Naturally occurring hazards:
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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
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:
- Deeper wells — 200 metres or more down — are less likely to be contaminated. They have to be installed carefully to prevent water from more superficial sources seeping in. In Bangladesh, the long-term sustainability of these sources may be an issue.
- Rain water harvesting: particularly suitable in areas of high rainfall such as Bangladesh, although the collection systems for the rainwater have to ensure that there is no risk of infection, or increasing other problems, such as mosquitoes breeding on the surface of tanks.
- Education programmes: people get used to a particular type of water supply and cannot be expected to change habits overnight. Any long term solution must include wide scale education and training about the harmful effects of arsenic and how to avoid them.
- Arsenic removal systems may be suitable for long term use, although this needs a centralised system to ensure good maintenance and regular disposal of the arsenic sludge. Domestic treatment systems can be used as an alternative to centralised arsenic removal, or to supplement such systems.
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.
Naturally occurring hazards:
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