The AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), Fifth Edition, uses approaches to respiratory system impairment rating that are similar to those of previous editions, with two significant changes: First, criteria for asthma impairment were updated to incorporate guidelines recently published by the American Thoracic Society (ATS), and, second, respiratory impairment criteria now incorporate the lower limit of normal for forced vital capacity, forced expiratory volume in the first second (FEV1), and diffusing capacity for carbon monoxide. In 1993, the ATS published Guidelines for the Evaluation of Impairment/Disability in Patients with Asthma that recommended evaluating three parameters: degree of airflow limitation (postbronchodilator FEV1); the degree of reversibility of the airflow limitation; and the minimum amount of medication needed to maintain maximal medical improvement (MMI). The ATS Guidelines contained a scoring system that was used to place the individual into one of six impairment classes. The Fifth Edition uses the same method of scoring the three clinical parameters and using the point score to determine which of the four impairment classes best describes the impairment. The Fifth Edition discussion of occupational asthma indicates the importance of removing the individual from further exposure to the sensitizing agent; MMI usually occurs within 2 years after removal from exposure.