Particulate matter present in air is divided into different categories depending on the size of the particles (aerodynamic diameter). Fine particles are airborne particles which are smaller than coarse particles. They have an aerodynamic diameter of 2.5 µm or less (PM2.5). The fine particles which are smaller than 0.1 µm are referred to as ultrafine particles (PM0.1). * Fine particles are largely formed from gases. * Ultrafine particles are formed by nucleation, which is the initial stage in which gas becomes a particle. These particles can grow up to a size of 1µm either through condensation, when additional gas condensates on the particles, or through coagulation, when two or more particles combine to form a larger particle. Please note that ultrafine particles (PM0.1) are part of the fine fraction (PM2.5).