Introduction Different metrics of daily levels of particulate matter were used to study the association between air pollution and primary health care visits of asthma sufferers in an area with air pollution levels well within the standard air quality guidelines.
Methods Hourly readings of ambient particulate matter (PM10, PM2.5), nitrogen dioxide (NO2), nitrogen oxide (NO), ozone (O3) and sulphur dioxide (SO2) from the Swedish Meteorological and Hydrological Institute (SMHI) from 2005 to 2010 were used. Six different metrics were created from these hourly readings: (1) daily 1-h maximum; (2) 24-h average; (3) commuting-period; (4) daytime average; (5) night-time average; and (6) the daily 8-h maximum. Outcome data were obtained from a regional health care database, covering approximately half a million people living in Malmö and neighbouring municipalities. Poisson generalised linear models were used to examine the relations between daily primary health care (PHC) visits due to asthma and air pollution metrics with different time lags.
Results Air pollutant levels throughout the study period remained well within the WHO air quality guidelines (PM10 daily mean of 16.4 μg/m3). A consistent relation between daily PM levels and PHC visits due to asthma was observed for all metrics. An increased risk of 11% was found with every 10-unit increase in daily mean levels of PM10. Commuting-period and daytime concentrations of PM10 were also associated with increased risks, of 10% and 9%, respectively. The increased risk was moderate for the 8-hour (8%) and night-time (7%) metrics. The lowest risk was observed for the 1-hour maximum value (3%).
Conclusions The results suggest that air pollution has adverse effects on respiratory health, even at very low concentrations. A significant variation in risk was observed during the day depending on the metric used. The results of this study highlight the need to re-assess air quality guidelines.
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