Original article Air pollution and seasonal asthma during the pollen season. A cohort study in Puertollano and Ciudad Real (Spain) There has been a significant increase in the prevalence and severity of allergic diseases in the industrialized countries, with a higher incidence of allergic symptoms in urban environments than in rural areas, particularly with respect to pollen-induced symptoms. Furthermore, the changes caused by industrialization have led to a pronounced increase in the levels of pollution in cities. We have an example of this in Japan, where the diesel vehicle fleet increased 500 fold and the quantity of aromatic hydro- carbons derived from the diesel exhaust increased 1000 fold (1, 2). Many studies in the United States, Europe and Australia, have shown that the number of asthma-related visits to emergency departments and hospital admissions increase proportionally with the increase in certain pollutants [ozone (O 3 ), particles (PM 10 ), nitrogen dioxide (NO 2 ) and sulphur dioxide (SO 2 )] (3–5). However, there are fewer studies which have evaluated other health indicators of lesser severity such as symptoms of asthma, spirometric changes and the use of asthma medication. These studies have enabled the relationship between pollutants and exacerbations to be observed in less severe (mild–moderate) asthma, which affects a broader spec- trum of the population. Similarly, several experimental and laboratory studies suggest a close relationship between pollens and environ- mental pollution. For example, exposure to ozone redu- ces to one half the dose of pollen necessary to produce a decrease of 15% in the forced expiratory volume (FEV 1 ) compared with noncontaminated air (6). Exposure to NO 2 and to particles (PM 10 ) (7, 8) increases the asthmatic response in subjects allergic to grass pollens. There are few epidemiological studies in which the relationship between pollutants, pollens and visits to emergency departments due to asthma crises has been analysed, and their results have been controversial; this may be because, as the study periods were not limited to the months of pollination, possible interactions between pollens and pollutants could be masked by the effects of the pollutants, which have a stable repercussion through- out the year (9, 10). The objective of this study was to determine the role of pollutants on the clinical course of patients with mild– moderate, pollen-allergic asthma, living in two cities with different levels of pollution. To reduce the confounding factors between pollens and pollutants, the study was limited exclusively to the pollination season. Background: Many studies have demonstrated a positive association between air pollutants and emergency visits for asthma. However, few studies have included pollen when analysing this relationship in mild–moderate asthmatic patients. Objective: To determine the importance of the pollutants such as ozone (O 3 ), particles (PM 10 ), nitrogen dioxide (NO 2 ) and sulphur dioxide (SO 2 ) in the clinical course of mild–moderate pollen-allergic asthmatic patients from two Spanish towns in La Mancha: Puertollano (high pollution levels) and Ciudad Real (low pollution levels). Methods: We used a Poisson regression model to study a cohort of 137 patients from Puertollano and Ciudad Real during two pollen seasons (2000–2001) and analysed the relationship between air pollutant and pollen levels and daily symptoms, the medication used and peak-flow measurements. Results: The number of asthma symptoms and the mean values of the PM 10 , SO 2 and NO 2 levels were higher in Puertollano than in Ciudad Real. In Puertollano, the risk of asthma increased by 6% with a 3-day lag for PM 10 , by 8% with a 3- day lag for O 3 , by 4% with a 1-day lag for SO 2 and by 15% with a 3-day lag for O 3 when its values exceeded the health threshold (P < 0.05). Conclusions: The air pollution levels in Puertollano were associated with an increased risk of asthma symptoms in pollen-allergic asthmatic patients com pared with a similar group from Ciudad Real. F. Feo Brito 1 , P. Mur Gimeno 2 , C. Martínez 3 , A. Tobías 4 , L. Suµrez 5 , F. Guerra 6 , J. M Borja 1 , A. M Alonso 1 1 Allergy Section, General Hospital, Ciudad Real; 2 Allergy Unit, Santa Barbara Hospital, Puertollano; 3 Investigation Unit, General Hospital, Ciudad Real; 4 National School of Public Health, Carlos III Health Institute, Madrid; 5 Castilla-La Mancha Environmental Department, Toledo; 6 Department of Allergy and Pathology, Medical School Unit, Reina Sofía University Hospital, Cordoba, Spain Key words: air pollution; asthma; ozone; PM 10 ; pollen allergy. Francisco Feo Brito Sección de Alergia Hospital General C/ Tomelloso, s/n 13004 Ciudad Real Spain Accepted for publication 20 April 2007 Allergy 2007: 62: 1152–1157 Ó 2007 The Authors Journal compilation Ó 2007 Blackwell Munksgaard DOI: 10.1111/j.1398-9995.2007.01438.x 1152