Most arsenic compounds can dissolve in water. It gets into air when contaminated materials are burned. It settles from the air to the ground. It doesn't break down, but can change from one form to another. Fish and shellfish build up organic arsenic in their tissues, but most of the arsenic in fish isn't toxic.
Inorganic arsenic is a human poison.
Organic arsenic is less harmful. High levels of inorganic arsenic in food or water can be fatal. Arsenic damages many tissues including nerves, stomach and intestines, and skin. Breathing high levels can give you a sore throat and irritated lungs. Direct skin contact may cause redness and swelling.