Table 4 Rating of the quality and strength of the evidence for studies assessing interactive effects between air pollution and pollen (n = 10)

Category

Summary of Criteria

Downgrades

Rationale

 Risk of Bias

Study limitations- a substantial risk of bias across body of evidence.

-1

Downgraded because of “high” or “probably high” risk of bias for air pollution exposure assessment for six studies and “high” or “probably high” risk of bias for pollen exposure assessment for six studies.

 Indirectness

Evidence was not directly comparable to the chosen population, exposure, comparator, and outcome.

0

Measured outcomes were assessed for humans in the populations for the duration of study periods, as outlined in the PECO statement.

 Inconsistency

Wide variability in estimates of effect in similar populations.

0

The studies were inconsistent in pollen types and air pollutants, precluding judgment as to whether reported effect estimates would be consistent or inconsistent.

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

Low

The overall quality of the evidence supporting interactive effects is low.

Limited

An association was shown in a few studies between air pollution and pollen and increased outcomes, however the results were inconsistent and there was a potentially high risk of bias from the exposure assessments in several studies.