Situation: A 24-year-old woman with asthma is brought to the emergency room (ER) with wheezing and breathlessness that began after a viral cold. At home she uses only a salbutamol (albuterol) inhaler, about 6 to 8 times a week. She has no other illness, takes no other medicines, and her oxygen saturation by pulse oximetry (SpO2) is monitored continuously. Before discharge, the nurse teaches her to use a peak expiratory flow (PEF) meter with a written asthma action plan. In the traffic-light zone system, which range of her personal best PEF defines the yellow zone?

In the traffic-light action plan, the green zone is 80% or more of personal best, the yellow zone is 50% to 79%, and the red zone is below 50%. In the yellow zone she uses her reliever, follows her written plan, and contacts her provider; in the red zone she uses her reliever and seeks emergency care.

Traffic-Light Zone System for PEF Monitoring

The written asthma action plan uses a traffic-light model to help patients interpret peak expiratory flow readings and respond appropriately. The zones are defined as percentages of the patient’s personal best PEF, which is established during a period of good asthma control.

Zone	PEF Range	Clinical Meaning	Typical Action
Green zone	80% to 100% of personal best	Good control; no significant airflow limitation	Continue daily controller; use reliever as needed
Yellow zone	50% to 79% of personal best	Caution; worsening airflow obstruction or impending exacerbation	Use reliever; follow written plan; contact provider
Red zone	Below 50% of personal best	Medical alert; severe airflow limitation	Use reliever; seek emergency care immediately

The yellow zone is the key early-warning band. A drop into the 50% to 79% range signals that airway inflammation and bronchoconstriction are increasing, even before the patient may perceive severe symptoms. This is especially important because patients with poor symptom perception may not recognize an impending exacerbation by clinical score or symptom questionnaires alone ^[2]. Objective PEF monitoring provides an earlier, effort-dependent but quantifiable signal of decline.

In the scenario, the patient uses salbutamol 6 to 8 times a week, which already suggests suboptimal control. Teaching her to identify the yellow zone allows her to act on a measurable threshold rather than waiting for breathlessness to become severe. The action plan in this zone typically includes increasing reliever use, possibly stepping up controller therapy per the written plan, and contacting the healthcare provider.

Watch out! Do not confuse the yellow zone lower boundary with the red zone threshold. The red zone begins below 50%, not at 60%. Option 1 (60% to 79%) is a narrower range that excludes the lower yellow-zone readings where early intervention is still possible.

Key point! The traffic-light system is standardized across most asthma action plan guidelines: green is 80% or above, yellow is 50% to 79%, and red is below 50% . The PEF meter converts a subjective sense of “not feeling well” into an objective number that guides stepwise self-management.