Arsenic is a toxic element that is classified as a metalloid. Metalloids exhibit some characteristics of metals and some characteristics of non-metals. Arsenic occurs naturally in rocks and in the earth's crust, but is often combined with other elements to make handling and dosing easier and safer. Arsenic has been known as a poison and used in rat poisons for hundreds of years.

...
A skull and crossbones can indicate a poison that may contain arsenic.

Compounds

In the environment, the element arsenic combines with oxygen, chlorine or sulfur to form inorganic arsenic compounds. In the manufacture of rat poison, these natural forms may be used or, in some cases, chemists may make other compounds based on the natural compounds. Some of those compounds may be used in rat poison.

Effects of Arsenic

Arsenic in rat poison can kill rats in a number of ways. Arsenic affects a rat's digestive system, heart and nervous system. It also damages red blood cells, bone marrow and the liver. Arsenic, consumed in large enough quantities, can kill rats very quickly. In smaller quantities, however, rats will often still die a slower death, often from kidney failure.

Over-the-Counter Sale

Many over-the-counter rat poisons contain arsenic. This has been true since before the 19th century. In fact, cases of accidental arsenic poisoning from homemade rat poison were common throughout the 19th century. During that time, people could buy concentrated arsenide compounds to use in homemade poisons. The commonness of these poisonings was likely because the ingredients – honey, flour and arsenic – also appealed to children who may not have recognized the contents of a packet of rat poison as anything other than a sweet.

Labeling of Rat Poisons

Because of the problems with accidental poisonings, a number of different methods were introduced to label rat poisons containing arsenic as toxic. The classic skull and crossbones is one method of indicating danger. Modern labeling, however, often lists the active toxic ingredients in rat poison, including percentage of arsenic by weight.

Environmental Breakdown

Arsenic, including the arsenide compounds in rat poison, does not break down in the environment. Although small amounts of arsenic naturally occur in the environment, arsenic cannot be broken down by natural processes. It can only change forms. Rat poisons will remain toxic indefinitely, and therefore must be treated carefully and disposed of properly.

How to Treat Accidental Poisoning

If you or someone else is accidentally poisoned by rat poison that contains arsenic, medical treatment is critical. Always take the rat poison package to the emergency room in case the poison contains other toxins in addition to arsenic. For arsenic-only poisoning, a doctor will use between 2.5 mg and 5 mg of dimercaprol per kilogram of body weight. This initial dose is given every four hours followed by reduced dosages for a total of five days.