Particulate matter consists of a complex mixture of solid and liquid particles and is classified into different fractions according to size. PM2.5 stands for particulate matter with a maximum diametre of 2.5 micrometres (µm). Due to their small size these particles can penetrate into the small bronchi and bronchioles when inhaled. Particulate matter consists of a complex mixture of solid and liquid particles and is classified into different fractions according to size. PM2.5 stands for particulate matter with a maximum diametre of 2.5 micrometres (µm). Due to their small size these particles can penetrate into the small bronchi and bronchioles when inhaled. Particulate matter consists of a complex mixture of solid and liquid particles and is classified into different fractions according to size. PM2.5 stands for particulate matter with a maximum diametre of 2.5 micrometres (µm). Due to their small size these particles can penetrate into the small bronchi and bronchioles when inhaled.