, widely used as solvents for cleaning metals, have significant ODP. These are carbon tetrachloride and methyl chloroform.

The main bromine-containing chemicals that destroy ozone are called halons. These are used in fire-extinguishing equipment. Some halons have an ODP ten times higher than that of the most potent CFCs.

Another chemical with a high ODP is methyl bromide, mainly used as an agricultural pesticide and to fumigate agricultural commodities.

These chemicals are collectively known as ozone-depleting substances (ODS). They can be identified by their trade names (e.g. FREON), refrigerant codes (e.g. CFC-113 or R-12) and by their chemical names (e.g. 1,1,1-trichlorotriflouroethane).

The ozone layer shields all life-form from the harmful effects of UV-B radiation by absorbing all but a small fraction of the harmful ultraviolet radiation (UV-B) emanating from the sun. Exposures to high levels of UV-B radiation is extremely dangerous and can cause the following :-

Skin cancers

Damage to eyes

Crop damage

Severe disruption of the marine ecosystems

Degradation of man-made materials like paints and plastics

Increased global warming and climate change

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Singapore became a signatory to the Stockholm Convention on Persistent Organic Pollutants on 23 May 2001.

In Singapore, the use of the 8 pesticide POPs namely, aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, mirex, and toxaphene, is already banned. Singapore does not need to use these POP pesticides as substitute chemicals and alternative methods for pest control are available.

The use of hexachlorobenzene as a pesticide is also banned in Singapore. Hexachlorobenzene is also not used as an industrial chemical by the industries in Singapore.

The import and use of PCBs, including electrical transformers and capacitors containing PCBs, has been prohibited in Singapore since 1980. The PCB-transformers and capacitors installed in Singapore prior to the ban had also been phased out.
Dioxins and furans are by-products produced from combustion processes such as refuse incineration. In Singapore, disposal of dioxin precursor compounds such as polyvinyl chloride wastes at our refuse incineration plants is not permitted. The levels of dioxins and furans emitted by the incineration plants in Singapore are low and comparable to those of developed countries.

With the use of advanced incineration technology, pollution control equipment and strict control on the disposal of dioxin precursor compounds in the waste stream, dioxins and furans generation and emissions have been minimised in our incineration plants.

Persistent organic pollutants (POPs) are chemicals that resist degradation by physical, chemical or biological pathways. Their persistence enables them to be transported by air, water or other means to remote regions where they have never been used. Since they have a propensity to bioaccumulate, they may concentrate in the living tissues of species higher up in the food chain and pose a risk to the wellbeing of human populations and wildlife.

12 chemicals, namely, aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, mirex, toxaphene, PCBs, hexachlorobenzene, dioxins and furans have been identified as POPs for control under the Stockholm Convention on Persistent Organic Pollutants (POPs). Based on their uses or sources of generation, these 12 POPs can be grouped into the following 3 categories:

Categories	Chemicals	Uses/Sources
Pesticides	Aldrin	To control termites
	Chlordane	To control termites
	DDT	To control malaria
	Dieldrin	To control termites
	Endrin	To control mice
	Heptachlor	To control malaria
	Mirex	To control fire ants and termites
	Toxaphene	To control ticks
Industrial chemicals	PCBs	Use as dielectric fluid and insulation material in electrical transformers and capacitors.
	Hexachlorobenzene	Use in fireworks, synthetic rubber and also as pesticide.
Unintended By-products	Dioxins	From waste incineration/ combustion process.
	Furans	Formed along with dioxins from waste incineration/ combustion process.