Weather-Driven Toxic Vapour Cloud Dispersion from an Accidental Railway Chemical Release: A Human Exposure Risk Case Study

Accidents involving hazardous materials during transport are rare but can have severe consequences for human health. This study evaluates accidental railway chemical release scenarios along the Leixões railway line in Matosinhos, Portugal, to assess the influence of diurnal and seasonal meteorological variations on toxic vapour cloud extent and population exposure. Daytime and nighttime, as well as seasonal, scenarios were developed using historical weather data, and a modelling framework integrating the ALOHA® dispersion software with geographic information system (GIS) tools quantified potential human impacts. Nighttime plumes with low wind speeds of 2.87 m/s and high atmospheric stability were wider and more elongated, particularly in low-risk zones and in confidence areas, resulting in higher population exposure. Daytime plumes with stronger winds of 4.24 m/s and enhanced vertical mixing were more compact and affected fewer individuals. Seasonal patterns further influenced dispersion, with winter and autumn producing moderately elongated plumes with substantial exposure, spring yielding the smallest footprints, and summer generating the largest horizontal plumes, especially in low-risk zones, leading to the highest number of exposed people. These findings highlight the importance of meteorological variability and site-specific conditions in risk assessment, emergency planning, and scheduling of hazardous material transport to minimise population exposure.