When referring to air pollution in health statistics, emissions is the term used to describe the gases and particles that are released into the air or emitted by various sources, for example road transport. The concentration of a specific air pollutant is the amount of material per unit volume of air. Concentrations are most commonly expressed as mass per unit volume (for example, micrograms per cubic meter, µg/m3). Human exposure refers to any contact between an airborne contaminant and a surface of the human body, either outer, for example the skin, or inner, for example the respiratory tract. Health impacts are negative changes in health resulting from exposure to a source of pollution, such as exacerbation of asthma, increases in respiratory and cardiovascular hospital admissions and mortality.