A study from the Harvard T.H. Chan School of Public Health showed that a large number of active commercial pilots are clinically depressed, and may even be suffering from suicidal thinking.

The pilots also expressed a reluctance to seek professional help due to concern that it could threaten their employment.

The study surveyed pilots who completed an anonymous, web-based survey that included a variety of topics so respondents were not aware of the mental health focus of the study. They were also asked whether a doctor had ever diagnosed them with depression.

Of the 3,500 participants, more than 1,800 completed the mental health questions, and nearly 13 percent of them (233 pilots) were considered depressed, compared to 7 percent of people in the U.S. who have experienced depression in the last year.

In addition, 75 of these pilots reported suicidal thinking during the previous two weeks.

Most people who express suicidal thinking do not act on those thoughts, but the results of this study argue for more effective ways to diagnose and treat depression to help pilots and protect public safety.