Children Exposed to Weed and Tree Pollen in Urban Areas May Experience Increased Risk of Asthma Onset


The study results also found that exposure to tree canopy area may protect children from asthma development; however, efficacy decreased as weed pollen exposure increased.

According to research published in European Respiratory Journal, children who are exposed to tree and weed pollen in urban areas are at an increased risk of respiratory health problems, including asthma. The authors note that green areas in urban settings decrease exposure to air pollution, allow kids to be active, and they also offer positive contact to diverse microbiota in children’s bodies, which may help the positive development of a child’s immune system. In turn, this could also lead to the development of childhood asthma.1

Tree pollen increasing asthma risk -- Image credit: Ingo Bartussek |

Image credit: Ingo Bartussek |

For this retrospective cohort study, investigators assessed early life exposure to urban vegetation and asthma incidence and whether modifications could be made through prenatal and early life exposure to airborne pollen. The authors linked mother and infant pairs using data from the Better Outcomes Registry and Network (BORN) Ontario, and exposure data were linked to the study cohort and assessed by the Institute of Clinical Evaluative Science (ICES). Additionally, the Ontario ASTHMA cohort database was utilized to identify incident cases of childhood asthma between birth and 6 years of age.2

Further, environmental exposures were gathered and linked to the BORN dataset using maternal postal code during pregnancy and following childbirth to gather residential mobility. Additionally, maternal exposure to residential greenness was assigned through a satellite-derived normalized difference vegetation index as well as using the proportion of tree canopy area—or crown area. Residential exposure to airborne pollen was assigned to different types (eg, total pollen, trees, grass, and weeds).2

Among the total 214,211 mother-child pairs enrolled in the study, 28,543 children (13.3%) with a mean age of 22 months were diagnosed with childhood asthma. The asthma cases had slightly higher prenatal average total pollen exposure to their non-asthma counterparts. The authors observed that normalized difference vegetation index (NDVI) exposure (per 0.08-unit increase) was associated with an increased risk of childhood asthma (HR 1.029, 95% CI 1.008–1.035), and there was a protective effect for the impact of increased tree canopy area on asthma incidence (HR 0.976, 95% CI 0.960–0.991). Additionally, a positive association between prenatal weed pollen and childhood asthma onset was found (IQR of 8 grains per m3: HR 1.021, 95% CI 1.008–1.035).2

Further, childhood exposures during the first 3 years of life demonstrated a greater magnitude in risks of childhood asthma for total pollen (HR 1.078, 95% CI 1.059–1.098); however, effects for total pollen appeared to be driven by weed pollen (HR 1.076, 95% CI 1.055–1.099) during the first 3 years of life.2

In addition, the protective effect of tree canopy area exposure disappeared when weed pollen concentration during childhood increased (first tertile of weed pollen concentration: HR 0.942, 95% CI 0.913–0.972, third tertile: HR 1.004, 95% CI 0.976–1.033; p = .01). Similarly, impact of NDVI on childhood asthma development was higher with increasing weed pollen concentration during childhood (first tertile: HR 0.998, 95% CI 0.973–1.024; third tertile: HR 1.053, 95% CI 1.029–1.079; p = .01). The authors also observed that the tree canopy area’s protective effects were significant for births that occurred during fall months (HR 0.949, 95% CI 0.920–0.979), and no differences in the effects of pollen types and vegetation measures on asthma incidence according to difference levels of socioeconomic statuses were found.2

“We found that tree canopy offers some protective effects towards the development of asthma in children. However, this protective effect goes away when there is increased exposure to weed pollen,” says lead author Éric Lavigne, adjunct professor, School of Epidemiology and Public Health, Faculty of Medicine at University of Ottawa, senior epidemiologist at Health Canada, in a news release.1

The authors also note that this can be significant in understanding the effect trees in urban areas can have. Additionally, any potential consequences of pollen on respiratory health, notably children’s, should be understand in the chance that pollen is the cause of an increased risk of asthma onset.1

“This could have an impact for urban planners and public health officials that work on redesigning our urban centers,” noted Lavigne in the news release. “Along with weed pollen control programs and deciding when to plan specific trees, it will be important to consider tree species that minimize allergic pollen in the air.”1


1. University of Ottawa. Early life exposure to weed pollen could increase childhood asthma risk: study. News release. June 11, 2024. Accessed June 12, 2024.
2. Stanescu, C, Talarico, R, Weichenthal, S, Villeneuve, PJ, Smargiassi, A, Stieb, DM, et al. Early life exposure to pollens and increased risks of childhood asthma: a prospective cohort study in Ontario children. European Respiratory Journal Apr 2024, 63(4)2301568 doi:10.1183/13993003.01568-2023
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