They refer to the observation of events which develop among human population under natural conditions, circumstances which form their most significant advantage. When measuring exposure to air pollution, many studies have used data from air pollution surveillance networks, but other approaches to asses exposure are increasing, ranging from consideration of residence as an approximation to the exposure level to pollution or exposure questionnaires, to the use of personal samplers or the determination of biomarkers (Ozkaynak, 1999). This Figure shows the main exposure measuring methods used in the field of epidemiology. A rise in the grade of sophistication makes measurements more valid, but it also affects their cost.