Particle characteristics in indoor environments are strongly related to the characteristics ofoutdoor particles. The aim of this work was to investigate how submicrometre particle numberconcentration changes with the distance from a road. This information is necessary forchoosing the optimal ventilation and filtration systems of buildings, to better protect theoccupants.This work involved measurements of the total number concentration of submicrometreparticles from vehicle emissions, at increasing distances from a major road, using a TSIScanning Mobility Particle Sizer in the size range 0.015 0.697micrometer.The measurements conducted at distances from the road ranging from 15 to 375m showed,that for conditions where the wind is blowing directly from the road, the concentration of fineand ultrafine particles decays to around half of the maximum (measured at the closest point tothe road) at a distance of approximately 100- 150m from the road. For the wind blowingparallel to the road, the reduction to half of the concentration occurs at 50 100m. There is noeffect on total particle number concentration at a distance greater than 15m from the road,when the wind is blowing towards the road and away from the sampling points.