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Table 2 Rating of the quality and strength of the evidence for studies assessing interactive effects between heat, air pollution, and pollen (n = 6)

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 because of “probably high” risk of bias for air pollution exposure assessment for four studies and for pollen exposure assessment for five studies.
 Indirectness Evidence was not directly comparable to the chosen population, exposure, comparator, and outcome. 0 Measured outcomes were assessed for humans in populations for the duration of study periods, as outlined in the PECO statement.
 Inconsistency Wide variability in estimates of effect in similar populations. 0 Some evidence of consistent effects, but the studies were too varied in definitions of risk factors and methods to judge consistency in effect estimates.
 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. 0 Studies did not report a consistent relationship between dose and response.
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 Low The overall quality of the evidence supporting interactive effects is low.
Overall Strength of Evidence Limited An association was sometimes observed for synergy between heat, air pollution, and pollen, but the potentially high risk of bias for air pollution exposure could have impacted results and there is a lack of consistently significant findings.