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The environmental health question
Does exposure to traffic-related air pollution cause children to develop asthma, or does it only trigger symptoms in children who already have it? The answer matters for where schools and housing are built and for how clinicians counsel families.
The evidence
Khreis et al. (2017) systematically reviewed epidemiological studies of children's exposure to traffic-related air pollution and the risk of developing asthma from birth to age 18. Forty-one studies met their criteria. Where at least four independent estimates existed for a pollutant, they pooled the results:
| Pollutant | Increment | Pooled risk estimate (95% CI) |
|---|---|---|
| Black carbon | 0.5 × 10⁻⁵ m⁻¹ | 1.08 (1.03–1.14) |
| Nitrogen dioxide (NO₂) | 4 μg/m³ | 1.05 (1.02–1.07) |
| Fine particulate matter (PM₂.₅) | 1 μg/m³ | 1.03 (1.01–1.05) |
| PM₁₀ | 2 μg/m³ | 1.05 (1.02–1.08) |
| Nitrogen oxides (NOx) | 30 μg/m³ | 1.48 (0.89–2.45), not significant |
Sensitivity analyses supported the findings. The authors concluded that the results support the hypothesis that childhood exposure to traffic-related air pollution contributes to the development of asthma.
Epidemiological appraisal
Strength of association. The individual risk estimates are small, but they apply to small increments of exposure, and exposure is widespread, so the population effect could be substantial.
Consistency. Associations were significant for four of five pollutants, with the least heterogeneity for black carbon and the most for NO₂ and NOx.
Exposure assessment. Studies estimated exposure in different ways, often from modeled pollution at home addresses, which can misclassify a child's true exposure at school or in transit. Nondifferential misclassification usually biases results toward no effect.
Confounding. Families living near busy roads may differ in income, housing quality, smoking and access to care. Studies adjusted for different confounders, and the authors called for harmonized confounder adjustment in future research.
Outcome definition. Definitions of asthma varied, from physician diagnosis to parent-reported wheeze, which adds heterogeneity.
Taken together, the evidence is consistent with a causal contribution, but not conclusive.
A population health plan
An elementary school in this plan sits 150 meters from an interstate highway, and the local clinic has noticed many asthma visits among its students. The school is illustrative.
- Surveillance: work with the school nurse to track asthma diagnoses, action plans and absences, and map cases against distance from major roads.
- Reduce exposure: advocate for high-efficiency air filtration in classrooms, moving outdoor recess and athletics to the side away from the highway, and anti-idling rules for buses and pickup lines.
- Clinical care: make sure every student with asthma has an action plan and controller medication when indicated.
- Policy: comment on local zoning so that new schools, child care centers and housing aren't sited next to major roads.
Conclusion
Traffic-related air pollution is a modifiable environmental exposure with growing evidence of a role in childhood asthma. DNP-prepared nurses can combine epidemiological appraisal with local action to reduce children's exposure.
Reference
Khreis, H., Kelly, C., Tate, J., Parslow, R., Lucas, K., & Nieuwenhuijsen, M. (2017). Exposure to traffic-related air pollution and risk of development of childhood asthma: A systematic review and meta-analysis. Environment International, 100, 1–31. https://doi.org/10.1016/j.envint.2016.11.012
