Agriculture Reference
In-Depth Information
pentavalent compounds incur the least hazard of the arsenicals that are used as
pesticides.
The pentavalent arsenicals are relatively water soluble and absorbable
across mucous membranes, while trivalent arsenicals, having greater lipid
solubility, are more readily absorbed across the skin. However, poisonings by
dermal absorption of either form have been extremely rare. Ingestion has been
the usual basis of poisoning; gut absorption efficiency depends on the physi-
cal form of the compound, its solubility characteristics, the gastric pH, gastro-
intestinal motility, and gut microbial transformations. Once absorbed, many
arsenicals cause toxic injury to cells of the nervous system, blood vessels,
liver, kidney, and other tissues. Two biochemical mechanisms of toxicity are
recognized: 1) reversible combination with thiol groups contained in tissue
proteins and enzymes, and 2) substitution of arsenic anions for phosphate in
many reactions, including those critical to oxidative phosphorylation. Be-
cause there are many uncertainties regarding the biotransformation of various
arsenicals in the gut and in the body, and because the toxic potentials of the
biotransformation products are not well established, it is generally safest to
manage cases of arsenic ingestion as though all forms of arsenic are highly
toxic.
Mammals, including humans, detoxify inorganic arsenic by methylation,
yielding cacodylic acid (dimethylarsinic acid) as the chief urinary excretion
product. Disposition by urinary excretion is usually prompt. Elimination of
the arsonic acid (monomethyl) compounds has not been extensively studied,
but urinary excretion of the unaltered compound and/or a further methylated
form would seem likely.
V. HERBICIDES
Weeds may be defined as plants growing out of place or where man does
not wish them to grow. Organized agriculture dates back some 10,000 years
and the extensive cultivation of crops is only about 5,000 years old. Weeds
have caused problems for at least 5,000 years by competing with crops for
moisture, nutrients, and light.
The idea of controlling weeds with chemicals is not new; for more than a
century chemicals have been employed for weed control. The removal of
plants from such places as railway tracks, fence lines, recreation areas, timber
yards, roadsides, and power lines with crude chemicals such as rock salt, salt
solutions, crushed arsenical ores, creosote, oil wastes, sulfuric acid, and copper
salts with massive does dates back to the nineteenth century and continued
into the early 1900s.
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