Arsenic has long been a commonly known poison. A move by the Environmental Protection Agency to decrease the acceptable concentration of the element in water from 0.05 mg/L to 0.01 mg/L was recently accepted by the Bush Administration after a year’s debate. The safety of treated wood, particularly for children’s playground equipment, has also come into question.

Wood treated with chromated copper arsenate (CCA) came into use in the 1930s and has essentially replaced more toxic methods of preserving wood with creosote and pentachlorophenol. During the CCA treatment process the chromium reacts with the wood fiber to enable "fixation" of the copper and arsenic with the wood cell walls. Leaching of arsenic from the treated wood is supposed to be minimal. However, at least one investigator claims that arsenic in quantities that are harmful can be detected from wipes of the wood surface.

Arsenic, per se, is not the issue; the dose is the issue. The element is ubiquitous; there is nothing in nature that doesn’t contain some amount. Arsenic ranks 20th of all elements in the Earth’s crust, at usual concentrations of 2 to 5 ppm. Most human and animal foods contain a trace up to 1 mg/Kg (dry wt. basis).

Foods of marine origin, however, are much richer in arsenic. Fish contains 2 to 80 mg/Kg, oysters 3 to 10 mg/Kg, and mussels as much as 10 to 120 mg/Kg (over 1 percent arsenic). The arsenic content of fish meals used in animal feed range up to approximately 20 mg/Kg. Normal daily intakes of the element are listed for populations of many countries in Trace Elements in Human and Animal Nutrition (Mertz, 1986) and the World Health Organization Series 18.

CCA-treated lumber has become a concern for some animal owners. CCA-treated lumber is very common in agriculture buildings and fences. A confirmed case of arsenic poisoning from the ingestion of intact treated boards (including by cribbing horses) has never been documented. However, arsenic poisoning has occurred on multiple occasions from the ingestion of the ashes from incinerated boards. Arsenic, for the most part, remains after combustion as part of the ash. Burning of CCA-treated boards is illegal in all 50 states; disposal is by approved landfill.

Contributed By: Dr. Gavin Meerdink, Retired
University of Illinois / College of Veterinary Medicine

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