Examples are mercury, lead, nickel, arsenic, and cadmium. Sulphur, phosphorus, arsenic, and chlorine compounds are permanent catalyst poisons. Toxic substances such as mercury, lead and arsenic may be released. Compounds of boron, silicon, arsenic, and antimony have been used as flame retardants. Treatment of chronic arsenic poisoning is possible. Chelation therapy is an antidote for poisoning by mercury, arsenic, and lead. Organic arsenic is less harmful than inorganic arsenic. Oxyhalides are more common for antimony than for arsenic and phosphorus. The contaminants including the poisons arsenic and lead. Analysis revealed the presence of small amounts of antimony, arsenic and lead. The mud contained elevated levels of arsenic and other pesticides. Effects may be similar to arsenic poisoning. Many aquifers contain high concentration of arsenic salts. Some Canadians are drinking water that contains inorganic arsenic. All five had died of poisoning, either arsenic or strychnine.