Naturally occurring hazards


Water gathers constituents from the rocks and ground through which it permeates. This process is unavoidable, although water treatment can reduce levels of natural contaminants. In the case of mineral waters, the rich variety of trace elements can be a highly prized asset, giving distinctive taste as well as potential health properties. However, contamination is not always so benign in its effects. This is demonstrated by the recent public health emergency in Bangladesh: high levels of arsenic in well water has caused widespread chronic poisoning. Also toxic in excess, fluoride is present in all waters at some concentration. At low levels it is undoubtedly beneficial for dental health. At high levels fluoride deposits on teeth and bones, a condition known as fluorosis. While some communities have too much natural fluoride in their drinking water, others have too little to have beneficial effects.


Arsenic is present in most waters, although usually in tiny amounts. Nevertheless, natural arsenic contamination is high enough to cause concern in parts of many countries including : Argentina, Chile, Bangladesh, China, India, Mexico, Thailand and the United States of America. The source is geological and affects the ground water: the water beneath the earth's surface that is collected from wells. Until the arsenic-related health problems became apparent in Bangladesh, the high levels of arsenic in the ground water had not been appreciated. The scale of the problem was realised only when the effects of poisoning were diagnosed in the population.

There are no easy community solutions for high natural arsenic contamination in Bangladesh because of the co-existing socio-economic and infrastructure problems. If arsenic contamination is recognised, relatively inexpensive water treatment can remove it, for example with candle filtration systems for use for a short period in the home or with a sachet of chemicals. Arsenic can also be removed before water distribution, but this requires a fairly sophisticated water treatment system.

Arsenic poisoning: an ancient—and modern—hazard

Arsenic compounds have been known since ancient times and the metallic form was isolated over 700 years ago. Inorganic arsenic is acutely toxic. Murderers have used its ability to slowly kill a victim from apparently natural causes: large doses—far higher than are found in water—cause rapid deterioration and death. Slow exposure, as in low-level water contamination causes several long-term effects. The effects of this arsenic poisoning, known as arsenicosis, can take a number of years (typically 5 – 20) to develop. Arsenic exposure via drinking water causes cancer in the skin, bladder and kidney, as well as skin changes such as hyperkeratoses (hard patches) and pigmentation changes. These and other health damaging effects are summarised in Table 1. It has been estimated that one in ten people who drink water containing >500 µg of arsenic per litre may ultimately die from cancers of the lung, bladder and skin. Occupational exposure by arsenic is mainly by inhalation and increased risks of lung cancer have been reported at cumulative exposure levels of = 0.75mg/cubic metre. This amounts to around 15 years exposure at a work-room concentration of 50µ/cubic metre. Tobacco smoking has been found to interact with arsenic in increasing the lung cancer risk. Because of multiple exposures and interaction with other toxic exposures, the relationship between arsenic and disease is not clear cut for all the postulated effects, such as diabetes and cerebrovascular disease.

Box 1: Long-term health effects of exposure to arsenic

  • Skin lesions and skin cancer
  • Internal cancers: bladder, kidney, lung
  • Neurological effects such as polyneuropathy and encephalopathy
  • High blood pressure and heart disease
  • Lung disease
  • Gastrointestinal symptoms
  • Bone marrow depression
  • Destruction of red blood cells
  • Enlarged liver
  • Disease of the blood vessels, including peripheral vascular disorders such as blackfoot disease in Taiwan, so named because it can result in gangrene of the feet
  • Diabetes

Who is at risk?

The very young are particularly vulnerable to the toxic effects, although all ages can be affected. Poverty and poor nutrition increase the chance of toxic effects. A vicious cycle can result, where people made sick by arsenic lose their jobs and become a burden to their family. Many of the long-term harmful effects are irreversible. In the early stages, drinking arsenic-free water and eating nutritious, vitamin-rich food can reverse some effects. Surface waters, such as lakes and rivers, are less likely to contain toxic levels of arsenic. While safer in terms of arsenic levels, such waters may carry a much greater risk of infection. Waterborne infection kills far more people than arsenic, so the use of alternative sources has to be carefully considered, taking the ability to limit or control infection hazards into account.

Unlike fluoride, arsenic has no apparent beneficial health effects for man and other animals. Unfortunately, we cannot completely remove all traces of this element from water. The acceptable upper limit for arsenic in water has been progressively lowered: before 1993, the WHO guideline value was 0.05 mg/ litre. Now it is 0.01 mg/litre [mg/l or milligrams per litre].

If arsenic can be reduced in water supplies, why is it a world wide water problem?

The first problem is knowing that it is there: this means testing water supplies. Apart from the cost of testing, someone has to be responsible for making sure the testing is done: this can be a problem in small supplies such as a village well. Education, training and monitoring are expensive and this is one of the greatest problems in controlling arsenic contamination and its effects. The cost can be kept down by restricting the testing to water used for drinking purposes: arsenic contaminated water may be used safely for bathing and laundry purposes. Quality control—making sure the analysis is correct—is also important.

While chemical or filtration treatment is effective, there can be problems in the use of chemicals to remove arsenic. For example, alum (aluminium sulphate) requires prolonged contact with the water to remove sufficient arsenic, which may be difficult in supplies without a water treatment works. Letting the water settle helps in iron rich waters, but only a proportion of the arsenic sinks with the iron, so this is not satisfactory for high levels of contamination. The amount removed varies according to several factors such as the concentrations of arsenic and iron and the standing time.

Arsenic is now little used in industrial and agricultural processes. In the past it was used as a pesticide, especially in orchards and as a component of wood preservative: small amounts could be leached from treated wood, such as electricity pylons. In the early stages of the investigation of the problem in Bangladesh, this was suggested as a possible cause. While arsenic can be found in minute amounts in air, food and water, the largest exposure is via the natural levels in water.

Long term solutions for arsenic in water

The Bangladesh arsenic emergency has shown the problem of using shallow wells in areas with high natural arsenic levels. The long term potential answers for Bangladesh include the following:

Precautions for controlling arsenic in water

In addition to the possible solutions, the Bangladesh arsenic problem has highlighted the value of testing and monitoring waters that may be at risk. This includes monitoring of water from vulnerable aquifers and conducting reconnaissance surveys to identify whether arsenic levels are a problem in previously unsuspected waters. Clinical monitoring for early signs of arsenic poisoning is also important: this is one of the ways in which the Bangladesh problem was identified. Clinical monitoring involves regular checks by doctors and nurses and surveillance systems to detect early signs of arsenicosis in the population. Common signs include hard patches on the palms and soles of feet and hyper-pigmentation (darker patches on the skin) and health workers can be quickly trained how to recognise them.


Fluoride in most groundwaters occurs as the anion F–. Waters with high fluoride content are found mostly in calcium-deficient ground waters in many basement aquifers, such as granite and gneiss, in geothermal waters and in some sedimentary basins. Groundwaters with high fluoride concentrations occur in many areas of the world including large parts of Africa, China, the Middle East and southern Asia (India, Sri Lanka). One of the best known high fluoride belts on land extends along the East African Rift from Eritrea to Malawi. There is another belt from Turkey through Iraq, Iran, Afghanistan, India, northern Thailand and China. The Americas and Japan have similar belts.

Fluoride is found in vegetables, fruit, tea and other crops. although drinking water is usually the largest contributor to the daily fluoride intake. Fluoride is also found in the atmosphere, originating from the dusts of fluoride-containing soils, from gaseous industrial wastes, from the burning of coal fires in populated areas and from gases of volcanic activity. Thus fluoride, in varying concentrations, is freely available in nature. Most of the studies of fluoride intake have been done in developed countries. In temperate climates, daily exposure is about 0.6mg/adult/day if the water is not fluoridated. The WHO guideline value for fluoride is 1.5mg/litre, with a target of between 0.8–1.2mg/l to maximise benefits and minimise harmful effects. Acceptable levels depend on climate, volumes of water intake and the likely intake of fluoride in other sources. Much depends on whether other sources, such as those mentioned above, also have high levels.

The effects of too little—and too much—Fluoride

Fluoride is a desirable substance: it can prevent or reduce dental decay and strengthen bones, thus preventing bone fractures in older people. Where the fluoride level is naturally low, studies have shown higher levels of both dental caries (tooth decay) and fractures. Because of its positive effect, fluoride is added to water during treatment in some areas with low levels. But you can have too much of a good thing; and in the case of fluoride, water levels above 1.5mg/litre may have long-term undesirable effects (Table 1: see also fact file on fluorosis). Much depends on whether other sources, such as vegetables, also have high levels. The risk of toxic effect rises with the concentration. It only becomes obvious at much higher levels than 1.5mg/l. The natural level can be as high as 95mg/l in some waters, such as in Tanzania where the rocks are rich in fluoride-containing minerals. Severe effects of excess fluoride have recently been reported from the Assam state in India (Box).

Table 1. Fluoride effects

Level in water Effects
0.8–1.2 mg/l Prevention of tooth decay, strengthening of skeleton
Above 1.5 mg/l Fluorosis: pitting of tooth enamel and deposits in bones
Above about 10 mg/l Crippling skeletal fluorosis

Box 2 : Too much natural fluoride in India

Nearly 100,000 villagers in the remote Karbi Anglong district in the north-eastern state of Assam were reported to be affected by excessive fluoride levels in groundwater in June 2000. Many people have been crippled for life. The victims suffer from severe anaemia, stiff joints, painful and restricted movement, mottled teeth and kidney failure. The first fluorosis cases were discovered in the middle of 1999 in the Tekelangiun area of Karbi Anglong. Fluoride levels in the area vary from 5-23 mg/L, while the permissible limit in India is 1.2 mg/L. Local authorities launched a scheme for the supply of fluoride-free water and painted polluted tube-wells red: they also put up notice boards warning people not to drink the water from these wells. (Times of India / UNI, 2 Jun 2000)

Removing excess fluoride

It is difficult and expensive to reduce a high natural level of fluoride in water. This means that the first option should be to find an alternative source with lower Fluoride levels. If there is no other possible or cost-effective source, de-fluoridation must be attempted to avoid the toxic effects. The best method depends on local circumstances (Table 2)

Table 2: Ways of removing fluoride

Method How it works
Bone charcoal; Filters out the fluoride, then column of charcoal disposed of
Precipitation method, for example, Nalgonda Aluminium sulphate and lime added daily and fluoride-rich sludge then removed
Activated alumina Filters out the fluoride, then column removed

Only the water for drinking and cooking needs to be de-fluoridated. The entire water demand is often ten times higher and to de-fluoridate this would be too expensive, as well as producing a large amount of toxic sludge.

Multiple fluoride exposure

In addition to the natural sources of fluoride, it may also be present in toothpaste and a range of other products aimed to reduce dental decay. Fluoride may reach high levels in fish because in builds up in the fish bones: a problem where soft bones are eaten, such as in salmon and sardines. Milk levels of Fluoride are typically low.

Other elements in water

Other natural elements in water include calcium (Ca) and magnesium (Mg). Where the natural level is high (>200mg/ litre), the water is ‘hard’ and does not lather easily with soap. High levels are believed to be generally beneficial to health: for example, a large study1 in the United Kingdom (UK) found that cardiovascular (heart-related) mortality was 10-15% higher in ‘soft’ water areas with low levels of Ca and Mg. The protective effect on the cardiovascular system may be due to the greater solubility of harmful trace elements, such as lead, in soft water.

Copper is an essential trace element for human physiology: copper pipes may increase natural levels in water in water distribution systems. Thus levels high enough to cause illness are rarely due to natural contamination alone. Liver disease has been reported in India, for example, due to copper pipes leading from a well. Lead pipes were used for centuries in water distribution systems, until it was realised that, like copper, lead could dissolve in the water. While new distribution systems have used other materials, lead pipes still cause problems in water.

Aluminium is also present in all waters to some degree. It only represents a health hazard if there is a mishap in the water treatment process.

Sodium salts are widely found in the environment. Some waters contain a naturally high level, including water reclaimed from the sea. This is an increasingly important issue for communities drawing their water supply near coasts, and for arid regions. Water with high sodium levels may raise blood pressure slightly, and this is of particular concern in people with heart, liver, kidney and other diseases where salt intake has to be restricted.

Uranium is a naturally occurring radioactive element, found in most rocks and soils in small amounts (2-4 parts per million), as well as in the oceans. Large reserves of uranium have been identified in Australia, Brazil, Canada, Kazakhstan, Namibia, South Africa and USA. Uranium decays to form radium and the gas radon. Uranium ore has been mined since ancient times, for example to colour glass, but its main modern use is in the nuclear power industry. It enters water supplies via leaching from natural sources, from industrial and nuclear use, from combustion of coal and other fuels, and from phosphate fertilisers used in agriculture. It is likely that food is the main source of uranium intake in most areas. While the naturally occurring isotopes of uranium are not highly radioactive, it has toxic effects on humans and animals, particularly affecting the kidney. Adequate short and long-term studies on the chemical toxicity of uranium are not available and most epidemiological studies have focused on the more radioactive decay products of radium and radon. The WHO recommends a provisional guideline value in drinking water of 2-µg l-1.

Radon occurs naturally in ground water, particularly where there are granite rocks. It is a radioactive gas, which is released from water when it comes to the surface. In rocky areas it can present in houses at high concentrations, increasing the risk of lung cancer.


The key points about the natural hazards in water are as follows:

References and further information

WHO, Fluorides and oral health. Report of a WHO Expert Committee on Oral Health Status and Fluoride Use. WHO Technical Report Series, 846. Geneva: WHO, 1994

Gleick P H, The World's Water 2000-2001: the biennial report on freshwater resources. Washington DC USA: Island Press, 2000

Murray JJ, Rugg-Gunn AJ, Jenkins GN, Fluorides in caries prevention. Third Edition. Butterwoth-Heinemann Ltd. 1991

Pocock SJ, Shaper AG, Cook DG, Packham RF, Lacey RF, Powell P, Russell PF, British Regional Heart Study: geographic variations on cardiovascular mortality, and the role of water quality. British Medical Journal. 1980; i:1243-1248

Smith A H, Lingas E O, Rahman M, Contamination of drinking-water by arsenic in Bangladesh: a public health emergency. Bulletin of the WHO, 2000; 78: 1093-1103

WHO, Guidelines for Drinking Water Quality, Second edition, volume 1, recommendations, Geneva, WHO 1993 p57/pp114-121,

See also

- Uranium

- WHO Guidelines for drinking-water quality

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