Fig. 1
From: Effects of particulate matter (PM) on childhood asthma exacerbation and control in Xiamen, China
Schematic view of the study design. Panel (a):For patients of acute exacerbation, the day two weeks before the exacerbation was considered as control. The PM exposures within 2 weeks before the exacerbation day and control day were recorded,respectively. Panel (b):The PM exposure within 4 weeks before the return visitwas recorded. Patients were assessed at follow-up visit based on the symptoms in the past 4 weeks