Peak expiratory flow tracks the fastest speed you can exhale. Comparing today's reading to your personal best places you in a green, yellow, or red asthma action-plan zone.
Green zone — good control, continue usual plan.
Yellow zone — caution; use reliever and follow the action plan.
Red zone — danger; seek medical care now.
Zones are set as a percentage of your personal best reading.
Individual variation is significant, so tracking against a person's own best measured value is generally considered more clinically useful than the population-predicted number for day-to-day asthma monitoring.
Green is at or above 80% of personal best (good control), yellow is 50–80% (caution — use reliever and follow the action plan), and red is below 50% (danger — seek medical care).
Record your highest peak flow reading over a 2–3 week period when your asthma is well controlled; that value becomes the reference for your zones.
Roughly 390–740 L/min depending on age, height, and sex — but the personal-best comparison matters more than the population range for monitoring.