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Saharan dust and association between particulate matter and case-specific mortality: a case-crossover analysis in Madrid (Spain)

Saharan dust and association between particulate matter and case-specific mortality: a... Background: Saharan dust intrusions are a common phenomenon in the Madrid atmosphere, leading induce exceedances of the 50 μg/m - EU 24 h standard for PM . Methods: We investigated the effects of exposure to PM between January 2003 and December 2005 in Madrid (Spain) on daily case-specific mortality; changes of effects between Saharan and non-Saharan dust days were assessed using a time-stratified case-crossover design. Results: Saharan dust affected 20% of days in the city of Madrid. Mean concentration of PM was higher during 3 3 3 dust days (47.7 μg/m ) than non-dust days (31.4 μg/m ). The rise of mortality per 10 μg/m PM concentration were always largely for Saharan dust-days. When stratifying by season risks of PM , at lag 1, during Saharan dust days were stronger for respiratory causes during cold season (IR% = 3.34% (95% CI: 0.36, 6.41) versus 2.87% (95% CI: 1.30, 4.47)) while for circulatory causes effects were stronger during warm season (IR% = 4.19% (95% CI: 1.34, 7.13) versus 2.65% (95% CI: 0.12, 5.23)). No effects were found for cerebrovascular causes. Conclusions: We found evidence of strongest effects of particulate matter during Saharan dust days, providing a suggestion of effect modification, even though interaction terms were not statistically significant. Further investigation is needed to understand the mechanism by which Saharan dust increases mortality. Background Among the diseases that would be affected by dust The increased levels of particulate matter in a given intrusions are those related to exacerbations of respira- geographical area are mainly influenced by the intrusion tory diseases, especially paediatric asthma [6-8]. of natural origin such as those related to the advection Recently, this subject is rising from studies conducted of dust from the desert [1]. Obviously, the areas which in Southern Europe, although they have not given con- are closer to the great deserts are the most affected by sistent results. In Spain, a study conducted in Barcelona this type of events. In the case of the Saharan desert, [9] reported strongest effects of coarse particles (PM 10- Southern European countries are largely influenced by ) on total daily mortality during Saharan dust intru- 2.5 dust intrusions [2]. Dust particles can also be trans- sions. In Italy, a study conducted in Rome [10] also ported over long distances by atmospheric circulation, found strongest effects of PM and coarse particles on reaching the Near East end the Americas [3]. daily mortality for cardiovascular causes, although others Saharan dust events can contribute to exceedances of in Emilia-Romagna region [11] did not found an PM daily European Union Limit of 50 μg/m [2,4]. increase of risk of total and case-specific mortality due Moreover, these particles can carry biological material to PM during Saharan dust days. Lastly, studies in that makes them potentially harmful to health [5]. Athens did not found effect modification of PM on total and case-specific mortality [12], although they did found an increase of risk on paediatric asthma exacerba- * Correspondence: [email protected] Institute of Environmental Assessment and Water Research (IDAEA), Spanish tion [8]. Council for Scientific Research (CSIC). C/JordiGirona, 18-26. 08034 Barcelona, The city of Madrid has high levels of pollution of Spain anthropogenic origin, which are strongly increased those Full list of author information is available at the end of the article © 2012 Díaz et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Díaz et al. Environmental Health 2012, 11:11 Page 2 of 6 http://www.ehjournal.net/content/11/1/11 days with Saharan dust intrusions, and its geographical Daily mean temperature and relative humidity regis- and meteorological conditions that did not facilitate the teredatthe Madrid-Retiro Observatory,situatedatcity dispersion of pollutants. The aim of this study is to eval- centre, were recorded by the National Meteorology uate the effect of Saharan dust on the association Agency (Agencia Nacional de Meteorología). between particulate matter and daily case-specific mor- tality in Madrid. Design and statistical analysis The association of daily concentrations of PM with Methods daily mortality was investigated using a case-crossover Setting design [13]. This uses the day on which the outcome of The city of Madrid constitutes a dense metropolitan interest (mortality) occurs as a case day. Exposure on area located on the central region of Spain. The main case days is compared with exposure on days on which emission source of atmospheric pollutants is road traffic. the outcome of interest does not occur (control days) Its geographical setting has climatological conditions [14]. A time-stratified approach used to represent expo- directly linked to emission levels, with frequent anticy- sure on control days from the same day of the week, clonic situations in summer and winter, which impede month, and year as case days, minimising bias from the dispersion of pollutants. Saharan dust events on the time trends in the exposure series and from other short- central region of Spain generally occur 50 to 100 days term time-varying confounders [14]. per year, mainly between spring and autumn, mostly We fitted a basic Poisson regression model to the caused by an atmospheric depression West or Southwest daily mortality data that included potential confounders: of Portugal or by anticyclonic conditions over Algeria weather, influenza, epidemics and time trends and sea- inducing Southern winds over the Iberian Peninsula [1]. sonality. A natural cubic spline with 3 degrees of free- dom was built to adjust for the potential confounding of Data mean temperature (4-day average) and a linear term to Daily mortality was obtained from the Madrid Regional adjust for relative humidity (4-day average). We also Inland Revenue Department (Consejería de Hacienda), controlled for gaseous pollutants and O , by means of a which is the department responsible for mortality regis- 4-day average linear term, and for public holidays and try, from 1st January 2003 to 31st December 2005. The influenza epidemics by means of categorical variables. outcomes of interest were organic causes except acci- Finally, we included a three-way interaction term dents (ICD-10: A00-R99), respiratory causes (ICD-10: between day of the week, month, and year to control for J00-J99), circulatory causes (ICD-10: I00-I99) and cere- both seasonality and time trends. This choice was moti- brovascular mortality (ICD-10: I60-I69). vated by the need to replicate the adjustment made by Daily mean concentrations of PM were obtained from the case-crossover design with the time-stratified the automated network of the Madrid’s City Comprehen- approach for the selection of control days [15]. To take sive Air-Pollution Monitoring, Forecasting and Information into account possible overdispersion of daily counts of System (Red de Control de Contaminación Atmosférica del deaths, we used quasi-likelihood estimation. Ayuntamiento de Madrid). Daily mean levels of gaseous TheeffectsofexposuretoPM were examined for pollutants (NO ,SO )and O were also collected. the same day (lag 0) to 4 days after exposure (lag 4). 2 2 3 Saharan dust intrusions, a dichotomous indicator vari- Estimated effects are reported as IR% and 95% CI asso- able used to show whether or not there had been a ciated with a 10 μg/m increase of PM . A dummy Saharan dust intrusion in the Madrid atmosphere on variable was created for the presence or absence of the date in question, with the pertinent data being Saharan dust outbreaks at exposure days and its interac- obtained from the Directorate-General for Environmen- tion with PM was fitted to test for effect modification tal Quality & Assessment (Dirección General de Calidad by Saharan dust outbreaks. Because of most of Saharan and Evaluación Ambiental)atthe Ministry for the dust intrusions occurs in Spain during the hot season, Environment and Rural & Marine Habitats (Ministerio as previously reported [9], separate estimates were de Medio Ambiente y Medio Rural y Marino) (available undertaken for the period May-September (warm sea- at: http://www.calima.ws/, last accessed February 17th, son) and October-April (cold season). All analyses were 2012). Briefly, an integrated methodological approach done using Stata, release 11, statistical software (Stata- was used to identify days on which air masses from the Corp, College Station, TX, 2010). Sahara-Sahel region were transported to the central region of Spain through back-trajectory analysis (Hysplit Results model), information from NRL SKIRON and During 2003-2005, Saharan dust affected 20% of days in BSCDREAM dust maps, and satellite images provided the city of Madrid (218 days). The proportion of days by the NASA SeaWiFS [1]. with a dust episode was maximal in the hot season Díaz et al. Environmental Health 2012, 11:11 Page 3 of 6 http://www.ehjournal.net/content/11/1/11 (28.4%) and lowest in the cold one (11.3%). The mean larger risk for Saharan dust days (3.34% [95% CI: 0.36, concentration of particles was higher during dust days 6.41] versus 2.87% [95% CI: 1.30, 4.47]) while for warm than during non-dust days (47.7 μg/m versus 31.4 μg/ season effects were stronger for circulatory causes and m ), while for gaseous pollutants did not differ substan- risk was larger for Saharan dust days (4.19% [95% CI: tially whereas levels of O were slightly larger during 1.34, 7.13] versus 2.65% [95% CI: 0.12, 5.23]). However, dust days. As expected, mean temperature was also none of the interaction terms were statistically signifi- higher during dust days (20.4°C versus 13.4°C), even cant. No effects were found for cerebrovascular causes though relative humidity was higher during non-dust for both seasons. days (46.2% versus 56.4%). Case specific mortality was similarly distributed between Saharan dust and non-dust Discussion days. Overall, organic causes represent 48% of the total Saharan dust transport events reaching the city of mortality, circulatory causes 30%, respiratory causes 15% Madrid showed strongest effects of daily concentrations and cerebrovascular causes 7% (Table 1). of PM on case-specific mortality due to organic, The rise of mortality per 10 μg/m PM concentra- respiratory and circulatory causes. Therefore, during tion increase was stronger, and statistically significant (p Saharan dust-days these effects were larger in the cold < 0.05) at lag 1 for both, Saharan dust and non-dust season for respiratory causes and in the warm season days (Figure 1), unless for cerebrovascular causes. Risks for circulatory ones. were always larger for Saharan dust-days providing a The distribution of daily concentrations of PM suggestion of effect modification, even though interac- reported in our city, with higher levels on Saharan dust tion terms between Saharan dust and non-dust were not intrusion days, are consistent with the increased contribu- statistically significant (Table 2). tion of particulate matter from the Sahara because of the Table 2 shows results also stratified by season at natural advections of this. Saharan dust intrusions tend to strongest lag previously found (lag 1). Risk of PM for occur more frequently in the warmer months, which organic causes was larger, and statistically significant, agrees with the frequency of synoptic-scale weather situa- during cold season for non-dust days (1.32% [95% CI: tions [16], mainly characterized by higher temperatures 0.67, 1.97]) whilst during warm season risk was larger and low concentrations in relative humidity. Similar for Saharan dust days (2.53% [95% CI: 1.01, 4.08]) also synoptic behaviour and frequency of Saharan dust intru- providing evidence of effect modification with interac- sions have been described in the study conducted in the tion terms marginally significant (p = 0.150 and p = Emilia-Romagna region [11], while in Athens are less fre- 0.066 for cold and warm season, respectively). Moreover, quent and have no clear seasonal pattern [12]. during cold season effects of PM were stronger, and The effects of PM on case specific mortality found 10 10 statistically significant, for respiratory causes being the in our study agreed with those previously reported in Table 1 Summary statistics for daily case-specific mortality, PM and weather variables during Saharan dust and non- dust days in Madrid, 2003-2005. Non-Saharan dust days (n = 878) Saharan dust days (n = 218) Mean (sd) Min. P P P Max. Mean (sd) Min. P P P Max. 25 50 75 25 50 75 Daily deaths Organic 61.5 (12.2) 32 53 60 75 109 59.6 (10.0) 34 53 59 66 88 Respiratory 9.6 (4.5) 0 7 9 12 32 8.8 (3.7) 2 6 8 11 22 Circulatory 18.8 (5.5) 5 15 18 22 40 17.9 (5.0) 6 14 17 22 32 Cerebrovascular 4.3 (2.1) 3 4 6 12 34 4.4 (2.1) 0 3 4 6 10 Particulate matter PM (μg/m ) 31.4 (15.4) 8 19 29 40 117 47.7 (19.1) 11 35 46 56 150 Gaseous pollutants NO (μg/m ) 59.7 (18.6) 19 47 57 72 133 60.7 (14.0) 28 51 60 72 100 SO (μg/m ) 12.2 (5.8) 5 8 10 16 36 10.1 (3.3) 5 8 9 12 25 Ozone (μg/m ) 33.2 (17.8) 5 18 33 48 79 41.4 (17.2) 6 29 42 55 89 Weather Temperature(°C) 13.4 (7.7) -2.2 6.8 12.2 20.3 30.1 20.4 (6.8) 2.4 14.0 22.1 25.9 30.2 Humidity(%) 56.4 (20.0) 13 39 57 74 96 46.2 (19.9) 15 28 43 63 94 Díaz et al. Environmental Health 2012, 11:11 Page 4 of 6 http://www.ehjournal.net/content/11/1/11 Figure 1 Percentage increases of case-specific mortality for an increase of 10 μ/m of PM , from lag 0 to lag 4, during Saharan dust (with circle) and non-dust days (back squares) with 95% confidence interval (vertical lines). Table 2 Association between particulate matter and daily case-specific mortality during Saharan dust and non-dust days, by season* Non-Saharan dust days Saharan dust days p-value for interaction (Lag 1) %IR (95% CI) %IR (95% CI) Organic Cold 1.32 (0.67, 1.97) 0.26 (-1.00, 1.54) 0.150 Warm 0.76 (-0.61, 2.15) 2.53 (1.01, 4.08) 0.066 Whole year 1.21 (0.63, 1.79) 1.70 (0.79, 2.62) 0.372 Respiratory Cold 2.87 (1.30, 4.47) 3.34 (0.36, 6.41) 0.792 Warm -0.59 (-4.22, 3.17) -0.05 (-3.95, 4.01) 0.830 Whole year 2.50 (1.05, 3.93) 3.48 (1.22, 5.79) 0.462 Circulatory Cold 0.83 (-0.33, 2.00) -0.16 (-2.41, 2.14) 0.453 Warm 2.65 (0.12, 5.23) 4.19 (1.34, 7.13) 0.391 Whole year 1.21 (0.16, 2.27) 1.95 (0.29, 3.64) 0.408 Cerebrovascular Cold 2.14 (-0.32, 4.70) -2.78 (-7.43, 2.10) 0.079 Warm 0.90 (-4.06, 6.12) 3.35 (-2.34, 9.37) 0.500 Whole year 1.76 (-0.42, 3.98) 0.46 (-2.93, 3.96) 0.528 *Cold season: October to April, warm season: May to September. Díaz et al. Environmental Health 2012, 11:11 Page 5 of 6 http://www.ehjournal.net/content/11/1/11 other European cities. The APHEA-2 study reported mortality by circulatory causes during the warm season, that an increase in PM by 10 μg/m was associated and on respiratory causes during cold season. This con- with increases of 0.76% in cardiovascular deaths and firms the seasonal pattern of case-specific mortality in 0.58% in respiratory deaths [17]. However, only few stu- Madrid, described in earlier studies more than two dec- dies have assessed the effect of Saharan dust events on ades ago [23]. This seasonal dependency was not found the association between PM and daily mortality, in the Emilia-Romagna region [11]. This could indicate reporting inconsistent findings. Our results, showing an that the activity of biological agents in Saharan air masses increase in mortality due to organic, respiratory and cir- differs depending on the season and the environmental culatory causes during Saharan dust days, are similar to conditions experienced by air masses travelling from the those recently found in Rome [10]. However, another Sahara to Southern Europe [4,11]. An interaction with study conducted in the Emilia-Romagna region [11] other environmental factors not included in the analyses concluded that Saharan dust outbreaks are an indepen- could be another feasible explanation. dent risk factor that increases respiratory mortality. On As limitation, we have not addressed for contextual the contrary, a latest study in Athens [12] did not found socioeconomic status and other potential modifiers, effect modification of PM on total and case-specific such as age and sex. However, the study design that mortality due to dust events. In addition, studies focused would have meant a spatial adjustment had been very on daily mortality related to dust of non-Saharan dust different to that conducted in our study [24]. From a origin, like those in Washington [18] and Seoul [19], pathophysiological point of view there it is well known also fail to find any effect on Saharan dust events. that the elderly group is most vulnerable to the short- Currently, some studies have been focus on the effect term effects of air pollution on health [25], we have not of coarse-fraction (PM ). A study conducted in Bar- considered age groups due to it was not available. 10-2.5 celona [9] found a stronger effect of PM on daily Furthermore, as well as in previous studies [10-12], we 10-2.5 total mortality during Saharan dust days, as well as the do nothaveavailabledataonchemicaland biological previously cited in Rome [10] that also found evidence components or on the copresence of other pollutants in of stronger effects on cardiovascular mortality. However, particulate matter during Saharan dust days, so cannot the health effects of coarse particles are still controver- determine specific components that could account for sial, since most of the available studies were not able to stronger estimated effects on Saharan dust days, or con- isolate an independent effect of coarse particles from firm the natural or anthropogenic origin of particulate fine particles [20]. Furthermore, previous studies of Afri- matter. Only the study in Barcelona [9] reported data can dust size distributions showed that long distant on chemical composition of fine and coarse particles for Saharan dust and non-dust days, concluding that transport reduces the amount of heavier and larger par- ticles in the Saharan air masses, thus increasing the rela- although coarse particles seem to be more hazardous tive contribution of smaller particles [21]. These data during Saharan dust days, differences in chemical com- suggested than the usual definition of coarse fraction position did not explain these observations. Inconsisten- (PM ) usually adopted in air pollution monitoring cies in the existing evidence suggests that characteristics 10-2.5 also in epidemiological studies should be use with cau- associated with the short-term effects of desert dust tion as specific marker of Saharan dust transports, should be further studied, with special attention to their which are probably better overall described by PM composition, biological properties, and anthropogenic 10-1 [11]. However, we did not consider other PM fractions components that may be associated with the dust parti- sincefromaprevious studywedid notfound differ- cles [10]. ences between Saharan and non-Saharan dust days for However, the results found in our study jointly with PM [22], and no PM levels were monitored during the knowledge of a possible biological mechanism that 2.5 1 the study period. Nevertheless, the issue of harmful indicates that particulate material from the desert carry effects of Saharan dust has recently been addressed by biological material which makes them particularly harm- taking PM into account, as recently did in the Emilia- ful to health [5,26,27], as well as the findings from other Romagna region [11] and in Athens [12]. Thus, our Mediterranean cities, such as Barcelona [9] and Rome study is entirely comparable with those in terms of out- [10], with similar settings like Madrid, make obvious the come and exposure evaluated, increasing the ongoing need to conduct further studies with a similar metho- evidence on the mortality induced by the influx of dology in Southern Europe. Saharan dust in southern Europe. A major strength of our study is that seasonal depen- Conclusions dency, only conducted previously in the study conducted Our study adds to existing evidence of short-term in the Emilia-Romagna region [11], has also been effects of PM on the risk of daily case-specific mortal- addressed. We found strongest effects of PM in daily ity, especially for organic, respiratory and circulatory 10 Díaz et al. Environmental Health 2012, 11:11 Page 6 of 6 http://www.ehjournal.net/content/11/1/11 8. Samoli E, Nastos PT, Paliatsos AG, Katsouyanni K, Priftis KN: Acute effects of causes. During Saharan dust intrusion we also observed air pollution on pediatric asthma exacerbation: Evidence of association effects of PM on mortality due to respiratory causes in and effect modification. Environ Res 2011, 111:418-424. the cold season and to circulatory causes in the warm 9. Pérez L, Tobías A, Querol X, Künzly N, Pey J, Alastuey A, Viana M, Valero N, González-Cabré M, Sunyer J: Coarse particles from Saharan dust and daily one. These shows the need that air quality standards mortality. Epidemiol 2008, 19:800-807. must consider the potential harmful effects of dust from 10. Mallone S, Stafoggia M, Faustini A, Gobbi GP, Marconi A, Forastiere F: natural sources. Furthermore, additional research is Saharan Dust and Associations between Particulate Matter and Daily Mortality in Rome, Italy. Environ Health Perspect 2011, 119:1409-1414. needed to understand the mechanism by which particu- 11. Sajani SZ, Miglio R, Bonasoni P, Cristofanelli P, Marinoni A, Sartini C, late matter from desert sources can increase mortality. Goldoni CA, De Girolamo G, Lauriola P: Saharan dust and daily mortality in Emilia-Romagna (Italy). Occup Environ Med 2011, 68:446-451. 12. Samoli E, Kougea E, Kassomenos P, Analitis A, Katsouyanni K: Does the Abbreviations presence of desert dust modify the effect of PM10 on mortality in PM Particulate matter < 10 μm in aerodynamic diameter; PM 10: 10-2.5: Athens, Greece? Sci Total Environ 2011, 409:2049-2054. Particulate matter size between 10 and 2.5 μm in aerodynamic diameter; 13. Maclure M: The case-crossover design: a method for studying transient PM Particulate matter size between 10 and 1 μm in aerodynamic 10-1: effects on the risk of acute events. Am J Epidemiol 1991, 133:144-153. diameter; NO Nitrogen dioxide; SO2: Sulphur dioxide; O Ozone; ICD-10. 2: 3: 14. Janes H, Sheppard L, Lumley T: Case-crossover analyses of air pollution International Classification of Diseases 10th revision; IR%: Percentage exposure data: referent selection strategies and their implications for increases in risk of death; CI: Confidence interval; p: P-value. bias. Epidemiol 2005, 16:717-726. 15. Lu Y, Symons JM, Geyh AS, Zeger SL: An approach to checking case- Acknowledgements crossover analyses based on equivalence with time-series methods. The study was supported by the Health Research Fund, Subdirectorate- Epidemiol 2008, 19:169-175. General for Assessment and Development of Research (Project PI080354) 16. Trigo RM, García-Herrera R, Díaz J, Trigo IF, Valente MA: How exceptional and by the Carlos III Institute of Health (Project SEPY1453/07). was the early August 2003 heat wave in France? Geophys Res Lett 2005, 32:L10701. Author details 17. Analitis A, Katsouyanni K, Dimakopoulou K, Samoli E, Nikoloulopoulos AK, National School of Public Health, Carlos III Institute of Health. Avda.Monforte Petasakis Y, Touloumi G, Schwartz J, Anderson HR, Cambra K, Forastiere F, de Lemos, 5. 28029 Madrid, Spain. Institute of Environmental Assessment Zmirou D, Vonk JM, Clancy L, Kriz B, Bobvos J, Pekkanen J: Short-term and Water Research (IDAEA), Spanish Council for Scientific Research (CSIC). effects of ambient particles on cardiovascular and respiratory mortality. C/JordiGirona, 18-26. 08034 Barcelona, Spain. Department of Environmental Epidemiology 2006, 17:230-233. Epidemiology and Cancer, National Centre of Epidemiology, Instituto de 18. Schwartz J, Norris G, Larson T, Sheppard L, Claiborne C, Koenig J: Episodes Salud Carlos III, Madrid, Spain. CIBER Epidemiología y Salud Pública of high coarse particles concentrations are not associated with (CIBERESP), Madrid, Spain. increased mortality. Environ Health Perspect 1999, 107:339-342. 19. 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Tobías A, Pérez L, Díaz J, Linares C, Pey J, Alastruey A, Querol X: Short-term Published: 8 March 2012 effects of particulate matter on total mortality during Saharan dust outbreaks: A case-crossover analysis in Madrid (Spain). Sci Total Environ 2011, , 412-413: 386-389. References 23. Alberdi JC, Díaz J: Modelización de la mortalidad diaria en la Comunidad 1. Rodríguez S, Querol X, Alastuey A, Viana MM, Mantilla E: Events affecting de Madrid de 1986 a 1991. Gac Sanit 1997, 11:9-15. levels and seasonal evolution of airborne matter concentrations in the 24. Barceló MA, Saez M, Saurina C: Spatial variability in mortality inequalities, Western Mediterranean. Environ Sci Technol 2003, 37:216-222. socioeconomic deprivation, and air pollution in small areas of the 2. Rodriguez S, Querol X, Alastuey A, Kallos G, Kakaliagou O: Saharan dust Barcelona Metropolitan Region, Spain. Sci Total Environ 2009, contributions to PM10 and TSP levels in Southern and Eastern Spain. 407:5501-5523. 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Kellogg C, Griffin D: Aerobiology and the global transport of desert dust. 20:459-477. Trends Ecol Evol 2006, 21:638e44. 6. Gyan K, Henry W, Lacaille S, Laloo A, Lamsee-Ebanks C, McKay S, Antoine RM, Monteil MA: African dust clouds are associated with doi:10.1186/1476-069X-11-11 increased paediatric asthma accident and emergency admissions on the Cite this article as: Díaz et al.: Saharan dust and association between Caribbean island of Trinidad. Int J Biometeorol 2005, 49:371-376. particulate matter and case-specific mortality: a case-crossover analysis 7. Kanatini KT, Ito I, Al-Delaimy WK, Adachi Y, Mathews WC, Ramsdell JW, in Madrid (Spain). Environmental Health 2012 11:11. Toyama Asian Desert Dust and Asthma Study Team: Desert dust exposure is associated with increased risk of asthma hospitalization in children. Am J Resp Crit Care Med 2010, 182:1475-1481. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Environmental Health Springer Journals

Saharan dust and association between particulate matter and case-specific mortality: a case-crossover analysis in Madrid (Spain)

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Copyright © 2012 by Díaz et al; licensee BioMed Central Ltd.
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Environment; Environmental Health; Occupational Medicine/Industrial Medicine; Public Health
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Abstract

Background: Saharan dust intrusions are a common phenomenon in the Madrid atmosphere, leading induce exceedances of the 50 μg/m - EU 24 h standard for PM . Methods: We investigated the effects of exposure to PM between January 2003 and December 2005 in Madrid (Spain) on daily case-specific mortality; changes of effects between Saharan and non-Saharan dust days were assessed using a time-stratified case-crossover design. Results: Saharan dust affected 20% of days in the city of Madrid. Mean concentration of PM was higher during 3 3 3 dust days (47.7 μg/m ) than non-dust days (31.4 μg/m ). The rise of mortality per 10 μg/m PM concentration were always largely for Saharan dust-days. When stratifying by season risks of PM , at lag 1, during Saharan dust days were stronger for respiratory causes during cold season (IR% = 3.34% (95% CI: 0.36, 6.41) versus 2.87% (95% CI: 1.30, 4.47)) while for circulatory causes effects were stronger during warm season (IR% = 4.19% (95% CI: 1.34, 7.13) versus 2.65% (95% CI: 0.12, 5.23)). No effects were found for cerebrovascular causes. Conclusions: We found evidence of strongest effects of particulate matter during Saharan dust days, providing a suggestion of effect modification, even though interaction terms were not statistically significant. Further investigation is needed to understand the mechanism by which Saharan dust increases mortality. Background Among the diseases that would be affected by dust The increased levels of particulate matter in a given intrusions are those related to exacerbations of respira- geographical area are mainly influenced by the intrusion tory diseases, especially paediatric asthma [6-8]. of natural origin such as those related to the advection Recently, this subject is rising from studies conducted of dust from the desert [1]. Obviously, the areas which in Southern Europe, although they have not given con- are closer to the great deserts are the most affected by sistent results. In Spain, a study conducted in Barcelona this type of events. In the case of the Saharan desert, [9] reported strongest effects of coarse particles (PM 10- Southern European countries are largely influenced by ) on total daily mortality during Saharan dust intru- 2.5 dust intrusions [2]. Dust particles can also be trans- sions. In Italy, a study conducted in Rome [10] also ported over long distances by atmospheric circulation, found strongest effects of PM and coarse particles on reaching the Near East end the Americas [3]. daily mortality for cardiovascular causes, although others Saharan dust events can contribute to exceedances of in Emilia-Romagna region [11] did not found an PM daily European Union Limit of 50 μg/m [2,4]. increase of risk of total and case-specific mortality due Moreover, these particles can carry biological material to PM during Saharan dust days. Lastly, studies in that makes them potentially harmful to health [5]. Athens did not found effect modification of PM on total and case-specific mortality [12], although they did found an increase of risk on paediatric asthma exacerba- * Correspondence: [email protected] Institute of Environmental Assessment and Water Research (IDAEA), Spanish tion [8]. Council for Scientific Research (CSIC). C/JordiGirona, 18-26. 08034 Barcelona, The city of Madrid has high levels of pollution of Spain anthropogenic origin, which are strongly increased those Full list of author information is available at the end of the article © 2012 Díaz et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Díaz et al. Environmental Health 2012, 11:11 Page 2 of 6 http://www.ehjournal.net/content/11/1/11 days with Saharan dust intrusions, and its geographical Daily mean temperature and relative humidity regis- and meteorological conditions that did not facilitate the teredatthe Madrid-Retiro Observatory,situatedatcity dispersion of pollutants. The aim of this study is to eval- centre, were recorded by the National Meteorology uate the effect of Saharan dust on the association Agency (Agencia Nacional de Meteorología). between particulate matter and daily case-specific mor- tality in Madrid. Design and statistical analysis The association of daily concentrations of PM with Methods daily mortality was investigated using a case-crossover Setting design [13]. This uses the day on which the outcome of The city of Madrid constitutes a dense metropolitan interest (mortality) occurs as a case day. Exposure on area located on the central region of Spain. The main case days is compared with exposure on days on which emission source of atmospheric pollutants is road traffic. the outcome of interest does not occur (control days) Its geographical setting has climatological conditions [14]. A time-stratified approach used to represent expo- directly linked to emission levels, with frequent anticy- sure on control days from the same day of the week, clonic situations in summer and winter, which impede month, and year as case days, minimising bias from the dispersion of pollutants. Saharan dust events on the time trends in the exposure series and from other short- central region of Spain generally occur 50 to 100 days term time-varying confounders [14]. per year, mainly between spring and autumn, mostly We fitted a basic Poisson regression model to the caused by an atmospheric depression West or Southwest daily mortality data that included potential confounders: of Portugal or by anticyclonic conditions over Algeria weather, influenza, epidemics and time trends and sea- inducing Southern winds over the Iberian Peninsula [1]. sonality. A natural cubic spline with 3 degrees of free- dom was built to adjust for the potential confounding of Data mean temperature (4-day average) and a linear term to Daily mortality was obtained from the Madrid Regional adjust for relative humidity (4-day average). We also Inland Revenue Department (Consejería de Hacienda), controlled for gaseous pollutants and O , by means of a which is the department responsible for mortality regis- 4-day average linear term, and for public holidays and try, from 1st January 2003 to 31st December 2005. The influenza epidemics by means of categorical variables. outcomes of interest were organic causes except acci- Finally, we included a three-way interaction term dents (ICD-10: A00-R99), respiratory causes (ICD-10: between day of the week, month, and year to control for J00-J99), circulatory causes (ICD-10: I00-I99) and cere- both seasonality and time trends. This choice was moti- brovascular mortality (ICD-10: I60-I69). vated by the need to replicate the adjustment made by Daily mean concentrations of PM were obtained from the case-crossover design with the time-stratified the automated network of the Madrid’s City Comprehen- approach for the selection of control days [15]. To take sive Air-Pollution Monitoring, Forecasting and Information into account possible overdispersion of daily counts of System (Red de Control de Contaminación Atmosférica del deaths, we used quasi-likelihood estimation. Ayuntamiento de Madrid). Daily mean levels of gaseous TheeffectsofexposuretoPM were examined for pollutants (NO ,SO )and O were also collected. the same day (lag 0) to 4 days after exposure (lag 4). 2 2 3 Saharan dust intrusions, a dichotomous indicator vari- Estimated effects are reported as IR% and 95% CI asso- able used to show whether or not there had been a ciated with a 10 μg/m increase of PM . A dummy Saharan dust intrusion in the Madrid atmosphere on variable was created for the presence or absence of the date in question, with the pertinent data being Saharan dust outbreaks at exposure days and its interac- obtained from the Directorate-General for Environmen- tion with PM was fitted to test for effect modification tal Quality & Assessment (Dirección General de Calidad by Saharan dust outbreaks. Because of most of Saharan and Evaluación Ambiental)atthe Ministry for the dust intrusions occurs in Spain during the hot season, Environment and Rural & Marine Habitats (Ministerio as previously reported [9], separate estimates were de Medio Ambiente y Medio Rural y Marino) (available undertaken for the period May-September (warm sea- at: http://www.calima.ws/, last accessed February 17th, son) and October-April (cold season). All analyses were 2012). Briefly, an integrated methodological approach done using Stata, release 11, statistical software (Stata- was used to identify days on which air masses from the Corp, College Station, TX, 2010). Sahara-Sahel region were transported to the central region of Spain through back-trajectory analysis (Hysplit Results model), information from NRL SKIRON and During 2003-2005, Saharan dust affected 20% of days in BSCDREAM dust maps, and satellite images provided the city of Madrid (218 days). The proportion of days by the NASA SeaWiFS [1]. with a dust episode was maximal in the hot season Díaz et al. Environmental Health 2012, 11:11 Page 3 of 6 http://www.ehjournal.net/content/11/1/11 (28.4%) and lowest in the cold one (11.3%). The mean larger risk for Saharan dust days (3.34% [95% CI: 0.36, concentration of particles was higher during dust days 6.41] versus 2.87% [95% CI: 1.30, 4.47]) while for warm than during non-dust days (47.7 μg/m versus 31.4 μg/ season effects were stronger for circulatory causes and m ), while for gaseous pollutants did not differ substan- risk was larger for Saharan dust days (4.19% [95% CI: tially whereas levels of O were slightly larger during 1.34, 7.13] versus 2.65% [95% CI: 0.12, 5.23]). However, dust days. As expected, mean temperature was also none of the interaction terms were statistically signifi- higher during dust days (20.4°C versus 13.4°C), even cant. No effects were found for cerebrovascular causes though relative humidity was higher during non-dust for both seasons. days (46.2% versus 56.4%). Case specific mortality was similarly distributed between Saharan dust and non-dust Discussion days. Overall, organic causes represent 48% of the total Saharan dust transport events reaching the city of mortality, circulatory causes 30%, respiratory causes 15% Madrid showed strongest effects of daily concentrations and cerebrovascular causes 7% (Table 1). of PM on case-specific mortality due to organic, The rise of mortality per 10 μg/m PM concentra- respiratory and circulatory causes. Therefore, during tion increase was stronger, and statistically significant (p Saharan dust-days these effects were larger in the cold < 0.05) at lag 1 for both, Saharan dust and non-dust season for respiratory causes and in the warm season days (Figure 1), unless for cerebrovascular causes. Risks for circulatory ones. were always larger for Saharan dust-days providing a The distribution of daily concentrations of PM suggestion of effect modification, even though interac- reported in our city, with higher levels on Saharan dust tion terms between Saharan dust and non-dust were not intrusion days, are consistent with the increased contribu- statistically significant (Table 2). tion of particulate matter from the Sahara because of the Table 2 shows results also stratified by season at natural advections of this. Saharan dust intrusions tend to strongest lag previously found (lag 1). Risk of PM for occur more frequently in the warmer months, which organic causes was larger, and statistically significant, agrees with the frequency of synoptic-scale weather situa- during cold season for non-dust days (1.32% [95% CI: tions [16], mainly characterized by higher temperatures 0.67, 1.97]) whilst during warm season risk was larger and low concentrations in relative humidity. Similar for Saharan dust days (2.53% [95% CI: 1.01, 4.08]) also synoptic behaviour and frequency of Saharan dust intru- providing evidence of effect modification with interac- sions have been described in the study conducted in the tion terms marginally significant (p = 0.150 and p = Emilia-Romagna region [11], while in Athens are less fre- 0.066 for cold and warm season, respectively). Moreover, quent and have no clear seasonal pattern [12]. during cold season effects of PM were stronger, and The effects of PM on case specific mortality found 10 10 statistically significant, for respiratory causes being the in our study agreed with those previously reported in Table 1 Summary statistics for daily case-specific mortality, PM and weather variables during Saharan dust and non- dust days in Madrid, 2003-2005. Non-Saharan dust days (n = 878) Saharan dust days (n = 218) Mean (sd) Min. P P P Max. Mean (sd) Min. P P P Max. 25 50 75 25 50 75 Daily deaths Organic 61.5 (12.2) 32 53 60 75 109 59.6 (10.0) 34 53 59 66 88 Respiratory 9.6 (4.5) 0 7 9 12 32 8.8 (3.7) 2 6 8 11 22 Circulatory 18.8 (5.5) 5 15 18 22 40 17.9 (5.0) 6 14 17 22 32 Cerebrovascular 4.3 (2.1) 3 4 6 12 34 4.4 (2.1) 0 3 4 6 10 Particulate matter PM (μg/m ) 31.4 (15.4) 8 19 29 40 117 47.7 (19.1) 11 35 46 56 150 Gaseous pollutants NO (μg/m ) 59.7 (18.6) 19 47 57 72 133 60.7 (14.0) 28 51 60 72 100 SO (μg/m ) 12.2 (5.8) 5 8 10 16 36 10.1 (3.3) 5 8 9 12 25 Ozone (μg/m ) 33.2 (17.8) 5 18 33 48 79 41.4 (17.2) 6 29 42 55 89 Weather Temperature(°C) 13.4 (7.7) -2.2 6.8 12.2 20.3 30.1 20.4 (6.8) 2.4 14.0 22.1 25.9 30.2 Humidity(%) 56.4 (20.0) 13 39 57 74 96 46.2 (19.9) 15 28 43 63 94 Díaz et al. Environmental Health 2012, 11:11 Page 4 of 6 http://www.ehjournal.net/content/11/1/11 Figure 1 Percentage increases of case-specific mortality for an increase of 10 μ/m of PM , from lag 0 to lag 4, during Saharan dust (with circle) and non-dust days (back squares) with 95% confidence interval (vertical lines). Table 2 Association between particulate matter and daily case-specific mortality during Saharan dust and non-dust days, by season* Non-Saharan dust days Saharan dust days p-value for interaction (Lag 1) %IR (95% CI) %IR (95% CI) Organic Cold 1.32 (0.67, 1.97) 0.26 (-1.00, 1.54) 0.150 Warm 0.76 (-0.61, 2.15) 2.53 (1.01, 4.08) 0.066 Whole year 1.21 (0.63, 1.79) 1.70 (0.79, 2.62) 0.372 Respiratory Cold 2.87 (1.30, 4.47) 3.34 (0.36, 6.41) 0.792 Warm -0.59 (-4.22, 3.17) -0.05 (-3.95, 4.01) 0.830 Whole year 2.50 (1.05, 3.93) 3.48 (1.22, 5.79) 0.462 Circulatory Cold 0.83 (-0.33, 2.00) -0.16 (-2.41, 2.14) 0.453 Warm 2.65 (0.12, 5.23) 4.19 (1.34, 7.13) 0.391 Whole year 1.21 (0.16, 2.27) 1.95 (0.29, 3.64) 0.408 Cerebrovascular Cold 2.14 (-0.32, 4.70) -2.78 (-7.43, 2.10) 0.079 Warm 0.90 (-4.06, 6.12) 3.35 (-2.34, 9.37) 0.500 Whole year 1.76 (-0.42, 3.98) 0.46 (-2.93, 3.96) 0.528 *Cold season: October to April, warm season: May to September. Díaz et al. Environmental Health 2012, 11:11 Page 5 of 6 http://www.ehjournal.net/content/11/1/11 other European cities. The APHEA-2 study reported mortality by circulatory causes during the warm season, that an increase in PM by 10 μg/m was associated and on respiratory causes during cold season. This con- with increases of 0.76% in cardiovascular deaths and firms the seasonal pattern of case-specific mortality in 0.58% in respiratory deaths [17]. However, only few stu- Madrid, described in earlier studies more than two dec- dies have assessed the effect of Saharan dust events on ades ago [23]. This seasonal dependency was not found the association between PM and daily mortality, in the Emilia-Romagna region [11]. This could indicate reporting inconsistent findings. Our results, showing an that the activity of biological agents in Saharan air masses increase in mortality due to organic, respiratory and cir- differs depending on the season and the environmental culatory causes during Saharan dust days, are similar to conditions experienced by air masses travelling from the those recently found in Rome [10]. However, another Sahara to Southern Europe [4,11]. An interaction with study conducted in the Emilia-Romagna region [11] other environmental factors not included in the analyses concluded that Saharan dust outbreaks are an indepen- could be another feasible explanation. dent risk factor that increases respiratory mortality. On As limitation, we have not addressed for contextual the contrary, a latest study in Athens [12] did not found socioeconomic status and other potential modifiers, effect modification of PM on total and case-specific such as age and sex. However, the study design that mortality due to dust events. In addition, studies focused would have meant a spatial adjustment had been very on daily mortality related to dust of non-Saharan dust different to that conducted in our study [24]. From a origin, like those in Washington [18] and Seoul [19], pathophysiological point of view there it is well known also fail to find any effect on Saharan dust events. that the elderly group is most vulnerable to the short- Currently, some studies have been focus on the effect term effects of air pollution on health [25], we have not of coarse-fraction (PM ). A study conducted in Bar- considered age groups due to it was not available. 10-2.5 celona [9] found a stronger effect of PM on daily Furthermore, as well as in previous studies [10-12], we 10-2.5 total mortality during Saharan dust days, as well as the do nothaveavailabledataonchemicaland biological previously cited in Rome [10] that also found evidence components or on the copresence of other pollutants in of stronger effects on cardiovascular mortality. However, particulate matter during Saharan dust days, so cannot the health effects of coarse particles are still controver- determine specific components that could account for sial, since most of the available studies were not able to stronger estimated effects on Saharan dust days, or con- isolate an independent effect of coarse particles from firm the natural or anthropogenic origin of particulate fine particles [20]. Furthermore, previous studies of Afri- matter. Only the study in Barcelona [9] reported data can dust size distributions showed that long distant on chemical composition of fine and coarse particles for Saharan dust and non-dust days, concluding that transport reduces the amount of heavier and larger par- ticles in the Saharan air masses, thus increasing the rela- although coarse particles seem to be more hazardous tive contribution of smaller particles [21]. These data during Saharan dust days, differences in chemical com- suggested than the usual definition of coarse fraction position did not explain these observations. Inconsisten- (PM ) usually adopted in air pollution monitoring cies in the existing evidence suggests that characteristics 10-2.5 also in epidemiological studies should be use with cau- associated with the short-term effects of desert dust tion as specific marker of Saharan dust transports, should be further studied, with special attention to their which are probably better overall described by PM composition, biological properties, and anthropogenic 10-1 [11]. However, we did not consider other PM fractions components that may be associated with the dust parti- sincefromaprevious studywedid notfound differ- cles [10]. ences between Saharan and non-Saharan dust days for However, the results found in our study jointly with PM [22], and no PM levels were monitored during the knowledge of a possible biological mechanism that 2.5 1 the study period. Nevertheless, the issue of harmful indicates that particulate material from the desert carry effects of Saharan dust has recently been addressed by biological material which makes them particularly harm- taking PM into account, as recently did in the Emilia- ful to health [5,26,27], as well as the findings from other Romagna region [11] and in Athens [12]. Thus, our Mediterranean cities, such as Barcelona [9] and Rome study is entirely comparable with those in terms of out- [10], with similar settings like Madrid, make obvious the come and exposure evaluated, increasing the ongoing need to conduct further studies with a similar metho- evidence on the mortality induced by the influx of dology in Southern Europe. Saharan dust in southern Europe. A major strength of our study is that seasonal depen- Conclusions dency, only conducted previously in the study conducted Our study adds to existing evidence of short-term in the Emilia-Romagna region [11], has also been effects of PM on the risk of daily case-specific mortal- addressed. We found strongest effects of PM in daily ity, especially for organic, respiratory and circulatory 10 Díaz et al. Environmental Health 2012, 11:11 Page 6 of 6 http://www.ehjournal.net/content/11/1/11 8. Samoli E, Nastos PT, Paliatsos AG, Katsouyanni K, Priftis KN: Acute effects of causes. During Saharan dust intrusion we also observed air pollution on pediatric asthma exacerbation: Evidence of association effects of PM on mortality due to respiratory causes in and effect modification. Environ Res 2011, 111:418-424. the cold season and to circulatory causes in the warm 9. Pérez L, Tobías A, Querol X, Künzly N, Pey J, Alastuey A, Viana M, Valero N, González-Cabré M, Sunyer J: Coarse particles from Saharan dust and daily one. These shows the need that air quality standards mortality. Epidemiol 2008, 19:800-807. must consider the potential harmful effects of dust from 10. Mallone S, Stafoggia M, Faustini A, Gobbi GP, Marconi A, Forastiere F: natural sources. Furthermore, additional research is Saharan Dust and Associations between Particulate Matter and Daily Mortality in Rome, Italy. Environ Health Perspect 2011, 119:1409-1414. needed to understand the mechanism by which particu- 11. Sajani SZ, Miglio R, Bonasoni P, Cristofanelli P, Marinoni A, Sartini C, late matter from desert sources can increase mortality. Goldoni CA, De Girolamo G, Lauriola P: Saharan dust and daily mortality in Emilia-Romagna (Italy). Occup Environ Med 2011, 68:446-451. 12. 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Published: Mar 8, 2012

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