title: Causality and temporality in the study of short-term effects of air pollution on health.
abstract: BACKGROUND
  The relationship between air pollution and health is usually analysed by time series analysis. However, temporality (i.e. looking to see whether the cause precedes the effect) is rarely examined in detail.
  
  
  METHODS
  Models were fitted to daily levels of black smoke, nitrogen dioxide, sulphur dioxide, ozone and mortality from Barcelona 1986-1995 to account for seasonality, environmental temperature, days of the week and influenza epidemics. Cross-correlations of the residuals were plotted for different lags.
  
  
  RESULTS
  Clear evidence of a temporal relationship between mortality and air pollution found for all four pollutants in that changes in the pollutant preceded changes in mortality, implying causality. However the pattern of dependence was different for each pollutant.
  
  
  CONCLUSIONS
  The cross-correlation plot is a useful tool in the analysis of air pollution time series.
authors.affiliation: University of Sheffield, UK. m.j.campbell@sheffield.ac.uk
authors.forename: M J
authors.initials: MJ
authors.lastname: Campbell
authors.forename: A
authors.initials: A
authors.lastname: Tobías
mesh_terms: D000397:Air Pollution / Q000009:adverse effects*; D015984:Causality; D003430:Cross-Sectional Studies; D006262:Health; D006801:Humans; D015233:Models, Statistical; D009026:Mortality; D012189:Retrospective Studies; D012621:Seasons; D013030:Spain / Q000453:epidemiology; D013696:Temperature; D013997:Time Factors