Toxicological Risk Assessment of Urban Motorcycle Noise Using WHO Night Noise Guidelines

In densely populated residential areas, increased motorcycle use for delivery has become a major noise source, yet its health impacts remain underexplored. We quantified motorcycle-related noise exposure in Seoul, South Korea, through measurements at 11 sites, primarily residential alleyways and roads adjacent to commercial areas with high motorcycle activity and frequent complaints. Equivalent continuous sound levels (L_eq) were recorded over 24 h and classified into daytime (06:00-22:00) and nighttime (22:00-06:00). Motorcycle volumes were determined via direct field observations and manual review of synchronized acoustic recordings. Results were evaluated against the WHO Night Noise Guidelines, which specify the no observed adverse effect level (NOAEL) of 60 dB(A) for daytime and 50 dB(A) for nighttime. A strong positive correlation was observed between hourly motorcycle volumes and L_eq, particularly at night (r = 0.91, p = 0.002). In several commercial-adjacent sites, nighttime levels exceeded WHO thresholds. Logistic regression indicated elevated odds of threshold exceedance: 3.81 for nighttime and 5.52 for daytime. These results, interpreted within a WHO-based toxicological risk assessment framework, identify nighttime motorcycle noise as a harmful environmental exposure with potential impacts on sleep and cardiovascular health.