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PM levels in Sydney are low compared with most cities where time series studies of the acute effects of air pollution have been conducted. The 50th and 90th percentiles of daily average PM10 (particulate matter £ 10mm) in Sydney are 16 and 26 mg/m3 respectively, and 8 and 14 mg/m3 for PM2.5 (particulate matter £ 2.5mm). Time series studies in Sydney in the early 1990's demonstrated associations between particles (measured by light scatter using integrating nephelometers), nitrogen dioxide and ozone with daily mortality and hospital admissions. Light scatter (BSP) is considered to be a proxy measure for PM2.5. From 1994 air pollution data in Sydney is available from a larger number of monitoring sites and includes daily data on PM10 (measured by TEOM). Daily PM2.5 data (measured by TEOM) is available from 1997.We investigate associations between the three ambient particulate measures (BSP, PM2.5 and PM10) and the gaseous pollutants (nitrogen dioxide and ozone) with daily mortality and hospital admissions for all ages and the elderly (65+years) in the Sydney metropolitan area from 1994 to 2000 using time series analysis controlling for a range of confounders. We conducted sensitivity analyses to investigate the effect of different modeling approaches including the use of various methods for smoothing long term and seasonal trends, and weather parameters (penalized splines compared with loess and natural splines).All three particulate measures (PM2.5, PM10 and BSP) were associated with all cause mortality, all cardiovascular mortality and all respiratory mortality. The magnitude of the PM2.5 effects were generally larger than those of BSP and PM10. Nitrogen dioxide was also associated with all cause mortality, all cardiovascular mortality and all respiratory mortality. Ozone was weakly associated with respiratory mortality. All three particulate measures were associated with all cardiac, IHD (ischaemic heart disease) and respiratory admissions, and weakly associated with COPD (chronic obstructive pulmonary disease). The magnitude of the effect for PM2.5 and BSP were generally larger than those for PM10. Nitrogen dioxide was also associated with all cardiac, IHD, all respiratory and COPD hosptial admissions.The relatively low levels of particulate air pollution in Sydney were consistently associated with both daily mortality and hospital admissions. These particulate associations were generally strongest for fine particles (ie: PM2.5 and BSP) compared with PM10, and persist even at the relatively low particulate levels seen in Sydney, indicating no threshold concentrations are present. These results are consistent with the international literature. Nitrogen dioxide is also consistently associated with both daily mortality and hospital admissions. Further investigation is required to determine the effects of these specific pollutants, compared to the air pollution mix in Sydney.