The site where particulates are absorbed into your body is a function of particle size. Larger particles are generally filtered by small hairs in the nose and throat and do not cause problems, but particulate matter smaller than about 10 micrometres, referred to as PM10, can settle in the bronchies and lungs and cause health problems. Particles smaller than 2.5 micrometres, PM2.5, can penetrate directly into the lungs, whereas particles smaller than 1 micrometer PM1 can penetrate into the alveolar region of the lung and tend to be the most hazardous when inhaled. Particles emitted from modern diesel engines are typically in the size range of 100 nanometres (0.1 micrometres), cannot be filtered by particulate traps and thus are deposited preferentially in the alveolar region. In addition, these soot particles also carry carcinogenic components like benzopyrenes adsorbed on their surface. In view of these deposition mechanisms, it is becoming increasingly clear that the legislative limits for engines, which are in terms of emitted mass, are not a proper measure of the health hazard.