Introduction
1.3 Maximum contamination level (MCL) and health effects of fluoride, iron and arsenic
Fluoride
Maximum contamination level (MCL) of fluoride in drinking water is 1.5 mg L-1 [21].
The endemic fluorosis in India is largely of hydrogeochemical origin. It has been observed that low calcium and high bicarbonate alkalinity favor high fluoride content in groundwater. Water with high fluoride content is generally soft, has high pH and contains large amount of silica. In groundwater, the natural concentration of fluoride depends on the geological, chemical and physical characteristics of the aquifer, the porosity and acidity of the soil and rocks, temperature, the action of other chemicals and the depth of wells. Due to large number of variables, the fluoride concentrations in groundwater range from well under 1.0 mg L-1 to more than 35.0 mg L-1. As the amount of water consumed and consequently the amount of fluoride ingested is influenced primarily by air temperature, USPHS [22] has set a range of concentrations for maximum allowable fluoride in drinking water for communities based on the climatic conditions as shown in Table 1.4.
Fluorine being a highly electronegative element has extraordinary tendency to get attracted by positively charged ions like calcium. Hence the effect of fluoride on mineralized tissues like bone and teeth leading to developmental alternations is of clinical
significance as they have highest amount of calcium and thus attract the maximum amount of fluoride that gets deposited as calcium–fluorapatite crystals. Tooth enamel is composed principally of crystalline hydroxylapatite. Under normal conditions, when fluoride is present in water supply, most of the ingested fluoride ions get incorporated into the apatite crystal lattice of calciferous tissue enamel during its formation.
Table 1.4: USPHS recommendations for maximum allowable fluoride in drinking water
Recommended fluoride concentration (mg L-1) Annual
average of maximum
daily air temperature (◦C)
Lower Optimum Upper
Maximum allowable fluoride concentration (mg L-1)
10 – 12 0.9 1.2 1.7 2.4
12.1 - 14.6 0.8 1.1 1.5 2.2
14.7 - 17.7 0.8 1.0 1.3 2.0
17.8 - 21.4 0.7 0.9 1.2 1.8
21.5 - 26.2 0.7 0.8 1.0 1.6
26.3 - 32.5 0.6 0.7 0.8 1.4
The hydroxyl ion gets substituted by fluoride ion since fluorapatite is more stable than hydroxylapatite. Thus, a large amount of fluoride gets bound in these tissues and only a
small amount is excretedthrough sweat, urine and stool. The intensity of fluorosis is not merely dependent on the fluoride content in water, but also on the fluoride from other sources, physical activity and dietary habits. The various forms of fluorosis arising due to excessive intake of fluoride are briefly presented in Table 1.5 [23].
Table 1.5: Effects of fluoride in water on human health Fluoride concentration
(mg L-1)
Effects
<1.5 Safe limit
1.5 – 3.0 Dental fluorosis (discoloration, mottling and pitting of teeth)
3.0 – 4.0 Stiffened and brittle bones and joints
4.0 – 6.0 and above Deformities in knee and hip bones and finally paralysis making the person unable to walk or stand in straight posture, crippling fluorosis
Dental fluorosis
Due to excessive fluoride intake, enamel loses its lustre. In its mild form, dental fluorosis is characterized by white, opaque areas on the tooth surface and in severe form, it is manifestated as yellowish brown to black stains and severe pitting of the teeth. This discoloration may be in the form of spots or horizontal streaks.
Skeletal fluorosis
Skeletal fluorosis affects children as well as adults. It does not easily manifest until the disease attains an advanced stage. Fluoride mainly gets deposited in the joints of neck, knee, pelvic and shoulder bones and makes it difficult to move or walk. The symptoms of
skeletal fluorosis are similar to spondylitis or arthritis. Besides skeletal and dental fluorosis, excessive consumption of fluoride may lead to muscle fibre degeneration, low hemoglobin.
Iron
Groundwater easily gets contaminated with iron usually in it’s (+2) valence state.
Although iron is an essential mineral for human, its presence in groundwater above a certain level make the water unusable mainly for aesthetic considerations such as discoloration, metallic taste, odor, turbidity, staining of laundry and plumbing fixtures.
Moreover, iron oxides, which are formed in reservoirs upon aerial oxidation of dissolved iron promotes growth of micro-organism in water. Considering the aesthetic impairment that may arise from the excessive iron in water supplies the revised Indian standard (IS:
10500-91) prescribes desirable and permissible limit of iron in drinking water as 0.3 mg L-1 and 1.0 mg L-1, respectively.
Iron is an essential element in human nutrition. Estimates of the minimum daily requirement for iron depend on age, sex, physiological status, and iron bioavailability and range from about 10 to 50 mg day-1. The average lethal dose of iron is 200–250 mg Kg-1 of body weight, but death has occurred following the ingestion of doses as low as 40 mg Kg-1 of body weight. Autopsies have shown hemorrhagic necrosis and sloughing of areas of mucosa in the stomach with extension into the submucosa. Chronic iron overload results primarily from a genetic disorder (hemochromatosis) characterized by increased iron absorption and from diseases that require frequent transfusions [24]. Adults have often taken iron supplements for extended periods without deleterious effects, and an
intake of 0.4–1 mg Kg-1 of body weight per day is unlikely to cause adverse effects in healthy persons [25].
Arsenic
Considering the lethal impact of arsenic on human health, environmental authorities of different countries have taken a more stringent attitude towards the presence of arsenic in water. Table 1.6 summarizes the maximum contamination level of arsenic in different countries.
The toxicity scale of arsenic decreases in the following order: arsine > inorganic arsenic(III) > organic arsenic(III) > inorganic arsenic(V) > organic arsenic(V) > arsonium compounds and elemental arsenic. The carcinogenic and mutagenic effects of arsenic have been established.
Health effects of arsenic on human are classified as acute and sub acute which are typically reversible and chronic effects. Acute and sub acute poisoning results from ingestion of large quantities of arsenic with lower exposure time whereas, chronic poisoning occurs due to consumption of arsenic contaminated water for a long time period. Arsenocosis is caused by drinking arsenic-tainted groundwater.
The disease causes many problems including vomiting and diarrhea; abdominal pain;
muscular pain; skin rashes; and swelling of the eyelids, feet and hands. Arsenocosis ultimately affects the heart, lungs, and kidneys and can be fatal. Hematological effects including anemia and leukaemia; and peripheral neuropathy might occur after weeks or month of exposure to high doses of arsenic (0.04 mg Kg-1 day-1 or higher) [29].
Consumption of large arsenic at a time may also cause stomach pain, nausea, vomiting or diarrhoea, which may lead to shock, coma, and even death. It has also been reported by many researchers that chronic arsenic poisoning causes hypertension, peripheral vascular diseases, cardiac vascular diseases, respiratory diseases, diabetes mellitus, malignancies including cancer of the lungs, bladder, kidney, liver, uterus and skin. The skin is quite sensitive to arsenic and skin lesion (hyperkeratosis and dyspigmentation) has been observed even at the exposure levels in the range of 5–10 µg L-1 arsenic in drinking water [29].
Table 1.6: World scenario of arsenic poisoning from drinking water Country name Permissible national limit (µg L-1) Reference
Bangladesh 50 [14]
India 50 [26]
Pakistan 50 [27]
Taiwan 50 [26]
China/Mongolia 50 [26]
USA 10 [26]
Germany 25 [26]
Vietnam 50 [28]
Japan 10 [26]
1.4 Existing processes for the separation of fluoride, iron and arsenic from