The element of arsenic has long been known to be acutely toxic. It is found in nature in low levels, mostly in compounds with oxygen, chlorine, and sulfur. These are called inorganic arsenic compounds. Copper and lead ores also contain small amounts of arsenic. Organic arsenic in plants and animals combines with carbon and hydrogen. Organic arsenic is usually less harmful than inorganic arsenic, which is a human poison. Most of these compounds have no particular smell or taste. 

How You Are Exposed

• Inorganic arsenic compounds are mainly used to preserve wood. Handling and applying these products can expose you.
• Breathing sawdust or smoke from burning wood containing (treated with) arsenic.
• Breathing workplace air.
• Handling or applying insecticides and weed killers containing arsenic.
• Ingesting contaminated water, air, or soil at industrial waste sites.
• Ingesting contaminated water, air, or soil in areas naturally high in arsenic.

How Can Arsenic Affect Your Health?

The Department of Health and Human Services has determined that arsenic is a known carcinogen. High levels of inorganic arsenic in food or water can be fatal. A high level is 60 ppm (parts per million). The World Health Organization (WHO) and US EPA set 10 parts per billion (daily exposure) as a recommended regulatory limit. Breathing inorganic arsenic increases the risk of lung cancer, at the very least resulting in a sore throat and irritated lungs. Ingesting inorganic arsenic increases the risk of skin cancer and tumors of the bladder, kidney, liver, and lung. Long-term exposure may lead to a darkening of the skin and the appearance of small "corns" or "warts" on the palms, soles, and torso. Low levels of exposure can result in:

• Nausea, vomiting, and diarrhea

• Decreased production of red and white blood cells

• Abnormal heart rhythm

• Blood vessel damage

• A "pins and needles" sensation in the hands and feet.

Do not to use any type of treated wood where the wood would come into direct or indirect contact with drinking water supplies. (Incidental contact, such as with docks or bridges, is considered acceptable.) There are other rot- and insect-resistant materials that can be used instead of treated lumber such as metal or plastic dock materials; stone, brick, or landscape blocks for gardening and landscaping; steel piling filled with concrete instead of creosote-treated pilings for underground construction; wood treated with less-toxic preservatives, such as ACQ, copper azole and ammoniacal copper citrate.
Source: http://www.moea.state.mn.us/campaign/garden/index.html

Avoiding Exposure: Tips and Alternatives   

• Exposure to higher-than-average levels of arsenic happens mostly in the workplace, near hazardous waste sites, and in areas with high natural levels. OSHA maximum permissible exposure limit for workplace airborne arsenic is 10 micrograms per cubic meter.
• Check labels of treated wood and insecticide/weed killer products to see if they contain arsenic.
• Avoid medicines containing arsenic. For many years arsenic was consumed in very low levels for medicinal purposes, notably in a product called "Fowler's Solution" (1% arsenite). Prolonged use could lead to chronic skin effects.
• Measure the arsenic level in any groundwater intended for human use.