Phosphine or phosphorus hydride (PH3)
(IUPAC name: phosphane)
Fig. 1: The chemical structure of phosphine1.
Phosphine gas has been making headlines in recent months with news stories such as “Phosphine pesticide used to kill bedbugs causes Fort McMurray baby's death”,2 and “Phosphine gas likely cause of Thailand deaths of Quebec sisters: coroner”.3 This is making news not only in Canada but in other countries around the world.4 The news of these tragic stories brings up questions such as: what is phosphine, what are its uses and what led to these outcomes?
Phosphine is a colourless, flammable, toxic gas. It is odorless when pure. The technical grade has an odor of garlic or rotting fish because of the presence of substituted phosphines and diphosphane (P2H4).1 The largest use of phosphine is as a fumigant and rodenticide but it is also extremely important as an intermediate in the synthesis of flame retardants for cotton fabrics, as a doping agent for n-type semiconductors, a polymerization initiator and a condensation catalyst. This is an extremely toxic gas with a concentration of only 2.8 mg/L (ca 2000 ppm in air) being lethal to humans in a very short period of time.5
Aluminum or magnesium phosphide are sources of phosphine. Aluminum phosphide when used as a pesticide is usually formulated in pellets, granules, or as a dust. Upon contact with moisture in the environment, aluminum phosphide undergoes a chemical reaction yielding phosphine gas which is the active fumigant component (Fig. 2).6
Fig. 2: Liberation of phosphine gas from aluminum phosphide.
These phosphides are used as fumigants primarily to control insects in stored grain and other agricultural commodities. They are also used to control burrowing rodents in outdoor agricultural and other non-domestic areas. In Canada, the fumigants are restricted to use by specially trained pesticide applicators. In fact, Health Canada requires that phosphine pesticides can be sold in Canada only to people holding an appropriate pesticide applicator certificate or license. In the case of the Fort McMurray family, the pesticide was brought back from a recent trip to Pakistan.
Research is still being conducted on phosphine to determine the exact mechanism of toxicity. Phosphine is a nucleophile, which also acts as a strong reducing agent and is capable of interacting with and inhibiting cellular enzymes involved in metabolic processes. The two exposure routes are inhalation of phosphine directly or from ingestion of aluminum phosphide, which can generate phosphine by the action of dilute hydrochloric acid content in the stomach. Three possible reasons for phosphine’s toxicity are currently being researched: its disruption of energy metabolism, its oxidative stress on the cell and its redox activity. Although these can be studied independently, they are most likely interrelated.7
A paper by Nisa Nath et al. describe the toxicity of not only phosphine (PH3) but also ammonia (NH3) and arsine (AsH3),8 which contain the elements located immediately above phosphorus (nitrogen) or below phosphorus (arsenic) in group 15 of the periodic table. All three of these hydrides are toxic gases. The chemical and biological differences and similarities between these three molecules would make an interesting online research project for your students.
References
- https://en.wikipedia.org/wiki/Phosphine
- http://www.cbc.ca/news/canada/edmonton/phosphine-pesticide-used-to-kill-bedbugs-causes-fort-mcmurray-baby-s-death-1.2969189
- http://www.cbc.ca/news/canada/montreal/phosphine-gas-likely-cause-of-thailand-deaths-of-quebec-sisters-coroner-1.2977667
- http://www.arabnews.com/saudi-arabia/news/760231;
http://www.haaretz.com/israel-news/1.570036,
http://www.safelawns.org/blog/2010/02/second-girl-dies-in-apparent-pesticide-poisoning-in-utah/ - Joseph Sherma, Ed. “Analytical Methods for Pesticides and Plant
Growth Regulators”. Academic Press, Inc., San Diego, 1988. - www.ncceh.ca/sites/default/files/Phosphine_Poisoning_Bed_Bug_Treatment_March_2015.pdf; http://npic.orst.edu/mcapro/Phosphine.pdf
- G. Bumbrah, K. Krishan, T. Kanchan, M. Sharma and G. Sodhi, “Phosphide poisoning: A review of literature”, Forensic Science International, Volume 214, 2012, pages 1-6.
- N. Nath, I. Bhattacharya, A. Tuck, D. Schlipalius and P. Ebert,
“Mechanisms of Phosphine Toxicity”, Journal of Toxicology, Volume 2011, 2011, pages 1-9.
