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Table 3 Rating of the quality and strength of the evidence for studies assessing interactive effects between heat and air pollution (n = 39)

From: Synergistic health effects of air pollution, temperature, and pollen exposure: a systematic review of epidemiological evidence

Category Summary of Criteria Downgrades Rationale
Initial Rating of Human Evidence = “Moderate”
 Risk of Bias Study limitations- a substantial risk of bias across body of evidence. -1 Downgraded due to “probably high” risk of bias for air pollution exposure assessment for 16 studies.
 Indirectness Evidence was not directly comparable to the chosen population, exposure, comparator, and outcome. 0 Measured outcomes were assessed for humans in the United States for the duration of the study periods, as outlined in the PECO statement.
 Inconsistency Wide variability in estimates of effect in similar populations. 0 There was not a wide variability in estimates of effects.
 Imprecision Studies had a small sample size and small outcome count. 0 The studies had large sample sizes with adequate samples for outcomes during study periods.
 Publication Bias Studies missing for body of evidence, resulting in an over or underestimate of true effects from exposure. 0 The studies were large studies that varied in year, data sources, and methods of statistical analysis that appeared to report outcomes found regardless of results.
Category Summary of Criteria Upgrades Rationale
Large magnitude of effects Study found confounding alone unlikely to explain association with large effect estimate as judged by reviewers. 0 Studies that reported positive associations of interactions reported effect estimates with low magnitudes.
Dose-response Consistent relationship between dose and response in one or multiple studies, and/or exposure response across studies 1 Exposure-response relationship was directionally consistent across 15 of the 34 studies in the category.
Confounding minimizes effect Upgraded if consideration of all plausible residual confounders or biases would underestimate the effect or suggest a spurious effect when results show no effect. 0 No evidence that residual confounders or biases would underestimate the effect or suggest a spurious effect when results show no effect
Overall Quality of Evidence Moderate The dose response relationships described in a number of studies did not warrant an upgrade for the overall quality rating.
Overall Strength of Evidence Sufficient An association was generally observed for synergistic effects of heat and air pollution exposure, specifically for ozone and PM, but the potentially high risk of bias from the air pollution exposure assessment methods in several studies could have impacted results.