Exploring the interaction between ambient air pollution and road traffic noise on stroke incidence in ten Nordic cohorts

Ambient air pollution and road traffic noise are stroke risk factors, but evidence on their potential joint effects remains limited. This study investigated the independent and joint associations of air pollution and road traffic noise on stroke incidence using both multiplicative and additive scales.

We followed stroke incidence in ten cohorts in Sweden, Denmark, and Finland. We modelled annual average levels of outdoor particulate matter < 2.5 µm (PM_2.5), nitrogen dioxide (NO₂) and road traffic noise at residential addresses. We applied Cox proportional hazards regression to evaluate their single association. We assessed multiplicative interaction with interaction terms in Cox models and additive interaction using the Relative Excess Risk due to Interaction method.

We followed 136,897 adults for 20 years, and 8.0 % experienced stroke incidence. PM_2.5, NO₂ and road traffic noise were associated with higher stroke risk in single-exposure models. Multiplicative models showed higher HRs between PM_2.5 and stroke at higher levels of noise and vice versa: HRs per 5 μg/m³ of PM_2.5 were 1.06 (95 % CI:0.94-1.21) at 40 dB and 1.11 (95 % CI:0.85-1.44) at 80 dB of road traffic noise; HRs per 12 dB of road traffic noise were 1.06 (95 % CI:1.01-1.11) at 4 μg/m³ and 1.17 (95 % CI:0.82-1.68) at 48 μg/m³ of PM_2.5. Additive models showed that the combined association of PM_2.5 and road traffic noise was 4 % (RERI = 0.04 (95 % CI:-0.05;1.12)) higher than the sum of their individual association.

PM_2.5 and road traffic noise showed a non-significant synergistic association on stroke incidence.