Arsenic exists in different chemical forms, which can be classified into two groups: organic arsenic and inorganic arsenic. Inorganic arsenic is considered to be the most toxic to human health, while organic arsenic is considered to be less toxic or non-toxic.

What is arsenic?

Arsenic is a naturally occurring element found throughout our environment. Arsenic compounds are used in manufacturing a variety of products and may enter the environment through the discharge of industrial wastes and by the deposit of arsenic particles in dust, or dissolved in rain or snow. These arsenic particles can enter the environment through:

Health effects of arsenic

Ingestion of arsenic

Inorganic arsenic has been recognized as a human poison since ancient times, and very large oral doses can result in death. Long-term exposure (over many years or decades) to high levels of arsenic may also cause:

Short-term exposure (days/weeks) to very high levels of arsenic can result in:

Health Canada and the International Agency for Research on Cancer (IARC) consider arsenic a human cancer-causing agent. Its effects have been studied in a population in Taiwan where the drinking water contains naturally high levels of arsenic (over 0.35 parts per million). The results suggest that consuming very high levels of arsenic over a lifetime can increase the risk of cancer in internal organs such as the bladder, liver and lungs.

Skin contact with arsenic

If you have direct skin contact with high concentrations of inorganic arsenic compounds, your skin may become irritated, with some redness and swelling. This normally occurs in industrial settings. However, it does not appear that skin contact is likely to lead to any serious internal effects.

Sources of arsenic


Arsenic can be found at very low levels in many foods. These trace amounts of arsenic generally are from normal accumulation in the environment. Higher levels of arsenic can be found in fish and shellfish, but in the organic form, which is not as much of a risk to human health. Health Canada has more information about arsenic in food.


Arsenic may enter lakes, rivers or underground water naturally, when mineral deposits or rocks containing arsenic dissolve, or through the discharge of industrial wastes and by the deposit of arsenic particles in dust, or dissolved in rain or snow.

Because of the health effects of arsenic, every effort should be made to keep arsenic levels in drinking water as low as possible. The Ontario Drinking Water Standards state the maximum amount of arsenic that should be in drinking water is .025 mg/L (25 parts per billion).

If you are not connected to a municipal drinking water system, and live in an area that is known to have high levels of arsenic in its water, you should have your water tested by an accredited laboratory. If the results show excessive levels of arsenic in your drinking water, you may want to:

More information on arsenic:


This item was last modified on May 14, 2024