To a physician, "cause" may refer only to direct or principal cause, but for the law of Workers' Compensation, a variety of contributing causes must be considered. Under the Federal Employees Compensation Act (FECA), any disease or disability is compensable when it is proximately caused or materially aggravated by an employment-related injury or condition of employment. Proximate cause is that which, in a natural and continuing sequence, produces the disability. However, natural progression of a disease while a person is working does not constitute cause or aggravation. For conditions of employment to bring about aggravation of any underlying disease, the employment factors must be capable of aggravating or accelerating the disease.

There are two kinds of aggravation: 
 The pre-existing condition is worsened or made more severe for a time with no residual alteration of the underlying condition and without leaving any continuing impairment beyond that time.

PERMANENT AGGRAVATION: There is a continuing and irreversible change in the underlying condition, thus adversely altering the course of the condition or disease process.

MEDICAL RATIONALE: A logical explanation for the physician's underlying opinions, fundamental reasons and beliefs concerning causal relationship.

PROXIMATE CAUSE: That which produces the injury in a natural and continuous sequence, unbroken by an efficient and intervening cause and without which the result would not have occurred.

AGGRAVATION: A documented physiological process by which a single occupational act, or series of acts over a period of time intensified the severity of a physical or mental problem which pre-existed the occupational disease.

ACCELERATION: A documented physiological process by which a single occupational act, or series of acts, can be shown to have increased the expected speed of progression in a pre-existing condition documented to be progressive in nature.

PRECIPITATION: Hastening the occurrence of an event or causing to happen or come to crisis suddenly, unexpectedly, or too soon.