Polar bears are superbly insulated by up to 10 cm (3.9 in) of blubber (a thick layer of vascularized fat found under the skin ), their hide and their fur; they overheat at temperatures above 10 °C (50 °F), and are nearly invisible under infrared photography. Polar bear fur consists of a layer of dense underfur and an outer layer of guard hairs, which appear white to tan but are actually transparent. The guard hair is 5–15 cm (2.0–5.9 in) over most of the body. Polar bears gradually moult from May to August, but, unlike other Arctic mammals, they do not shed their coat for a darker shade to camouflage themselves in the summer conditions. The hollow guard hairs of a polar bear coat were once thought to act as fiber-optic tubes to conduct light to its black skin, where it could be absorbed; however, this theory was disproved by recent studies.

Solar water disinfection, also known as SODIS is a method of disinfecting water using only sunlight and plastic PET bottles. SODIS is a free and effective method for decentralized water treatment, usually applied at the household level and is recommended by the World Health Organization as a viable method for household water treatment and safe storage. SODIS is already applied in numerous developing countries. Educational pamphlets on the method are available in many languages, each equivalent to the English language version.

In solar water disinfection (SODIS), microbes are destroyed by temperature and UVA radiation provided by the sun. Water is placed in a transparent plastic PET bottles, which is first oxygenated by shaking partially-filled capped bottles prior to filling the bottles all the way. The completely water-filled and capped bottles are exposed to sunlight, preferably on a corrugated metal roof, slanted slightly to maximize the exposure to solar radiation. In practice, the water-filled bottles are placed for six hours in full sun, or for two days in partial sunlight for weather conditions involving partially overcast days, which raises the temperature of the water and gives an extended dose of solar radiation to the water in the bottles, killing almost all microbes that may be present. The combination of the two effects (UVA and heat) provides a simple method of disinfection for use in tropical developing countries, or in survival situations. The use of glass bottles may or may not provide the same degree of SODIS disinfection as using PET bottles. This is because most glass bottles are non-transparent or opaque over the wavelengths of sunlight required for successful UV disinfection from the solar spectrum required for SODIS to work, and glass bottles are usually thicker than PET bottles, which further reduces the dose of UVA to the water inside glass bottles versus PET bottles. For cases where the UVA is blocked, or reduced, only the heating effects without adequate UVA exposure are typically at work if glass bottles are used, potentially leaving dangerous amounts of bacterial and viral loads within the water.

See also:
https://en.wikipedia.org/wiki/Polar_bear
https://en.wikipedia.org/wiki/SODIS

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