of food and drink that is made in fluoridated areas and consumed in unfluoridated ones. a 2000 uk systematic review (york) found that water fluoridation was associated with a decreased proportion of children with cavities of 15% and with a decrease in decayed, missing, and filled primary teeth (average decreases was 2.25 teeth). the review found that the evidence was of moderate quality: few studies attempted to reduce observer bias, control for confounding factors, report variance measures, or use appropriate analysis. although no major differences between natural and artificial fluoridation were apparent, the evidence was inadequate for a conclusion about any differences. a 2007 australian systematic review used the same inclusion criteria as york's, plus one additional study. this did not affect the york conclusions. a 2011 european commission systematic review based its efficacy on york's review conclusion. a 2015 cochrane systematic review estimated a reduction in cavities when water fluoridation was used by children who had no access to other sources of fluoride to be 35% in baby teeth and 26% in permanent teeth. the evidence was of poor quality. a 2020 study in the journal of political economy found that water fluoridation significantly improved dental health and labor market outcomes, but had non-significant effects on cognitive ability. fluoride may also prevent cavities in adults of all ages. a 2007 meta-analysis by cdc researchers found that water fluoridation prevented an estimated 27% of cavities in adults, about the same fraction as prevented by exposure to any delivery method of fluoride (29% average). a 2011 european commission review found that the benefits of water fluoridation for adult in terms of reductions in decay are limited. a 2015 cochrane review found no conclusive research regarding the effectiveness of water fluoridation in adults. a 2016 review found variable quality evidence that, overall, stopping of community water fluoridation programs was typically followed by an increase in cavities. most countries in europe have experienced substantial declines in cavities without the use of water fluoridation due to the introduction of fluoridated toothpaste and the large use of other fluoride-containing products, including mouthrinse, dietary supplements, and professionally applied or prescribed gel, foam, or varnish. for example, in finland and germany, tooth decay rates remained stable or continued to decline after water fluoridation stopped in communities with widespread fluoride exposure from other sources. fluoridation is however still clearly necessary in the u.s. because unlike most european countries, the u.s. does not have school-based dental care, many children do not visit a dentist regularly, and for many u.s. children water fluoridation is the primary source of exposure to fluoride. the effectiveness of water fluoridation can vary according to circumstances such as whether preventive dental care is free to all children. fluorosis a mild case of dental fluorosis, visible as white streaks on the subject's upper right central incisor. fluoride's adverse effects depend on total fluoride dosage from all sources. at the commonly recommended dosage, the only clear adverse effect is dental fluorosis, which can alter the appearance of children's teeth during tooth development; this is mostly mild and is unlikely to represent any real effect on aesthetic appearance or on public health. in april 2015, recommended fluoride levels in the united states were changed to 0.7 ppm from 0.7–1.2 ppm to reduce the risk of dental fluorosis. the 2015 cochrane review estimated that for a fluoride level of 0.7 ppm the percentage of participants with fluorosis of aesthetic concern was approximately 12%. this increases to 40% when considering fluorosis of any level not of aesthetic concern. in the us mild or very mild dental fluorosis has been reported in 20% of the population, moderate fluorosis in 2% and severe fluorosis in less than 1%. the critical period of exposure is between ages one and four years, with the risk ending around age eight. fluorosis can be prevented by monitoring all sources of fluoride, with fluoridated water directly or indirectly responsible for an estimated 40% of risk and other sources, notably toothpaste, responsible for the remaining 60%. compared to water naturally fluoridated at 0.4 mg/l, fluoridation to 1 mg/l is estimated to cause additional fluorosis in one of every 6 people (95% ci 4–21 people), and to cause additional fluorosis of aesthetic concern in one of every 22 people (95% ci 13.6–∞ people). here, aesthetic concern is a term used in a standardized scale based on what adolescents would find unacceptable, as measured by a 1996 study of british 14-year-olds. in many industrialized countries the prevalence of fluorosis is increasing even in unfluoridated communities, mostly because of fluoride from swallowed toothpaste. a 2009 systematic review indicated that fluorosis is associated with consumption of infant formula or of water added to reconstitute the formula, that the evidence was distorted by publication bias, and that the evidence that the formula's fluoride caused the fluorosis was weak. in the u.s. the decline in tooth decay was accompanied by increased fluorosis in both fluoridated and unfluoridated communities; accordingly, fluoride has been reduced in various ways worldwide in infant formulas, children's toothpaste, water, and fluoride-supplement schedules. safety fluoridation has little effect on risk of bone fracture (broken bones); it may result in slightly lower fracture risk than either excessively high levels of fluoridation or no fluoridation. there is no clear association between water fluoridation and cancer or deaths due to cancer, both for cancer in general and also specifically for bone cancer and osteosarcoma. series of research concluded that concentration of fluoride in water doesn't associate with osteosarcoma. the beliefs regarding association of fluoride exposure and osteosarcoma stem from a study of us national toxicology program in 1990, which showed uncertain evidence of association of fluoride and osteosarcoma in male rats. but there is still no solid evidence of cancer-causing tendency of fluoride in mice. fluoridation of water has been practiced around the world to improve citizens' dental health. it is also deemed as major health success. fluoride concentration levels in water supplies are regulated, such as united states environmental protection agency regulates fluoride levels to not be greater than 4 milligrams per liter. actually, water supplies already have natural occurring fluoride, but many communities chose to add more fluoride to the point that it can reduce tooth decay. fluoride is also known for its ability to cause new bone formation. yet, further research shows no osteosarcoma risks from fluoridated water in humans. most of the research involved counting number of osteosarcoma patients cases in particular areas which has difference concentrations of fluoride in drinking water. the statistic analysis of the data shows no significant difference in occurrences of osteosarcoma cases in different fluoridated regions. another important research involved collecting bone samples from osteosarcoma patients to measure fluoride concentration and compare them to bone samples of newly diagnosed malignant bone tumors. the result is that the median fluoride concentrations in bone samples of osteosarcoma patients and tumor controls are not significantly different. not only fluoride concentration in bones, fluoride exposures of osteosarcoma patients are also proven to be not significantly different from healthy people. more recent studies have disputed any relationship to consumption of fluoridated drinking water during childhood. fluoride can occur naturally in water in concentrations well above recommended levels, which can have several long-term adverse effects, including severe dental fluorosis, skeletal fluorosis, and weakened bones; water utilities in the developed world reduce fluoride levels to regulated maximum levels in regions where natural levels are high, and the who and other groups work with countries and regions in the developing world with naturally excessive fluoride levels to achieve safe levels. the world health organization recommends a guideline maximum fluoride value of 1.5 mg/l as a level at which fluorosis should be minimal. in rare cases improper implementation of water fluoridation can result in overfluoridation that causes outbreaks of acute fluoride poisoning, with symptoms that include nausea, vomiting, and diarrhea. three such outbreaks were reported in the u.s. between 1991 and 1998, caused by fluoride concentrations as high as 220 mg/l; in the 1992 alaska outbreak, 262 people became ill and one person died. in 2010, approximately 60 gallons of fluoride were released into the water supply in asheboro, north carolina in 90 minutes—an amount that was intended to be released in a 24-hour period. like other common water additives such as chlorine, hydrofluosilicic acid and sodium silicofluoride decrease ph and cause a small increase of corrosivity, but this problem is easily addressed by increasing the ph. although it has been hypothesized that hydrofluosilicic acid and sodium silicofluoride might increase human lead uptake from water, a 2006 statistical analysis did not support concerns that these chemicals cause higher blood lead concentrations in children. trace levels of arsenic and lead may be present in fluoride compounds added to water, but no credible evidence exists that their presence is of concern: concentrations are below measurement limits. the effect of water fluoridation on the natural environment has been investigated, and no adverse effects have been established. issues studied have included fluoride concentrations in groundwater and downstream rivers; lawns, gardens, and plants; consumption of plants grown in fluoridated water; air emissions; and equipment noise. mechanism fluoride exerts its major effect by interfering with the demineralization mechanism of tooth decay. tooth decay is an infectious disease, the key feature of which is an increase within dental plaque of bacteria such as streptococcus mutans and lactobacillus. these produce organic acids when carbohydrates, especially sugar, are eaten. when enough acid is produced to lower the ph below 5.5, the acid dissolves carbonated hydroxyapatite, the main component of tooth enamel, in a process known as demineralization. after the sugar is gone, some of the mineral loss can be recovered—or remineralized—from ions dissolved in the saliva. cavities result when the rate of demineralization exceeds the rate of remineralization, typically in a process that requires many months or years. demineralization and remineralization of dental enamel in the presence of acid and fluoride in saliva and plaque fluid. all fluoridation methods, including water fluoridation, create low levels of fluoride ions in saliva and plaque fluid, thus exerting a topical or surface effect. a person living in an area with fluoridated water may experience rises of fluoride concentration in saliva to about 0.04 mg/l several times during a day. technically, this fluoride does not prevent cavities but rather controls the rate at which they develop. when fluoride ions are present in plaque fluid along with dissolved hydroxyapatite, and the ph is higher than 4.5, a fluorapatite-like remineralized veneer is formed over the remaining surface of the enamel; this veneer is much more acid-resistant than the original hydroxyapatite, and is formed more quickly than ordinary remineralized enamel would be. the cavity-prevention effect of fluoride is mostly due to these surface effects, which occur during and after tooth eruption. although some systemic (whole-body) fluoride returns to the saliva via blood plasma, and to unerupted teeth via plasma or crypt fluid, there is little data to determine what percentages of fluoride's anticavity effect comes from these systemic mechanisms. also, although fluoride affects the physiology of dental bacteria, its effect on bacterial growth does not seem to be relevant to cavity prevention. fluoride's effects depend on the total daily intake of fluoride from all sources. about 70–90% of ingested fluoride is absorbed into the blood, where it distributes throughout the body. in infants 80–90% of absorbed fluoride is retained, with the rest excreted, mostly via urine; in adults about 60% is retained. about 99% of retained fluoride is stored in bone, teeth, and other calcium-rich areas, where excess quantities can cause fluorosis. drinking water is typically the largest source of fluoride. in many industrialized countries swallowed toothpaste is the main source of fluoride exposure in unfluoridated communities. other sources include dental products other than toothpaste; air pollution from fluoride-containing coal or from phosphate fertilizers; trona, used to tenderize meat in tanzania; and tea leaves, particularly the tea bricks favored in parts of china. high fluoride levels have been found in other foods, including barley, cassava, corn, rice, taro, yams, and fish protein concentrate. the u.s. institute of medicine has established dietary reference intakes for fluoride: adequate intake values range from 0.01 mg/day for infants aged 6 months or less, to 4 mg/day for men aged 19 years and up; and the tolerable upper intake level is 0.10 mg/kg/day