Urbanisation, asthma and allergies

Differences in asthma in urban and rural environments were first reported from Germany in 1965, followed by many reports of higher rates of asthma symptoms in urban than rural environments around the world including Africa, Asia and Papua New Guinea. Authors have suggested that wealth, lifestyle, housing and urban environmental exposures may contribute to increasing asthma prevalence. In relation to possible protective factors in the rural environment, studies have emerged on farming lifestyles in Westernised high-income countries, showing an inverse relationship between farm exposures early in life and asthma and rhinitis, but not eczema symptoms. 8 The potential protective effect of microbial exposures from animals or unpasteurised milk on these diseases was highlighted. A recent review of these and other potential factors suggested a ‘package’ of factors may influence the lower prevalence in rural areas. In Thorax, two articles 11 present new findings related to urbanisation in Latin America. Most of the readers of Thoraxwill not have visited the locations studied in these papers, as they are not on popular travel routes. However, they may represent the living circumstances of many of the world’s children in lowand middleincome countries where the prevalence of asthma, rhinitis and eczema is mainly increasing. Rodríguez et al undertook an ecological study in 59 communities in Esmeraldas Province, a humid, tropical, coastal region in Ecuador. This is one of the poorest and remotest areas of the country undergoing changes related to urbanisation over the last 20 years. From 2005 to 2008 all the 7e15-year-olds were studied within the communities using parent completed questionnaires and reported community infrastructure. This study is robust with a high number of participants (n1⁄44183) and a 95% response rate. Asthma was defined as a positive response to the question ‘Has your child had wheeze in the chest in the last 12 months?’, which I define as ‘current asthma symptoms’. The overall prevalence of current asthma symptoms was 10.1%. Three of four urbanisation indices were significantly associated with current asthma symptom prevalence: the socioeconomic, lifestyle and summary urbanisation indices. Using multivariate analyses, infrastructure indicators explained 17.7% of variability, socioeconomic indicators 17.4% and lifestyle indicators 19.8%. Prevalence of current asthma symptoms decreased in communities with dirt roads compared with river access, and children living in cement-built rather than wood and bamboo houses; prevalence increased for communities with an electric grid connection, those with more than 15 shops compared with zero or one shops, houses with access to electricity, households with motor vehicles and adolescents consuming fizzy drinks. Robinson et al in 2009e2010 studied 13e15-year-old adolescents living in two coastal regions of Peru: Pampas de San Juan de Miraflores, a ‘peri-urban’ shanty town in Lima with high population density centred on a heavy traffic route, and 23 rural villages with little traffic outside Tumbes city in northern Peru. Subjects were a sex-stratified sample in peri-urban Lima, and all the 13e15-yearolds in rural Tumbes. The adolescents completed their own questionnaires, they had some testing and concentrations of particulate matter of <2.5 mm diameter (PM2.5) were measured in a subset of households. The number of subjects was relatively low (peri-urban Lima n1⁄4725, rural Tumbes (n1⁄4716)), as was the participation rate (78% overall). Significantly higher prevalence was found in peri-urban Lima than rural Tumbes for current symptoms of asthma (12% vs 3%), rhinitis (23% vs 12%) and eczema (12% vs 0.4%). The peri-urban to rural gradient was also present for positive skin prick tests (56% vs 38%), personal history of cigarette smoking (7.4% vs 1.3%) and mean indoor PM2.5 (31 vs 13 mg/m). Living in peri-urban Lima was associated with a 2.6-fold greater odds (95% CI 1.3 to 5.3) of current asthma symptoms in multivariable regression. It is difficult to interpret the slightly higher exhaled nitric oxide levels in peri-urban Lima, which the authors attribute to asthma and atopy. Differences were found between sites: in peri-urban Lima adolescents lived with fewer wood burning stoves and lower secondhand smoking exposure, but more active smoking, greater body mass index and were more likely to live in households with adequate sanitation, higher indoor and outdoor air pollution than in rural Tumbes. The Ecuador study was an ecological study of a whole population aged 7e15 years, with questionnaires answered by parents, so direct comparisons of prevalence should not be made with the Peruvian study where adolescents self completed the questionnaires. The focus of the Ecuador study was to investigate factors of importance to asthma in rural communities undergoing transition, and some new aspects were explored such as dirt roads compared with power driven river transport. The Peruvian study compared symptom prevalence between urban and rural locations and sought risk factors, but this study was limited by smaller numbers. They studied not only asthma, but also symptoms of rhinitis and eczema, atopy and some other factors. The high prevalence of current symptoms of asthma in children from a peri-urban shanty town in Lima has been previously reported. These are among the first studies of rural versus urban environments in Latin America, and therefore are exciting and add useful information. The investigators have completed studies in environments that would be very challenging for researchers in high-income countries. The Ecuador study found the prevalence among the communities varied from 0% to 31.4%dalmost the full range of prevalence values reported among the 233 centres in 97 countries in the global International Study of Asthma and Allergies in Childhood, consistent with that study ’s findings of extremes of prevalence among some lowand middle-income centres. The finding of such high prevalence among socioeconomically disadvantaged populations within Latin America has led to questioning of the relevance of This paper is freely available online under the BMJ Journals unlocked scheme, see http://thoraxjnl.bmj.com/ site/about/unlocked.xhtml