People spend the majority of their time indoors, mostly in the domestic environment, where there may be significant air pollution sources. Indoor air pollutants can be derived from the ingress of contaminated air from outdoors (e.g., heavily trafficked roads, presence of industry, allergens) and from specific indoor sources such as combustion appliances, building materials, and consumer products. The activities and behaviour of individuals, for example, cooking, heating, cleaning, smoking, pets, can also influence the production and levels of indoor air pollutants. There is a body of literature concerning indoor concentrations of pollutants such as CO, NO2, formaldehyde, volatile organic compounds, house dust mites, fungi, bacteria and their health effects. However, there have been few, if any studies, conducted in the UK to date to compare and contrast indoor and outdoor particulate matter (PM). Epidemiological studies consistently suggest that increases in outdoor PMIO (particulate matter with an aerodynamic diameter of less than l0μm) concentrations are linked to health effects (morbidity and mortality in persons with cardiopulmonary diseases).