on his qualitative observations of the volunteer staff (including himself) at a free clinic for drug addicts. He characterized burnout by a set of symptoms that includes exhaustion resulting from work's excessive demands as well as physical symptoms such as headaches and sleeplessness, "quickness to anger", and closed thinking. He observed that the burned-out worker "looks, acts, and seems depressed." After the publication of Freudenberger's paper, interest in the concept grew. In 1976, Christina Maslach noted the term being used by California lawyers working with the poor, and began to study the concept. In 1981, Maslach and Susan E. Jackson published an instrument for assessing burnout, the Maslach Burnout Inventory (MBI). It was the first such instrument of its kind and the most widely used burnout instrument. The two researchers described burnout in terms of emotional exhaustion, depersonalization (feeling low-empathy towards other people in an occupational setting), and reduced feelings of work-related personal accomplishment. In 2010, the Swedish Board of Health and Welfare adopted a refined conceptualisation of severe burnout it described as "exhaustion disorder." In 2015, The World Health Organization adopted a conceptualisation of burnout that is consistent with Maslach's. It adopted a modified version of this in 2022. However, "is not itself classified by the WHO as a medical condition or mental disorder." Also in 2015, Bianchi and colleagues brought to bear evidence that the construct of burnout substantially overlaps with depression. As of 2017, nine European countries considered burnout as being a recognised medical disorder. Diagnosis Classification The two main classification systems of psychological disorders are the Diagnostic and Statistical Manual of Mental Disorders (DSM, used in North America and elsewhere) from the American Psychiatric Association (APA), and the International Classification of Diseases (ICD, used in Europe and elsewhere) from the World Health Organisation (WHO). Burnout is not recognized as a distinct mental disorder in the DSM-5 (published in 2013). Its definitions for Adjustment Disorders, and Unspecified Trauma- and Stressor-Related Disorder in some cases reflect the condition. 2022's update, the DSM-5-TR, did not add a definition of burnout. As of 2017, nine European countries considered burnout as being a recognised medical disorder. These countries classify it under one or more codes of the ICD. The ICD-10 (current 1994–2021) classified "burn-out" as a type of non-medical life-management difficulty under code Z73.0. It was considered to be one of the "factors influencing health status and contact with health services"