Asthma action plans recommend classifying respiratory status as green, yellow or red, depending on symptoms and peak expiratory flow. The green zone is 80% of the patient's usual peak flow and above; yellow is 50-80% and red is less than 50%. The usual (asymptomatic) peak flow should be measured for the individual patient, but can be estimated based on patient height (Hsu et al. (1979) Ventilatory functions of normal children and young adults—Mexican-American, white and black. I. Spirometry. Journal of Pediatrics 95:14-23).