Properties Of Metal Phosphides

There are several properties of metal phosphide fumigant products that must be considered in any application.

1. Its density is such that any leakages will result in loss of gas.

2. Hydrogen phosphide gas can self-ignite at a concentration of 17,900 ppm if there is an ignition source available. The reaction between a metal phosphide and atmospheric moisture, which yields hydrogen phosphide, is exothermic and, therefore, the piling of tablets during fumigation is to be avoided as the heat generated from this exothermic reaction can be the ignition source.

3. Hydrogen phosphide reacts with certain metals, especially with copper, so care must be taken during fumigation to prevent corrosion of electrical and electronic equipment and instruments.

4. Note that 1 g of hydrogen phosphide gas per 1,000 ft3 will result in a concentration of 25 ppm. There fore, if a dose of 20 tablets/1,000 ft3 is applied, the theoretical maximum concentration would be 500 ppm (20 g X 25 = 500 ppm), which is equivalent to a dose of 0.7 g/m3.

5. Aluminum or magnesium phosphide tablets and pellets that contain ammonium carbamate also release ammonia and, therefore, can be phytotoxic to living plants if sufficient concentration is present as well as darkening some nut meats when direct contact is made with the product. The darkening of nut meat is usually reversible. It would be better to use the magnesium phosphide impregnated in a plastic matrix (Fumi-Cel/Fumi-Strip) for some of these fumigations because it contains no ammonium salts.

6. Hydrogen phosphide has no effect on germination and, therefore, can safely be used to fumigate various seeds.

7. Hydrogen phosphide has no real CT product and fumigations must be no less than three days for complete control.

8. Do not fumigate if commodity temperature is below 5°C (40°F).

9. A fumigated commodity or space must be aerated until there is 0.3 ppm or less of hydrogen phosphide before reentry is allowed without respiratory protection.

10. Repeated fumigation with hydrogen phosphide will not result in a buildup of residues if the product is properly aerated following fumigation.

Finally, it must also be noted that hydrogen phosphide produced from metal phosphide product has a garlic or carbidelike odor. One word of caution in this regard is to emphasize that pure hydrogen phosphide has no odor, and the odor noted during a fumigation is an impurity and can be sorbed on a commodity, therefore, the lack of odor cannot be considered as a key for the lack of hydrogen phosphide being present. A measuring device must be used to determine concentration.

Metal phosphide products can be applied directly to raw agricultural commodity and animal feeds and have a tolerance of 0.1 ppm for this purpose. No direct contact can be made with these products on processed food so packaged metal phosphide products such as prepacs, bags, sachets, or Fumi-Cel/Fumi-Strip are used for these applications, and the approved tolerance is 0.01 ppm. Care should always be taken when deactivating residual materials as fumigation conditions could have resulted in having some unreacted phosphides in the residuals. The manufacturer's applicator's manual should be consulted prior to initiating this activity.

The half-life of hydrogen phosphide in the atmosphere is short and depends on climatic condition; it is in the range of 5-28 h. Hydrogen phosphide is known to be oxidized to phosphate both in the atmosphere and during fumigation.

Toxicology

Hydrogen phosphide has been shown to be an acute toxic material and has no chronic effects. Effects shown by ex perimental animals as a result of subchronic exposure were shown to be reversible. This substance also had no teratological or mutagenic action when these studies were completed.

Aluminum phosphide, a fumigant used extensively in India, is extremely toxic to humans with a fatal dose reported to be 1.5 g (2). Gupta and Ahlawat (3) attributed aluminum phosphide poisoning to the release of a cytotoxic phosphine gas to the major organs of the body including heart, lungs, and kidneys.

Respiratory Protection

If the application is made inside a structure, then gas readings must be taken and a gas mask with approved canister must be worn if the levels exceed 0.3 ppm. An air pack must be worn when the levels exceed 15 ppm. These same requirements also apply to aeration.

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