Washington: Pilots killed in aeroplane crashes in 2012 tested positive for drugs four times more often than those killed in 1990, according to a study by the National Transportation Safety Board, although drawing a causal link between drugs and crashes was difficult.

The board, in a study approved on Tuesday, said that doctors and the Federal Aviation Administration were not doing enough to warn pilots about how their performance could be affected by drugs — prescription, over-the-counter and illicit. Perhaps the most worrisome category was sedating antihistamines, present in 9.9 per cent of pilots who died from 2008 to 2012, compared with 5.6 per cent from 1990 to 1997. The ingredients in Vicodin and Valium also showed up more frequently.

Nearly all the cases involved private pilots. One of the few commercial cases took place in March 1983, when a Lear Jet crashed at Newark Airport. The safety board attributed the accident in part to marijuana and other drugs.

The board cites the use of drugs, legal or illegal, as a factor in about 3 per cent of crashes.

Toxicology reports on pilots killed in crashes showed a positive result for one drug 39 per cent of the time in 2012, up from 9.6 per cent in 1990. The percentage with more than one drug rose to 20.5 per cent from 2 per cent, and more than two drugs rose to 8.3 per cent in 2012 from zero in 1990.

The board’s study said that the pilot population, like the nation itself, was getting older and sicker and thus more likely to be taking medication, some of which could affect performance and others used to treat “potentially impairing conditions,” like heart problems.

Over the years of the study, which covered 6,677 fatalities, the average age of pilots who died increased to 57.3 from 46.2. The pilots who died were, on average, about 15 years older than pilots as a whole.

The safety board’s staff said that there was no evidence that pilots were more likely to use drugs than drivers or other transportation operators, but that the database for pilots was better; toxicology reports were done on 88 per cent of pilots who died, and the tests screened for 1,300 separate drugs.

“This particular study allowed us to look at it in a way that we can’t look at it in other modes of transportation,” said Dr. Mary Pat McKay, a public health specialist who is the board’s chief medical officer. She added that the increase in drug use, including prescription drug use, in the general population was “pretty alarming.”

The acting chairman of the safety board, Christopher A. Hart, noted that he was from Colorado, where he pointed out that marijuana had recently been legalised, and he asked about what the study found about marijuana use. Loren Groff, a staff specialist, said it had increased to 3 per cent from 1.6 per cent, and was more likely to be found in younger pilots, opposite from the trend of other drugs, which were more common in older pilots. McKay said it was more potent when eaten, instead of smoked.

Hart, a private pilot, deadpanned, “I’ll keep that in mind in my future decision-making.”