Table 3.

Ratings and rationale for downgrading and upgrading factors applied to studies that evaluated PM_2.5, and overall quality of the body of evidence. Table S8 presents effect estimates and confidence intervals for the group of studies that evaluated PM_2.5

Factor	Rating	Rationale
Downgrading factors
Risk of bias	-1	The reviewers conclude that there is a moderate, but not substantial, risk of bias across the included studies
Indirectness	0	The included studies directly assessed the population, exposure, and outcome of interest
Inconsistency	-2	There was inconsistency in results within similar populations in more than three pollutant/outcome pairs
Imprecision	0	Included studies had adequate sample sizes and confidence intervals were not considered wide (all reported 95% confidence intervals)
Publication bias	0	The reviewers found no indication of publication bias. The search was extensive and comprehensive and there is no reason to believe that studies were missing from the body of evidence
Upgrading factors
Large magnitude of effect	0	The estimated effects across studies were not considered to be large
Concentration-response	+2	Relationships between concentration and response were identified in ten or more studies
Confounding minimizes effect	0	The reviewers did not find evidence to suggest that residual confounding or additional biases would reduce effect estimates
Overall rating of quality of evidence:	Low Quality Evidence	Low = -1: -1 downgrade for risk of bias; -2 downgrade for inconsistency amongst pollutant effects on thyroid outcomes; and + 2 upgrade for concentration-response relationship.