through the inside of the wing's leading edge, heating it to a temperature above freezing, which prevents the formation of ice. The air then exits through small holes in the wing edge. On propeller-driven aircraft, it is common to use bleed air to inflate a rubber boot on the leading edge, breaking the ice loose after it has already formed. Bleed air from the high-pressure compressor of the engine is used to supply reaction control valves as used for part of the flight control system in the Harrier family of military aircraft. Contamination On about 1 in 5,000 flights, bleed air used for air conditioning and pressurization can be contaminated by chemicals such as oil or hydraulic fluid. This is known as a fume event. While those chemicals can be irritating, such events have not been established to cause long-term harm. Certain neurological and respiratory ill health effects have been linked anecdotally to exposure to bleed air that has been alleged to have been contaminated with toxic levels on commercial and military aircraft. This alleged long-term illness is referred to as aerotoxic syndrome, but it is not a medically recognized syndrome. One potential contaminant is tricresyl phosphate. Many lobbying groups have been set up to advocate for research into this hazard, including the Aviation Organophosphate Information Site (AOPIS) (2001), the Global Cabin Air Quality Executive (2006) and the UK-based Aerotoxic Association (2007). Cabin Environment Research is one of many functions of the ACER Group, but their researchers have not yet established any causal relationship. Although a study made for the EU in 2014 confirmed that contamination of cabin air could be a problem, that study also stated: "A lot of reported fume events caused comfort limitations for the occupants but posed no danger. A verification of cabin air contamination with toxic substances (e.g. TCP/TOCP) was not possible with the fume events the BFU investigated." While no scientific evidence to date has found that airliner cabin air has been contaminated to toxic levels (exceeding known safe levels, in ppm, of any dangerous chemical), a court in Australia in March 2010 found in favor of a former airline flight attendant who claimed she suffered chronic respiratory problems after being exposed to oil fumes on a trip in March 1992. Such testing is infrequent due to Boeing's refusal to install air quality sensors in its planes, fearing lawsuits from crew or passengers over