Table 2.

The rating and downgrading criteria of the appraisal approach components.

Component	Levels	Rating criteria	Downgrading criteria
Study design	serious/not serious	Rate “serious” if any of the eight questions is answered “NO”.	Downgrade one level of evidence certainty if rated “serious”.
Risk of Bias	serious/not serious	Rate “serious” if more than half of the original studies had “high” or “some” risk of bias.	Downgrade one level of evidence certainty if rated “serious”.
Inconsistency	serious/not serious	Rate “serious” if more than half of the relevant results had high heterogeneity.	Downgrade one level of evidence certainty if rated “serious”.
Indirectness	very serious/serious/not serious	Rate “serious” if any one of the following descriptions were not met, and rate “very serious” if more than two descriptions were not met. 1.The outcome is COVID-19 infection. 2.The original studies were conducted in healthcare settings. 3.Comparison is N95 respirator vs. medical mask.	Downgrade one level of evidence certainty if rated “serious”, and two levels if rated “very serious”.
Imprecision	serious/not serious	Rate “serious” if more than half of the relevant results had confidence intervals across 1.	Downgrade one level of evidence certainty if rated “serious”.
Publication bias	serious/not serious	Rate “serious” if the authors mentioned concerns about publication bias.	Downgrade one level of evidence certainty if rated “serious”.