based its decommissioning decisions on estimates of the quantities of various pollutants, including PCBs, crude oil, heavy metals, and scale, which it had calculated based on the operating activities of the platform, and metal that would remain in the structure after decommissioning. Scale is a by-product of oil production and, because of the radioactivity found in the rocks from which the oil is extracted, is considered to be low-level radioactive waste. It is regularly dealt with on-shore by workers wearing breathing masks to prevent inhalation of dust. Disposal options Shell examined a number of options for disposing of the Brent Spar, and took two of these forward for serious consideration. On-shore dismantling The first option involved towing the Brent Spar to a shallow water harbour to decontaminate it and reuse the materials used in its construction. Any unusable waste could be disposed of on land. Technically, this option was more complex and presented a greater hazard to the workforce. This option was estimated to cost £41M. There was some concern that the facility would disintegrate in shallow coastal water, having a much more economically and environmentally significant effect. Deep sea disposal The second option involved towing the decommissioned platform into deep water in the North Atlantic, positioning explosives around the waterline, then detonating them, in order to breach the hull and sink the platform. The facility would then fall to the seabed and release its contents over a restricted area. Due to the uncertainty associated with detonating explosives, a number of possible scenarios were envisaged. First, the structure would fall to the seabed in one piece, releasing its contaminants slowly, and affecting the seabed for around "down-current". Second, the structure might disintegrate as it fell through the water column. This would release contaminants in a single burst, and have an effect for "down current" of the final resting place, although this would last for a shorter time than in the first instance. Third, the structure could fail catastrophically when the explosives detonated, releasing its contaminants into the surface waters. This would affect sea birds and the fishing industry in that area. The cost of this option was estimated at between £17M and £20M. Shell proposed that deep sea disposal was the best option for Brent Spar, arguing that their decision had been made on sound scientific principles and data. Dismantling the platform on-shore was more complex from an engineering point