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Particle number concentrations and size distributions resulting from the firework displays held in Budapest, Hungary every year on St. Stephen's Day were studied over a period of seven years. In the year most impacted, the total particle number concentration reached its peak measured level of 369 × 103 cm−3 5 min after the end of the display, and returned to the pre-event state within 45 min. The fireworks increased hourly mean concentrations by a factor of 5–6, whereas the concentrations in the diameter range of 100–1000 nm, in which the magnitude of the increase was the greatest, were elevated by a factor of 20–25. An extra particle size mode at 203 nm was manifested in the size distributions as result of the fireworks. The PM 10 mass concentrations at peak firework influence and as 1-h mean increased 123 and 58 times, respectively, relative to the concentration before the display. The smoke was characterised by a relatively short overall atmospheric residence time of 25 min. Spatiotemporal dispersion simulations revealed that there were substantial vertical and horizontal concentration gradients in the firework plume. The affected area made up a large part of the city. Not only the spectators of the display at the venue and nearby areas, but the population located further away downwind of the displays and more distant, large and populous urban quarters were affected by the plume and its fallout. The fireworks increased the deposition rate in the respiratory system of females by a factor of 4, as a conservative estimate. The largest surface density deposition rates were seen in the segmental and sub-segmental bronchi, which represents an excessive risk to health. Compared to adults, children were more susceptible to exposure, with the maximum surface density deposition rates in their case being three times those of adults in the trachea. [Display omitted] • Concentrations during fireworks increase drastically with respect to those before them. • Median diameter and residence time of firework particles are 203 nm and 25 min. • Fallout from smoke plume affects many inhabitants and large urban quarters. • Children are more susceptible to the firework smoke exposure than adults. [ABSTRACT FROM AUTHOR]