Definition of the study period. The study period is defined according to the availability of environmental and health data. Whenever possible, environmental and health data should be collected for the same period. If a common period does not exist, the lag between the collection period of environmental and health data should not exceed two years. The objective is to obtain data representative of the usual situations observed in the cities. The minimum study period should cover a whole calendar year. However a period of two to three consecutive years is advisable in order to smooth year-to-year variations in pollutant concentrations associated with meteorological conditions. Considering environmental data, it is advisable to favour periods when several monitoring stations are available for one pollutant. Doing so will allow calculating an average exposure across the city. Considering health data, it is advisable to select those years for which the coding methods for mortality or morbidity are identical. The study period should exclude years that have been associated with unusual health events that may interfere with the health baseline (e.g., the management of the H1N1 pandemics may have interfered with hospital admissions data). Similarly, years associated with unexpected meteorological episodes (e.g., heat waves), or unusual air pollution (e.g., forest fires lasting several weeks) should be excluded. In practice: - Select a common study period for environmental and health data. If not possible, the lag between the two periods should not exceed two years. - Favour two to three consecutive years with more than one air-pollution monitoring station available. - Check that no unusual health, environmental or meteorological events were observed during the study period. - Compare the meteorological characteristics of the study period with the usual climatic means.
