Association between airborne pollen and epidemic asthma in Madrid, Spain: a caseecontrol study In ˜aki Gala ´n, 1 Alicia Prieto, 2 Marı ´a Rubio, 2 Teresa Herrero, 2 Patricia Cervigo ´n, 3 Jose Luis Cantero, 1 Maria Dolores Gurbindo, 4 Marı ´a Isabel Martı ´nez, 5 Aurelio Tobı ´as 1,6 ABSTRACT Background Despite the fact that airborne pollen is an important factor in precipitating asthma attacks, its implication in increases of epidemic asthma in usual meteorological conditions has not been reported. A study was undertaken to estimate the relationship between various types of aeroallergens and seasonal epidemic asthma in the region of Madrid, Spain. Methods A caseecontrol study was carried out in individuals aged 4e79 years who received emergency healthcare for asthma during 2001 in a base hospital covering a population of 750 000 inhabitants of Madrid. A skin prick test was performed with grass pollen, plantain pollen, olive pollen, cypress pollen, plane tree pollen, dust mites and Alternaria and the prevalence of skin reactivity was compared between subjects with asthma requiring emergency care for asthma within (cases) and outside (controls) the seasonal epidemic period. Data were analysed using logistic regression adjusting for age and sex. Results The response rate was 61.7% for cases (n¼95) and 51.6% for controls (n¼146). The OR of sensitisation to grass pollen for cases compared with controls was 9.9 (95% CI 4.5 to 21.5); plantain pollen: 4.5 (95% CI 2.5 to 8.2); olive pollen: 7.3 (95% CI 3.5 to 15.2); plane tree pollen: 3.6 (95% CI 2.0 to 6.4); cypress pollen: 3.5 (95% CI 2.0 to 6.2); dust mites: 1.1 (95% CI 0.6 to 1.9); Alternaria: 0.9 (95% CI 0.5 to 1.9). The association with grasses was maintained after adjusting simultaneously for the remaining aeroallergens (OR 5.0 (95% CI 1.5 to 16.4)); this was the only one that retained statistical significance (p¼0.007). Conclusions These results suggest that allergy to pollen, particularly grass pollen, is associated with the epidemic increase in asthma episodes during the months of May and June in the Madrid area of Spain. INTRODUCTION People with asthma are frequently more allergic than the general population and it is estimated that, in our environment, the population attributable risk of atopy in explaining asthma is 42%. 1 Sensitisation to seasonal allergens in Spain, including grass pollen, is an independent risk factor for developing asthma. 2 The region of Madrid has botanical and climatic characteristics which predispose to very high grass pollen concentrations. The continental climate, in which there is an abrupt temperature change during the months of April and May moving quickly from cold to hot days, favours the simul- taneous flowering of most grass species. 3 As a result, and depending on the meteorological conditions, 4 very high concentrations of grass pollen are reached for 2e3 weeks between May and June, exceeding the threshold level above which most people sensitised to this pollen develop symptoms related to the disease. 5 6 During this period and coinciding with the presence of high atmospheric pollen concentrations, an epidemic increase in asthma episodes or attacks is observed. 7 Several studies based on time-series design have evaluated the short-term effects of aeroallergens on the healthcare demand for asthma exacerbations. Many have observed positive associations 8e16 but others have found no relationship. 17e19 In Madrid, a strong association between grass pollen and hospital emergency room admissions for asthma has also been shown. 7 Apart for exceptional meteorological circum- stances, such as thunderstorms in the presence of high grass pollen concentrations which have been associated with epidemic days of asthma, 20 it has not been shown from individual observational studies that airborne pollen levels in usual meteo- rological conditions could cause epidemic asthma. 21 The objective of this study was to estimate the association between various aeroallergens and seasonal epidemic asthma in the region of Madrid, comparing the prevalence of skin reactivity between subjects with asthma who require emer- gency healthcare for asthma within and outside the seasonal epidemic period. METHODS Study design A caseecontrol study was carried out in a popula- tion aged 4e79 years who received emergency healthcare for asthma in the emergency room of the Hospital General Universitario Gregorio Marañón during 2001 (n¼900, 359 men and 541 women). This hospital is the reference hospital for a healthcare area of the region of Madrid and includes a population of 750 000 inhabitants. Only the clinical diagnosis of asthma (ICD-9-CM diag- nosis code 493) was considered, excluding terms such as bronchospasm, bronchial hyper-reactivity or asthmatic bronchitis. People who received emergency healthcare for asthma during the epidemic period of May and June 2001 were defined as cases. The identification of epidemic days was based on those days of May and June which exceeded by three standard devia- tions the prediction made from a Poisson distribu- tion for the time series between 1994 and 2001. 22 Controls were defined as asthma episodes seen in 1 Department of Epidemiology, Consejerı ´a de Sanidad de la Comunidad de Madrid, Spain 2 Department of Allergy, Hospital General Universitario Gregorio Maran ˜o ´n, Consejerı ´a de Sanidad de la Comunidad de Madrid, Madrid, Spain 3 Department of Environmental Health, Consejerı ´a de Sanidad de la Comunidad de Madrid, Madrid, Spain 4 Department of Immunopediatrics, Hospital General Universitario Gregorio Maran ˜o ´n, Consejerı ´a de Sanidad de la Comunidad de Madrid, Madrid, Spain 5 Department of Child Allergy, Hospital General Universitario Gregorio Maran ˜o ´n, Consejerı ´a de Sanidad de la Comunidad de Madrid, Madrid, Spain 6 Institute of Environmental Assessment and Water Research (IDAEA), Spanish Council for Scientific Research (CSIC), Barcelona, Spain Correspondence to In ˜aki Gala ´n Labaca, Servicio de Epidemiologı ´a, Consejerı ´a de Sanidad, Comunidad de Madrid, C/ Julia ´n Camarillo 4 B 28037 Madrid, Spain; inaki.galan@salud.madrid.org Received 5 May 2009 Accepted 13 February 2010 398 Thorax 2010;65:398e402. doi:10.1136/thx.2009.118992 Asthma group.bmj.com on May 3, 2010 - Published by thorax.bmj.com Downloaded from