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/features/arsenic.pdf