Associations between airborne pollen, pollen-related allergic rhinitis and blood pressure: A systematic review and meta-analysis

Allergic rhinitis (AR) due to pollen affects an estimated 18% of adults globally, but is often trivialized. While there is evidence for an association between high pollen exposure and cardiovascular morbidity and mortality, as well as multiple plausible mechanisms suggesting pathways via blood pressure (BP) elevation, no overview exists to date. This systematic review and meta-analysis aims to investigate whether 1) pollen-related AR and 2) exposure to airborne pollen are associated with BP and/or hypertension. We further evaluated which personal characteristics may modify a potential association and identified research gaps. We searched the PubMed, ScienceDirect, EMBASE, and Web of Science databases and systematically reviewed studies focusing on human populations published between 1 January 1990 and 20 September 2023 that presented data on the association between pollen-related AR or pollen exposure and BP or hypertension. We assessed the risk of bias with a customized version of the Risk of Bias tool by the Office of Health Assessment and Translation of the National Toxicology Program (OHAT), conducted random effects meta-analyses of comparable studies and assessed the certainty of evidence according to OHAT recommendations. We carried out sensitivity analyses, excluding studies with a high bias rating. We identified 24 studies, ranging in sample size from 41 to 9'548'939 participants. Of these, 23 had a cross-sectional design, and the remaining article included both cross-sectional and longitudinal analyses. Investigated exposures were binary pollen-related AR status (23 studies) and within and outside of pollen season (one study), but no study investigated measured pollen exposure (zero studies). Around half of the studies (13/24) were afflicted with high selection and confounding bias. Data from 22 studies using pollen-related AR status as exposure could be included in meta-analysis, to which eleven studies contributed hypertension data, four studies BP data, and seven studies both. In meta-analysis, we found an adjusted odds ratio of 1.16 (95 % confidence interval (CI): 0.91, 1.47) for hypertension and mean differences of 0.99 (95 % CI: -0.09, 2.07) and 0.40 (95 % CI: -0.41, 1.21) for adjusted systolic and diastolic BP, respectively. Results for crude pooled meta-estimates were similar, and none showed statistically significant differences. We rated the overall certainty in the evidence as very low, mainly due to cross-sectional designs, selection and confounding bias and indirectness of exposure. This review shows that there is insufficient evidence to decide whether pollen-related AR is associated with BP or hypertension to date, and therefore does not support a clear mediating role of BP in the relationship between pollen allergy and cardiovascular events. Studies that 1) investigate BP or hypertension as the main outcome and provide adjusted estimates to avoid selection and confounding bias, 2) define AR as solely triggered by pollen and 3) have longitudinal designs were not identified. The certainty of the evidence is therefore limited by (1) reliance on cross-sectional studies, which prevents conclusions about causality, (2) high risk of selection and confounding biases which could not be adjusted for, (3) heterogeneous and indirect definitions of pollen-related AR, and (4) an exclusively binary exposure assessment (with or without pollen-related AR), which limited the ability to study an exposure gradient. Given these limitations, further cross-sectional studies are unlikely to yield clearer results, whereas repeated measurement designs could offer more valuable insights. This review was primarily funded by the Swiss National Science Foundation (SNSF, grant No. 185864) and the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 853568). We pre-published the protocol and search strategy on the PROSPERO website (registration number CRD42022381907).