The modifying effect of pollen on the association between particulate matter and respiratory mortality: a multi-city analysis in Kyushu, Japan

BACKGROUND AND AIM: Pollen, a biogenic pollutant of growing concern due to climbing global temperatures, exacerbates the relationship between particulate matter and allergenicity-related adverse health outcomes. This study investigated the modifying effect that pollen may have on the short-term association between respiratory mortality (total and older adults aged ≥65) and suspended particulate matter (SPM), the volume of aerodynamic particles with a 100% cutoff at 10μm in terms of diameter, in eight Kyushu cities from 1989 to 2014 during spring months (February to April). METHODS: A two-stage time-series regression analysis was conducted. First, we used a quasi-Poisson generalized linear model incorporating an interaction term between pollen and particulate matter and adjusted for long-term time trend, day-of-season, day-of-week, holidays, and mean temperature for each city. Pollen days were categorized into quartiles by distribution. City-specific estimates were combined with meta-regression analysis. RESULTS:In total, 43,557 respiratory deaths were included in this study. A majority of total mortality was attributable to the older adults age group. Mortality risks increased by 2.9% (95% CI: 0.5%–5.3%) for total and 2.7% (95% CI: 0.2%–5.1%) for older adults per interquartile range increase in SPM on the previous day (lag1) at the highest pollen quartile. In contrast, risks were estimated toward the null at the lowest pollen quartile for both total (0.3%, 95% CI: -2.6%–3.3%) and older adults (0.1%, 95% CI: -2.7%–2.9%) mortality. Despite the potential differences between the lowest and highest pollen quartiles, no definitive evidence of an interaction was gleaned. CONCLUSIONS:While no decisive evidence of a modifying effect of pollen on the relationship between air pollution and mortality exists, we observed some pattern of an exacerbating impact between SPM and pollen. Further research, preferably in heavily populated cities with high pollen and air pollutant concentrations, is required to confirm these conclusions. KEYWORDS: air pollution, particulate matter, pollen, mortality, elderly mortality, respiratory mortality