The potential risk can be assessed only once the concentration of arsenic and its chemical speciation are known. The WHO Drinking Water Guidelines, 4th Edition (2011) give a figure of 0.01 mg/l (10 microgram/l) as a guideline value for elemental arsenic concentration, designated as provisional on the basis of treatment performance and analytical achievability. A guideline value normally represents the concentration of a constituent that does not result in any significant risk to health over a lifetime of consumption. There is a large safety margin incorporated in calculating such values and they can be regarded as "no risk" values. Because of the safety margin, even concentrations above the guideline value may be safe. High "risky" concentrations of elemental arsenic in drinking water must be assessed with regard to local conditions and this requires considerable chemical and toxicological expertise as the elemental concentration alone does not have a simple relationship to likely toxicity.

Note: Provisional guideline values have been established at concentrations that are reasonably achievable through practical treatment approaches or in analytical laboratories; in these cases, the guideline value is above the concentration that would normally represent the calculated health-based value. Guideline values are also designated as provisional when there is a high degree of uncertainty in the toxicological and health data.

Individuals may be occupationally exposed to arsenic, otherwise the most important route of exposure is through the oral intake of food and drinking-water, including food cooked in water such as rice, and beverages made from drinking-water. This is a particular problem in areas where the groundwater contains high levels of arsenic. The mean daily intake of arsenic from drinking-water is generally less than 10 microgram, however it can be much higher when the water source is contaminated. The estimated intake from air is generally less than 1 microgram.

Arsine is considered to be the most toxic form or arsenic, followed by the arsenites (arsenic(III)), the arsenates (arsenic(V)) and organic arsenic compounds. Lethal doses in humans range from 1.5 mg/kg of body weight (diarsenic trioxide) to 500 mg/kg of body weight (DMA). Acute arsenic intoxication associated with the ingestion of well water containing 1.2 and 21.0 mg of arsenic per litre has been reported. MMA(III) and DMA(III) are more toxic than arsenate in vivo and in vitro.

Early clinical symptoms of acute intoxication include abdominal pain, vomiting, diarrhoea, muscular pain and weakness, with flushing of the skin. These symptoms are often followed by numbness and tingling of the extremities, muscular cramping and the appearance of a papular erythematous rash. Within a month, symptoms may include burning paraesthesias of the extremities, palmoplantar hyperkeratosis, Mees lines on fingernails and progressive deterioration in motor and sensory responses.

Signs of chronic arsenicism, including dermal lesions such as hyperpigmentation and hypopigmentation, peripheral neuropathy, skin cancer, bladder and lung cancers and peripheral vascular disease, have been observed in populations ingesting arsenic contaminated drinking-water. Dermal lesions were the most commonly observed symptom, occurring after minimum exposure periods of approximately 5 years. Effects on the cardiovascular system were observed in children consuming arsenic-contaminated water (mean concentration 0.6 mg/l) for an average of 7 years.

This information has been extracted from the background document for the arsenic entry in the WHO Guidelines for Drinking Water Quality (http://www.who.int/water_sanitation_health/dwq/chemicals/arsenic.pdf).

Please find information about arsenic compiled by WHO from
http://www.who.int/ipcs/features/arsenic.pdf

Further information is available from the references mentioned there and from
http://www.who.int/ipcs/assessment/public_health/arsenic/en/index.html