Evaluating associations between environmental and meteorological exposures and IPD risk using a case-crossover approach, we identified an inverse association between ambient levels of sulphur oxides and disease risk during the likely incubation period (Table 3) . No other significant associations between environmental exposures and risk were identified either during the incubation period, or in the period immediately prior to incubation. In particular, occurrence of IPD was not affected by daily changes in clear-sky UV index, temperature or coolingdegree days, in contrast to associations on longer time scales described above.