Significant Associations between Meteorological and Air Quality Parameters and the Daily Number of Emergency Calls because of Breathing Difficulties in Graz (Austria) – A Time Series Analysis from 2001 to 2009

Introduction

On the one hand, meteorological conditions influence the human body directly, but on the other hand, they have an indirect effect, because sources and spatial distribution of air pollutants in the atmosphere depend on those conditions. A great number of studies documented an association between different environmental factors and the daily number of emergency calls due to a variety of diseases.

In this study we analyzed the pattern of the relations between the environmental factors and the daily frequency of emergency ambulance calls in Graz (Austria) during the period from January 2001 to December 2009. The analysis was performed for emergency calls with focus on breathing difficulties. In addition to the identification of environmental predictors the aim of the study was to assess whether the relationship was linear or nonlinear.

Data and Methods

To describe the morbidity of patients with breathing difficulties, we analyzed the correspondent cases of emergencies (provided by the Medical University of Graz, Austria, G. Prause) which were diagnosed by the emergency physician. Diseases were coded according to the international classification of diseases (10th version; ICD10). For data analysis we used the disease considered as the subject's major disorder which was coded as first or second diagnose and included all cases of asthma, chronic obstructive pulmonary disease (COPD), pneumonia, bronchitis, acute spasmodic laryngitis, epiglottitis and other reasons of dyspnea.

The environmental data were provided by the meteorological measurement station of the University of Graz (IGAM, E. Putz), Austria. Daily mean values of temperature, air pressure and relative humidity as well as particulate matter (PM10), ozone, nitrogen monoxide and dioxide were used, either as values at the day of emergency call and with time lags of up to three days for meteorological parameters and up to seven days for air pollutants. All data sets were available for January 2001 to December 2009.

To account for the expected large influence of anthropogenic and social factors, a number of formal variables such as the day of the week, public and school holidays, and season were included as covariates.

After descriptive analyses two different kinds of variable selection procedures were performed to select variables with statistically significant relationship to emergency calls due to breathing difficulties. Using generalized additive models (GAMs) as well as distributed-lag nonlinear models it was checked, whether these variables had a linear or nonlinear impact on the emergency frequency in the observation period. All statistical analyses were performed in R (version 2.8.0) using different packages.

Results

In total, about 14,300 calls to the emergency ambulance were available of which only 520 were due to breathing difficulties diagnosed by an emergency physician.

The variable selections and the regression analyses identified relative humidity, PM10 and nitrogen dioxide as significant predictors for the daily number of emergency calls because of airway diseases.

Not only the values of these parameters of the actual day had a significant association to the selected emergency calls but also different time lags: relative humidity 2 days before, 5-day lagged values of PM10 and 3, 6 and 7 days before the actual day of nitrogen dioxide.

The results of the GAM indicated that only 2-day lagged values of relative humidity and values of three days before the actual day of nitrogen dioxide had a linear relation to the outcome variable. The other significant predictors had a nonlinear relation to the emergency calls because of airway diseases. Thus, the associations with these environmental predictors had to be described by functions involving more than two parameters.

Conclusion

The results indicate that several environmental parameters had an impact on the daily number of emergency calls due to breathing difficulties in Graz (Austria) as derived from a data set covering the period from 2001 to 2009. The effects of the meteorological predictors were considerably smaller than those of air pollutants. The results of this study are in accordance to other studies which analyzed hospitalizations instead of emergency calls.