Early exposure to pollens may increase risk for childhood asthma
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Key takeaways:
- Early exposure to weed pollen counteracts benefits of urban green spaces in instances of childhood asthma.
- Tree canopies may provide a protective effect.
Early exposure to tree and weed pollens in urban environments increases the risk for developing childhood asthma, according to a study published in the European Respiratory Journal.
“We know we want to increase urban planning in the cities with an emphasis on planting trees and increasing the amount of vegetation,” Éric Lavigne, MSc, PhD, research scientist at the Environmental Health Science and Research Bureau at Health Canada and one of the authors of the study, told Healio.
“But at the same time, people are developing allergies to pollen and so we wanted to evaluate whether green space could cause side effects, perhaps by increasing exposure to pollen,” Lavigne, who is also an adjunct professor in the University of Ottawa School of Epidemiology and Public Health, continued.
The study included 214,211 mother-child pairs, among whom 28,543 (13.3%) children were diagnosed with childhood asthma. The mean age of diagnosis was 22 months with asthma cases having a slightly higher prenatal average total pollen exposure vs. non-asthma cases.
Researchers used the Ontario ASTHMA cohort database to identify cases of childhood asthma between birth and age 6 years. Maternal environmental exposures to greenness were measured using the normalized difference vegetation index (NDVI), and tree canopy estimates were measured in a 250 m buffer around the central area of the postal code at birth.
Exposure to airborne pollen was measured using a land use regression model that captures exposure at a 1 km-by-1 km space during the course of pregnancy and infancy. A directed acyclic graph was used to identify variables for adjustment, including ambient air pollution exposure to particulate matter with aerodynamic diameter less than 2.5 µm, nitrogen dioxide, and ozone; maternal age at delivery; infant sex; parity; breastfeeding status; maternal smoking; maternal asthma; year of birth; season of birth; and socioeconomic status.
The association between pollen exposure, vegetation and development of childhood asthma was assessed with Cox proportional hazards models.
Maternal exposure to greenness per 0.08-unit increase on the NDVI was associated with an increased risk for childhood asthma (HR = 1.029; 95% CI, 1.008-1.035), with exposure to tree canopies providing a protective effect (HR = 0.976; 95% CI, 0.96-0.991).
Researchers also noted a positive association between prenatal weed pollen and childhood asthma onset (for an interquartile range of 8 grains per m3, HR = 1.021; 95% CI, 1.008-1.035). A window of susceptibility was found between the later part of the first trimester to the beginning of the third trimester for weekly weed pollen concentrations.
Exposure to pollen concentrations during the first year of life showed total pollen (HR = 1.023; 95% CI, 1.01-1.035) and tree pollen per 81 grains per m3 (HR = 1.004; 95% CI, 1.002-1.006) were associated with childhood asthma risk. Exposure during the first 3 years of life showed greater risk for asthma for total pollen (HR = 1.078; 95% CI, 1.059-1.098), with the effects being driven by weed pollen (HR = 1.076; 95% CI, 1.055-1.099).
The protective effect of the tree canopy disappeared when childhood weed pollen concentrations increased (first vs. third tertile of weed pollen concentration, HR = 0.942; 95% CI, 0.913-0.972 vs. HR = 1.004; 95% CI, 0.976-1.033; P = .01).
Researchers further stated that the impact of NDVI on the instance of childhood asthma was higher with the increase of exposure to weed pollen concentrations in childhood (first vs. third tertile of weed pollen concentration, HR = 0.998; 95% CI, 0.973-1.024 vs. HR = 1.053; 95% CI, 1.029–1.079; P = .01). The tree canopy protective effect was significant only for children born during the fall (HR = 0.949; 95% CI, 0.92-0.979).
“What’s surprising was finding that specifically when weed pollen was elevated, we found that the protective impact of the tree canopy could actually go away,” Lavigne told Healio. “So, that was a little surprising. We weren’t necessarily expecting that weed pollen would be the driver of the increased risk of asthma.”
Lavigne further called for the involvement of scientists and the incorporation of these kinds of study results in urban planning projects for cities aiming to increase their green space, specifically the consideration of weed pollen control programs.