Arsenic is a naturally occurring substance found in rocks and soil. It comes in organic and inorganic forms. Organic arsenic is combined with carbon and is considered safe. Inorganic carbon is considered harmful to humans.
The largest source of arsenic exposure is through drinking water that contains arsenic. You might also inhale pollution from some types of mining, smelting or manufacturing plants that contain arsenic. If you are consuming drinking water with a high level of arsenic over a long period of time, you are at a greater risk of several types of cancer. There is also a link between LOW LEVELS of arsenic in tap water and diabetes being investigated.
New studies suggest that arsenic is a potent hormone disruptor that interferes with glucocorticoids - hormones involved in most of the human body's basic functions. They help regular the immune system, nervous sytem and changes in blood, bones and kidneys and how the body uses sugar, starch, fat and protien. These disruptions to hormones may be how arsenic promotes cancer and possibily diabetes.
In Canada, the national guidelines state there should be less than 10 parts per billion of arsenic in tap water. These national guidelines are not legally binding, but the water quality laws in many provinces and territories generally follow these guidelines. It of course comes down to the municipality you live in that ensures the guidelines and regulations are carried out.
The World Health Organization set the 10 ppb as the recommended limit for arsenic in drinking water but also estimates that lifetime exposure to water containing 10 ppb of aresenic will lead to six cases of skin cancer per 10,000 people. In other words, according to the WHO, there is no completely "safe" level for arsenic in drinking water.
What can you do?
Call your local government water treatment works and ask what the current levels of aresenic in the tap water is. If you have a private well, test it for arsenic. Arsenic is removed by reverse-osmosis water filtration systems, as are many other heavy metals. It is not removed by activated carbon filtration which is what you would find in pitcher-based filtration systems. As with any type of pollutant, it's not always possible to completely avoid arsenic, but reducing your exposure could lead to better health in the long-term.