Diseases of the respiratory system are a leading cause of morbidity, mortality and disability worldwide. The lungs are constantly exposed to a myriad of noxious agents present in ambient air, such as particles, chemicals and infectious organisms. At least 2 billion people are exposed globally to the toxic smoke produced by combustion of biomass fuel, inefficiently burned in poorly ventilated indoor stoves or fireplaces used for cooking or warming. One billion people inhale polluted outdoor air, and another billion are exposed primarily or secondarily to tobacco smoke. As a consequence, respiratory disease is a major cause of morbidity, disability and death worldwide primarily affecting individuals of low socioeconomic status, who are exposed to crowding, environmental exposures and poor living conditions.