Many freight container units and bulk cargo holds on ships are treated with chemical pesticides termed fumigants. Fumigant must be applied according to the correct instructions. As holds are always unventilated for a time after fumigation, there may be a risk of excessive condensation, which can produce sweating or dripping. This can lead to cargo damage as well as fire and explosion risks, warns the Swedish P&I Club.
, in a previous edition on the causes and prevention of cargo fires, the Club says that agricultural products in bulk may be
fumigated in ships’ holds to prevent insect infestation. Solid aluminium phosphide (or similar) is often used for fumigation. Aluminium phosphide reacts with water vapour (humidity) in air to produce phosphine, a toxic and flammable/explosive gas, which kills insects.
Heat is also given off during the reaction. The solid fumigant may be applied in fabric ‘socks’ or as pellets on the surface, just before closing holds. Holds are then kept closed for a period before ventilating. and people must keep out of holds that are being fumigated due to the toxic fumigant.
If there is an excessive amount of fumigant in one place, or if the fumigant is contacted by liquid water e.g. from sweating or condensation, then the fumigant can react too quickly. This can evolve excessive heat and lead to ignition of cargo and/ or packaging such as bags or paper placed over the top of the cargo. Under certain conditions the fumigant gas itself may ignite, producing an explosion.
It is important that fumigant is applied according to the correct instructions. As holds are always un-ventilated for a time after fumigation, there may be a risk of excessive condensation, which can produce sweating or dripping. This can lead to cargo damage as well as the fire and explosion risks mentioned above. The weather conditions and cargo conditions, such as moisture content, therefore need to be considered properly before fumigation, which is often carried out by specialist companies.
In addition, the Guide includes the following case study to highlight associated risks.
A bulk carrier was fully loaded with yellow corn. After loading was completed, fumigation technicians came on board and fumigated the cargo holds with fumigation pellets. The fumigation pellets should, as per the requirements, be applied subsurface. However the technicians poured the pellets on top of the cargo.
This work took a little more than an hour and afterwards all cargo hatches were closed and the vessel sailed. A couple of hours later an explosion occurred in hold number 3. The crew noted that the hatch covers had moved slightly and blue-grey smoke was seen coming from under the edges of the hatch covers. About an hour later another explosion occurred. This time it was from hold number 4 and a couple of minutes later an explosion in hold number 6 occurred.
The remaining holds 1, 2, 5 and 7 had explosions shortly afterwards. Fumigation pellets and similar fumigants are formulated with some 55% aluminium phosphide, ammonium carbamate and inert materials. The aluminium phosphide reacts with water to produce phosphine, which is extremely toxic and an effective fumigant. Under normal conditions, phosphine is a gas that is slightly denser than air.
It is colourless, highly toxic, and has an odour variously described as ‘fishy’, ‘garlic-like’ or ‘like carbide’. Phosphine gas will form an explosive (or flammable) mixture when mixed with air in a concentration exceeding about 1.8% to 2% by volume – this is its lower explosive limit.
The probability of the phosphine concentration exceeding the lower explosive limit concentration is increased if pellets are allowed to accumulate in a heap, as opposed to being spread out across the surface of a cargo, or buried in the bulk ofthe cargo. The explosive phosphine/ air mixture mentioned above spontaneously ignited.
The method of application mentioned above had likely permitted the accumulation of the pellets in limited areas and promoted relatively rapid reaction of the fumigation pellets with moisture, thereby generating concentrations of phosphine gas above the lower explosive limit concentration.