Dangerous phosphine practices in West Africa

Following liberalisation, highly toxic phosphine for grain preservation is readily available in West Africa. Peter Golob and Bruno Tran report on the hazardous practices among smallholders and retailers and explain why these are also ineffective in protecting grain from pest damage. 

Aluminium phosphide tablets being sold by market traders in Tamale, Northern Ghana. Photo Peter Golob

Phosphine is a gas that is commonly used to disinfect durable food commodities before storage. It is released from either aluminium or magnesium phosphide, which are available in a variety of formulations, the most common being three gram tablets. The grain warehousing industry is becoming ever more reliant on phosphine as the phase out of the ozone depleting methyl bromide draws closer. The gas is acutely toxic and in the past, in many African countries its use was severely restricted to registered pest control companies and to large-scale warehousing organisations. However, since agricultural marketing liberalisation, restrictions on the distribution and sale of phosphine formulations have ceased and the gas is now widely used by small-scale farmers and consumers both to protect grain in store and for rodent control.
    Potentially, phosphine presents a greater health hazard than organophosphates or pyrethroid insecticides because of its very high acute toxicity. Gas is liberated from aluminium phosphide when tablets are exposed to water vapour in air. In hot humid conditions in West Africa, phosphine starts to be released within 30 minutes of the tablets being removed from their air-tight containers. The limit of gas concentration that is regarded as safe for humans is one part per million (ppm) for exposures not exceeding 15 minutes, with no more than four such exposures a day. For more continuous exposure, the limit is 0.3 ppm. Under normal fumigation conditions, gas is released continuously for 24 hours by which time concentrations of 2,000 ppm are achieved. Clearly, even within a short period of tablets being removed from their container lethal concentrations of gas will be released.
    Effective phosphine fumigations take at least seven days to be completed. Over the period gas concentration has to be retained above a level of 150 ppm (parts per million parts of the grain) for it to be biologically active. This can only be achieved by conducting fumigations inside gas-tight enclosures, which can be in well sealed silos, warehouses, rail wagons or containers, or under specially designed fumigation sheets. In most of the developing world it is most effective to use fumigation sheets, which can be used with relatively little effort to create effective gas-tight chambers. However, skill and care is needed to maintain gas-tight conditions because even small openings, such as found around closed windows or doors, can result in sufficient gas loss to make the fumigation fail.

The case of northern Ghana
Phosphine has been available in Northern Ghana for more than a decade. Insecticide wholesalers, who also retail, obtain their supplies of aluminium phosphide tablets from distributors in Kumasi market. They in turn are supplied by Chemico Limited, a company based in Tema, which is the main agent for Gastoxin, an aluminium phosphide preparation manufacturered in Brazil. However, unofficially smuggled chemical from United Phosphorus of India, sold through the Oyo (Nigeria) State Corporation, is also on sale in Tamale, the main trading town in northern Ghana. Tamale wholesalers supply itinerant traders who carry tubes of tablets from one local market to another, selling individual tablets to whoever will buy. 
    Tablets are packed by the manufacturers into tubes of 30 (enough to treat 10 tonnes of grain) or into resealable flasks of 100 or 250. These packs contain far too many tablets for the average farmer who, storing no more than one tonne of grain, will need perhaps three at most. The trader usually puts individual tablets, or two or three, into a small polythene bag (150-300 gauge; minimum of 500 gauge is needed for gas tightness) before handing them to the purchaser, who pockets the packet and returns home. In an area where the average daytime temperature is above 280C such practices are very hazardous as phosphine may well be released whilst travelling, putting the farmer and fellow passengers at risk.

Dangerous conditions on farms
Farmers do not have access to stores that can be made gas-tight. Instead, they attempt to fumigate their own traditional stores. These are either made of woven cereal stalks or grass, which may be plastered with mud, or are small silos built of mud and cow dung. Air can diffuse between the inner and outer walls of these stores and so they are by no means suitable for fumigation. The mud silo can be converted to a reasonable fumigation chamber by painting both internal and external surfaces with two coats of oil-based paint, but only at great expense which is beyond contemplation for poor rural families. Nevertheless, farmers do attempt to fumigate these structures at their homesteads. Although most storage structures are located outside of the main accommodation, leaking gas will put both family members and domestic livestock at risk. Furthermore, many people attempt to fumigate commodities that are stored in rooms inside the house, which may be adjacent to sleeping quarters. Escaping gas easily penetrates mud and wattle walls through the building, putting sleeping adults and children at risk from intoxication.
    People also try to fumigate grain inside jute, hessian or woven polypropylene sacks, none of which are suitable for this purpose. Sacks are becoming increasingly popular for storage and the Plant Protection and Regulatory Services Department (PPRSD) of the Ministry of Food and Agriculture (MoFA) has recommended that tablets can be used in grain sacks that have an inner polythene liner. However, farmers do not use sack liners because of the extra cost. Instead, they simply place the tablets, wrapped in a piece of cotton, straight into the sacks from where the gas is free to leak straight back out into the environment.

Ineffective unless sealed
The normal recommended dosage rate is three to five tablets per tonne (each tablet releases one gram of phosphine). Into each individual 100kg sack of maize, farmers add one tablet, a potentially significant overdose. However, this treatment has only a cosmetic effect on the pest population, only killing those insects exposed on the surfaces. Much of the gas diffuses out through the walls of the sack too quickly for all of the insects to be intoxicated. Effective treatment of produce can be undertaken by covering the sacks with a proper fumigation sheet, not a typical tarpaulin, which is permeable to gas, but one which will prevent gas escaping. Such sheets do not need to be expensive, they can be made from heavy duty polythene.
    Gas loss leads to insects being exposed to sub-lethal dosages. This results in the pest becoming resistant to the gas and then more difficult to control. Resistance to phosphine has been increasing in the past decade and is now of significant importance because of the phase out of methyl bromide, which at present is the only viable alternative for rapid disinfestations of grain.
    Although tablets are readily available and relatively cheap (one tablet costs 700 cedis, so the cost of treating a tonne of grain is approximately $0.30) there is insufficient knowledge available locally to ensure that they are used safely and effectively. Retailers and wholesalers verbally instruct purchasers on the methods to employ when using tablets. However, the suppliers themselves are not sufficiently conversant with the properties of the gas to enable them to provide correct and adequate information. For example, they all recommended treating individual sacks. One retailer, who stocks both tablets and sachets of powdered aluminium phosphide, believed the formulations functioned in different ways, he was unaware that both produced phosphine gas. This lack of knowledge is symptomatic of farmers as well. One group of producers said they put tablets in sacks, but that after a few days, when a grey powder remained (after decomposition to aluminium hydroxide), they mixed the powder with the grain and so provided protection for several months. These farmers, as well as some retailers, were unaware that the only active component is a gas.

Information and training vital
The culture of poor and dangerous fumigation practice is further reinforced by various NGOs and MoFA extension staff whose responsibility it is to advise and assist farmers. Insufficient care is given to using suitable fumigation enclosures or to ensuring these are made and kept gas-tight. Staff do not appreciate the need for care as they do not possess adequate training or experience of good fumigation practice.
    It is difficult to estimate the extent of accidental poisoning that occurs as a result of phosphine use. There are no hospital records confirming phosphine poisoning within northern Ghana. Furthermore, it is very unlikely that medical practitioners would be capable of recognising the symptoms. Reports of people suffering from headaches and feeling nauseous when they handled tablets are quite common. In a few villages the entire population is reluctant to use phosphine because of the 'bad scent'. Many people interviewed said they vacated rooms where the tablets had been used but several felt that it was not too worrying if they did come in contact with the odour. One large farmer, who is also a retailer in Damongo, said there were on average two fatalities every year as a result of people using phosphine to commit suicide. His own wife also became affected when she unknowingly entered a room in which grain had just been dosed with tablets. She became nauseous and vomited; she fully recovered after a night in hospital.
    In order to stay healthy, people should not be attempting to fumigate grain in individual sacks. Sack fumigation will be ineffective anyway. Also, farmers should not be fumigating traditional storage structures. 
    However, it would be extremely difficult to eliminate the use of phosphine even if legislation was introduced. It is therefore vital that actions are taken to enable people to use phosphine effectively and safely. To begin, this must encompass a comprehensive training programme directed at all levels of society in order to create awareness of the risks involved and of good storage pest management. Additionally, appropriate methods of application need to be developed and extended, perhaps based on the use of fumigation sheets and gas-tight containers such as heavy duty, high density polythene water tanks. 
    Similar poor fumigation practices are occurring in other West African countries and increasingly in East and Southern Africa where the effects of market liberalisation are also being felt. Unquestionably, if measures are not put into place then inevitably there will be major human tragedies occurring before very long.

Until recently Peter Golob and Bruno Tran worked at the Natural Resources Institute, University of Greenwich, UK.

[This article first appeared in Pesticides News No. 53, September 2001, pages 4-5]