When exposure is by ingestion, the clearest effect is increased risk of skin cancer. For most people, the most significant route of exposure to arsenic is through food. It is a normal component of the diet. Ingestion of drinking water can be a source of arsenic exposure. Several national surveys of drinking water systems have found arsenic in 3 to 39 percent of all samples averaging less than 10 ppb.
Arsenic concentrations are generally highest in groundwater.
Surface water concentrations, although generally low, also may be at levels of regulatory concern. Low levels of arsenic can also be detected in ambient air. However, inhalation is generally not a significant source of exposure for the general public. Members of the NRC panel expressed concern that reducing the federal standard below 10 ppb is unsupportable. The World Health Organization has established an arsenic standard for drinking water of 10 ppb.