purification, even being included in accessory packs for c-rations until 1945. sodium dichloroisocyanurate (nadcc) has largely displaced halazone tablets for the few remaining chlorine-based water purification tablets available today. it is compressed with effervescent salts, usually adipic acid and sodium bicarbonate, to form rapidly dissolving tablets, diluted to 10 parts per million available chlorine (ppm av.cl) when drinking water is mildly contaminated and 20ppm when visibly contaminated. chlorine bleach tablets give a more stable platform for disinfecting the water than liquid bleach (sodium hypochlorite) as the liquid version tends to degrade with age and give unregulated results unless assays are carried out – not practical on the spot. still, despite chlorine-based halazone tablets falling from favor for portable water purification, chlorine-based bleach may nonetheless safely be used for short-term emergency water disinfection. two drops of unscented 5% bleach can be added per liter or quart of clear water, then allowed to stand covered for 30 to 60 minutes. after this treatment, the water may be left open to reduce the chlorine smell and taste. guidelines are available online for effective emergency use of bleach to render unsafe water potable. the centers for disease control & prevention (cdc) and population services international (psi) promote a similar product (a 0.5% - 1.5% sodium hypochlorite solution) as part of their safe water system (sws) strategy. the product is sold in developing countries under local brand names specifically for the purpose of disinfecting drinking water. bleach common bleach including calcium hypochlorite (ca2) and sodium hypochlorite (naocl) are common, well-researched, low-cost oxidizers. the epa recommends two drops of 8.25% sodium hypochlorite solution (regular, unscented chlorine bleach) mixed per one quart/liter of water and let stand 30 minutes. two drops of 5% solution also suffices. double the amount of bleach if the water is cloudy, colored, or very cold. afterwards, the water should have a slight chlorine odor. if not repeat the dosage and let stand for another 15 minutes before use. neither chlorine (e.g., bleach) nor iodine alone is considered completely effective against cryptosporidium, although they are partially effective against giardia. chlorine is considered slightly better against the latter. a more complete field solution that includes chemical disinfectants is to first filter the water, using a 0.2 µm ceramic cartridge pumped filter, followed by treatment with iodine or chlorine, thereby filtering out cryptosporidium, giardia, and most bacteria, along with the larger viruses, while also using chemical disinfectant to address smaller viruses and bacteria that the filter cannot remove. this combination is also potentially more effective in some cases than even using portable electronic disinfection based on uv treatment. chlorine dioxide chlorine dioxide can come from tablets or be created by mixing two chemicals together. it is more effective than iodine or chlorine against giardia, and although it has only low to moderate effectiveness against cryptosporidium, iodine and chlorine are ineffective against this protozoan. the cost of chlorine dioxide treatment is higher than the cost of iodine treatment. mixed oxidant (miox) a simple brine solution in an electrolytic reaction produces a powerful mixed oxidant disinfectant (mostly chlorine in the form of hypochlorous acid (hocl) and some peroxide, ozone, chlorine dioxide). chlorine tablets (nadcc) sodium dichloroisocyanurate or troclosene sodium more commonly shortened as nadcc, is a form of chlorine used for disinfection. it is used by all major ngo's such as unicef to treat water in emergencies, and widely by social marketing organisations for household water treatment where household sources of water may not be safe. nadcc tablets are available in a range of concentrations to treat differing volumes of water to give the world health organization's recommended 5ppm available chlorine. they are effervescent tablets allowing the tablet to dissolve in a matter of minutes. silver ion tablets an alternative to iodine-based preparations in some usage scenarios are silver ion/chlorine dioxide-based tablets or droplets. these solutions may disinfect water more effectively than iodine-based techniques while leaving hardly any noticeable taste in the water in some usage scenarios. silver ion/chlorine dioxide-based disinfecting agents will kill cryptosporidium and giardia, if utilized correctly. the primary disadvantage of silver ion/chlorine dioxide-based techniques is the long purification times (generally 30 minutes to 4 hours, depending on the formulation used). another concern is the possible deposition and accumulation of silver compounds in various body tissues leading to a rare condition called argyria that results in a permanent, disfiguring, bluish-gray pigmentation of the skin, eyes, and mucous membranes. hydrogen peroxide one recent study has found that the wild salmonella which would reproduce quickly during subsequent dark storage of solar-disinfected water could be controlled by the addition of just 10 parts per million of hydrogen peroxide. ultraviolet purification main article: ultraviolet germicidal irradiation ultraviolet (uv) light induces the formation of covalent linkages on dna and thereby prevents microbes from reproducing. without reproduction, the microbes become far less dangerous. germicidal uv-c light in the short wavelength range of 100–280 nm acts on thymine, one of the four base nucleotides in dna. when a germicidal uv photon is absorbed by a thymine molecule that is adjacent to another thymine within the dna strand, a covalent bond or dimer between the molecules is created. this thymine dimer prevents enzymes from "reading" the dna and copying it, thus neutering the microbe. prolonged exposure to ionizing radiation can cause single and double-stranded breaks in dna, oxidation of membrane lipids, and denaturation of proteins, all of which are toxic to cells. still, there are limits to this technology. water turbidity (i.e., the amount of suspended & colloidal solids contained in the water to be treated) must be low, such that the water is clear, for uv purification to work well - thus a pre-filter step might be necessary. a concern with uv portable water purification is that some pathogens are hundreds of times less sensitive to uv light than others. protozoan cysts were once believed to be among the least sensitive, however recent studies have proved otherwise, demonstrating that both cryptosporidium and giardia are deactivated by a uv dose of just 6 mj/cm2 however, epa regulations and other studies show that it is viruses that are the limiting factor of uv treatment, requiring a 10-30 times greater dose of uv light than giardia or cryptosporidium. studies have shown that uv doses at the levels provided by common portable uv units are effective at killing giardia and that there was no evidence of repair and reactivation of the cysts. water treated with uv still has the microbes present in the water, only with their means for reproduction turned "off". in the event that such uv-treated water containing neutered microbes is exposed to visible light (specifically, wavelengths of light over 330-500 nm) for any significant period of time, a process known as photo reactivation can take place, where the possibility for repairing the damage in the bacteria's reproduction dna arises, potentially rendering them once more capable of reproducing and causing disease. uv-treated water must therefore not be exposed to visible light for any significant period of time after uv treatment, before consumption, to avoid ingesting reactivated and dangerous microbes. recent developments in semiconductor technology allows for the development of uv-c light emitting diodes (leds). uv-c led systems address disadvantages of mercury-based technology, namely: power-cycling penalties, high power needs, fragility, warm-up time, and mercury content. solar water disinfection in solar water disinfection (often shortened as "sodis"), microbes are destroyed by temperature and uva radiation provided by the sun. water is placed in a transparent plastic pet bottle or plastic bag, oxygenated by shaking partially filled capped bottles prior to filling the bottles all the way, and left in the sun for 6–24 hours atop a reflective surface. solar distillation solar distillation relies on sunlight to warm and evaporate the water to be purified which then condenses and trickles into a container. in theory, a solar (condensation) still removes all pathogens, salts, metals, and most chemicals but in field practice the lack of clean components, easy contact with dirt, improvised construction, and disturbances result in cleaner, yet contaminated water. homemade water filters water filters can be made on-site using local materials such as sand and charcoal (e.g. from firewood burned in a special way). these filters are sometimes used by soldiers and outdoor enthusiasts. due to their low cost they can be made and used by anyone. the reliability of such systems is highly variable. such filters can do little, if anything, to mitigate germs and other harmful constituents and can give a false sense of security that the water so produced is potable. water processed through an improvised filter should undergo secondary processing such as boiling to render it safe for consumption. prevention of water contamination human water-borne diseases usually come from other humans, thus human-derived materials (feces, medical waste, wash water, lawn chemicals, gasoline engines, garbage, etc.) should be kept far away from water sources. for example, human excreta should be buried well away (>60 meters/200 feet) from water sources to reduce contamination. in some wilderness areas it is recommended that all waste be packed up and carted out to a properly designated disposal point.