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Disability is commonly defined as a decrease in or inability to perform some or all functions related to personal, social or workplace demands due to a physical or mental impairment. This view is distinct from impairment, defined as a reduction in normal function. Impairment should not necessarily imply that work disability is present, unless there is a functional limitation that precludes the worker from performing some or all of the tasks required in their job, after considering available work accommodations.

The AMA's Guides to the Evaluation of Permanent Impairment states that its impairment ratings are not intended to be used to rate disability because disability reflects a combination of medical and nonmedical factors. Critics point out that the numeric quantification of impairment, the aspect of the guides that encourages its expanding use, is not evidence based. State workers' compensation programs inappropriately use the impairment ratings as a proxy for the extent of disability.

Work disability has not been consistently defined nor measured, and thus population data on incidence, prevalence, and causes are inconsistent. In developed countries, around one in six workers has or develops a significant health-related limitation in ability to work each year, and a third has a period of work absence as a result—mostly short-term absences due to temporary conditions. A small but important group develops long-term work disability. Although these persons are just a few percent of all workers with health-related work limitations, they account for the majority of total days lost from work due to health conditions.

Nearly one in five US adults is disabled from mental and behavioral disorders, musculoskeletal problems, and neurological conditions. About half of the disabled are severely limited and unable to work. Musculoskeletal disorders are the most frequent type of disability, but in some working age groups are now exceeded by mental health disorders. The relationships between work, disability, and mental health are interrelated with each one having a potential impact on the other (see Chapter 35).

Work disability is a complex issue that can involve multiple participants and concerns beyond a particular health-related condition and its treatment. Personal, workplace, medical, and societal issues can affect the extent of a disability and how return to work may succeed or fail. Social and environmental factors usually have a greater impact on disability and its prevention than health-related dimensions. The organizational work environment with supervisors, coworkers, unions, and management roles along with insurers, family, and society are recognized as major potentially modifiable influences on disability and return to work; the immediate workplace response to a worker with a potentially disabling condition is especially important. Health care providers and their medical systems (facilities, staff, insurers, administrators, medical records) must understand these influences and be able to collaborate with and at times influence nonmedical personnel, to achieve optimal prevention and management of work disability.

One model used to elaborate the ...

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