Which metric of ambient ozone to predict daily mortality?
Highlights
► We performed a time-series analysis over 19 years of ozone and daily mortality. ► Within-day variability of ozone was the best predictors of same day mortality. ► Previous day nitrogen dioxide did not confound the results.
Introduction
The adverse health effects of the gaseous air pollutant ozone are well established (WHO, 2006, Bates, 2005), but the magnitude of the effect of an incremental change in ozone concentration is still controversial (Smith et al., 2009). Goodman (2005) pointed out that the choice of different averaging times in the studies of Bell et al., 2005, Levy et al., 2005, and Ito et al. (2005) might partly explain the differences in effect estimates. But other methodological differences in the three papers preclude firm conclusions as to the impact of the choice of different ozone metrics. Also Fann et al. (2011) underline the concern that the optimal ozone metric still has to be determined.
We have previously reported an increase in daily mortality in Vienna, Austria, due to particulate air pollution in a time series analysis 1999–2004 (Neuberger et al., 2007). For the sake of comparability the methodology of this study followed that of the APHEA project (Samoli et al., 2003) which performed time series analyses in a range of European cities. In fact, the Viennese results were broadly in accordance with those from APHEA and similar studies: The increase of (all causes) daily deaths per 10 μg m−3 increase of PM10 (average of same and previous day) amounted to 0.2% (CI = −0.4; 0.7) and was higher for specific causes of death and when longer lags (up to 14 days) were considered. Daily maximum ozone 8 h moving average was analysed as well for the same period but no consistent effect was found even when the analysis was restricted to the summer seasons. This null finding for ozone effects is not uncommon in the Austrian situation where ozone both temporally and spatially is negatively correlated with the indicators of traffic related air contaminations PM10, CO, NO, and NO2. Under equilibrium conditions between NOX and ozone and no other precursor substances there is no net production of ozone and during the day with enough solar energy to fuel the reaction positively its concentration is correlated with NOX. However, in the presence of precursors like CO, methane or other carbohydrates there is an additional production of ozone that leads to an increase of ozone while NO2 is decreasing due to photo dissociation or remains stable. NO2 proved to be a good predictor of health effects in several studies in Austria (Neuberger et al., 2007, Neuberger et al., 2002, Moshammer et al., 2006).
Ozone is an irritant gas with potent oxidative capacity. It also serves as an indicator of a reactive mixture of secondary pollutants that includes also very short-lived radicals that are not easily directly monitored. The association between ozone and these radicals is not constant. In relatively clean air ozone formed under the influence of UV radiation is quite stable and only few radicals are generated while in more polluted air the generation and consumption of ozone is accelerated and thus more radicals are formed (Reid et al., 1996, Solomon et al., 2000).
Therefore, this study sets out to test the hypothesis that daily peak levels of ozone or even the difference between night time low and day time high ozone are better predictors of health effects than daily mean values or even maximal 8 h moving averages.
In fact the idea of that study resulted from a meeting of a working group of the Austrian Academy of Sciences that was charged with the drafting of a guideline document regarding air quality requirements for Austrian spas. In spas health-seeking people also expect good air quality. But in some well-known alpine mountain spas daily average ozone levels are sometimes rather high during summer month in spite or even because of otherwise very clean air. It was hypothesised that higher average ozone levels (measured as daily mean levels) with less daily variation are not as dangerous as the same average levels but with higher daily fluctuations.
Section snippets
Material and methods
Mortality data for the years 1991–2009 were provided by Statistics Austria. For each death in the database the following data were extracted: Day of death, age of deceased person (in completed years of life), sex, district of home address, primary cause of death: before 2002 according to ICD9 (WHO, 1977), starting with 2002 according to ICD10 (WHO, 1992). Analysis was restricted to all cases with a home address district in Vienna, the capital city of Austria with approx. 1.5 million inhabitants
Results
The monitoring station ZA performed best of all 4 stations for all metrics except for SO2 (Table 1). For the sake of consistency also for SO2 the data from ZA were used because the correlation coefficients were only slightly lower at ZA (range: 0.86–0.93) compared to the data from STE (r range: 0.89–0.93). For all metrics but one the correlation coefficients between the 4 stations were very high. Only NO2 displayed a somewhat stronger spatial heterogeneity.
Daily overall mortality in Vienna
Discussion
In this time series over nearly 19 years O3 and NO2 were adversely associated with daily mortality. The effect estimates were comparable with previous findings (Goldberg et al., 2001, Gryparis et al., 2004, Anderson et al., 2004, Bell et al., 2004, Bell et al., 2005, Bell et al., 2006, Levy et al., 2005, Ito et al., 2005, Parodi et al., 2005, Zhang et al., 2006, Zanobetti and Schwartz, 2008, Garrett and Casimiro, 2011). Table 3, Table 4 provide effect estimates per 10 μg m−3. For a 10 μg m−3
Conclusion
The results of this analysis indicate that the daily variability of ozone or the daily maximal ozone concentration are better predictors of short term health effects than daily mean values. Legally defined health based limit values for ozone (e.g. European Air Quality Directive: EP, 2008) are often based on 8-h moving average data. Should the findings of this investigation be confirmed for other regions these recommendations may have to be reconsidered.
Disclosure statement
The authors declare no conflict of interest. The data for this study were obtained from routine data sources. Therefore no financial support was needed and no ethical considerations regarding clinical studies were applicable.
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