vessels. The International Tanker Owners Pollution Federation has tracked 9,351 accidental spills that have occurred since 1974. According to this study, most spills result from routine operations such as loading cargo, discharging cargo, and taking on fuel oil. 91% of the operational oil spills are small, resulting in less than 7 metric tons per spill. On the other hand, spills resulting from accidents like collisions, groundings, hull failures, and explosions are much larger, with 84% of these involving losses of over 700 metric tons. Following the Exxon Valdez spill, the United States passed the Oil Pollution Act of 1990 (OPA-90), which excluded single-hull tank vessels of 5,000 gross tons or more from US waters from 2010 onward, apart from those with a double bottom or double sides, which may be permitted to trade to the United States through 2015, depending on their age. Following the sinkings of (1999) and Prestige (2002), the European Union passed its own stringent anti-pollution packages (known as Erika I, II, and III), which also require all tankers entering its waters to be double-hulled by 2010. The Erika packages are controversial because they introduced the new legal concept of "serious negligence". Air pollution Air pollution from engine operation and from cargo fires is another serious concern. Large ships are often run on low quality fuel oils, such as bunker oil, which is highly polluting and has been shown to be a health risk. Ship fires may result in the loss of the ship due to lack of specialized firefighting gear and techniques and fires can burn for days. See also Hydraulic tanker List of oil spills List of replenishment ships of the Royal Fleet Auxiliary List of tankers List of Type T2 tankers Marine transfer operations Merchant vessel Petroleum transport Slosh dynamics T1 tanker T2 tanker T3 Tanker Type C1 ship Type C2 ship Type C3 ship United States Navy oiler References Notes Bibliography . Further reading External links : Saudi shipper Bahri plans to increase VLCC fleet to 46 ship-photos.de: Private homepage of categorized ship photos including tankers of all kinds Oil tanker by Picture "Floating Oil Tanks" Popular Mechanics, March 1930, pp. 370–374 article on the oil tankers between the World Wars Bill Willis. Supertankers Intertanko—the society of International Tanker Operators The International Maritime Organization—Tanker Safety (for double-hulls) Ship photos of tankers, ULCCs, VLCCs, barges Information on crude oil tankers and other forms of