Asthma, rhinoconjunctivitis and eczema in childhood have become three of the more important public health problems worldwide. Although in the past it was thought that these diseases occurred more frequently in populations from developed countries, it has been evident since the first global report from the International Study of Asthma and Allergies in Childhood (ISAAC) that prevalence of those conditions in some low-resourced countries was similar or even higher than in developed ones and that a wide variability in their prevalence occurred at regional and even at country level.1–4

Since then, a large body of new national, regional and global information on the prevalence, severity, risk factors, trends and several other aspects related to asthma, rhinoconjunctivitis and eczema in childhood, has been reported by ISAAC.5 The recently completed third ISAAC Phase included the largest number of centres from the main world regions ever studied in regard to these conditions, with about 1,200,000 schoolchildren surveyed; detailed information on the global prevalence and severity of asthma, rhinitis and eczema has been reported in the ISAAC world map journal articles.6–8

The well-known methodological consistency of the ISAAC programme, its originality and especially the inclusion of countries with different cultures, socioeconomic development and lifestyles, has led to its results being employed by several governmental and academic institutions at country and regional level. It has constituted the epidemiological basis for many of the well-known global initiatives on asthma, rhinitis and eczema management in childhood, and for guidelines and recommendations from international health organisations.

This article presents new information on disease overlap (asthma, rhinoconjunctivitis, eczema), socioeconomic and geospatial considerations from ISAAC Phase Three (written questionnaires) concerning the prevalence of asthma, rhinoconjunctivitis and eczema in children aged 6–7 and 13–14 years.

The detailed methodology employed by ISAAC to obtain and analyse data for Phase Three, has already been published in detail elsewhere.5,9 Briefly, written questionnaires were self-completed at school by 13–14-year olds and completed at home by parents of 6–7-year olds. Samples of 3000 children for each age group (with a minimum of 1000) within each centre were selected by randomly sampling schools within the study area. Almost all centres studied the 13–14-year age group, but only some centres studied the optional 6–7-year age group. Data checks were undertaken by the ISAAC International Data Centre in Auckland, New Zealand which ensured adherence to the protocol. The English language questionnaire was translated where required, and a back translation to English was provided. In this paper, we focus on the current symptoms (past 12 months) of asthma, rhinoconjunctivitis and eczema; and reported asthma ever.

Associations between prevalence of current symptoms of asthma, rhinoconjunctivitis and eczema, and a range of geo-climatic factors were assessed using maps prepared in the ArcGIS geographic information system (version 9.3.1, ESRI Inc., Redlands, CA, USA). The factors presented are altitude,10 air pollution as indicated by the predicted concentrations (micro grams per cubic metre) of particulate matter less than 10μm in diameter (PM10),11 and Gross National Income per capita (GNI).12 High altitude was defined as ≥300m. High PM10 was defined as ≥50μg/m3. Data for altitude and air pollution were available at centre level whereas the GNI data were only available at country level. Non-affluent countries were those classified by the World Bank as low, lower middle, or upper middle income, and affluent countries are those classified as high income. The maps (Figs. 1–3) illustrate the distribution of low and high values of prevalence and GNI, altitude and PM10. For current symptoms of asthma (current wheeze) and current symptoms of eczema, high prevalence centres were defined as those with prevalence of 10% or greater. For current symptoms of rhinoconjunctivitis, high prevalence centres were defined as those with prevalence of 15% or greater for the 13–14-year age group and 7.5% or greater for the 6–7-year age group. These definitions are based on the levels used in the maps presented in the ISAAC prevalence papers.1–4,6–8 Spearman correlation (rho) was used to study the association of altitude, latitude and longitude with the prevalence of current symptoms of asthma, rhinoconjunctivitis and eczema.

Figure 1.

Geospatial distribution of prevalence of current symptoms of asthma and country income, 13–14-year age group. Each symbol represents a centre. The four symbols depict different levels of prevalence of symptoms combined with their categorisation as affluent or non-affluent. Red stars indicate high prevalence of current symptoms of asthma (≥10%) and affluent (GNI>$9265); yellow diamonds indicate high prevalence of current symptoms of asthma and non-affluent; green circles indicate low prevalence of current symptoms of asthma and affluent; and blue squares indicate low prevalence of current symptoms of asthma and non-affluent. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

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Figure 2.

Geospatial distribution of prevalence of current symptoms of asthma and altitude, 13–14-year age group. Each symbol represents a centre. The four symbols depict different levels of prevalence of symptoms combined with their categorisation as low or high altitude. Red stars indicate high prevalence of current symptoms of asthma (≥10%) and high altitude (≥300m); yellow diamonds indicate high prevalence of current symptoms of asthma and low altitude; green circles indicate low prevalence of current symptoms of asthma and high altitude; and blue squares indicate low prevalence of current symptoms of asthma and low altitude. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

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Figure 3.

Geospatial distribution of prevalence of current symptoms of asthma and particulate matter (PM10), 13–14-year age group. Each symbol represents a centre. The four symbols depict different levels of prevalence of symptoms combined with their categorisation as low or high (PM10). Red stars indicate high prevalence of current symptoms of asthma (≥10%) and high PM10 (≥50μg/m3); yellow diamonds indicate high prevalence of current symptoms of asthma and low PM10; green circles indicate low prevalence of current symptoms of asthma and high PM10; and blue squares indicate low prevalence of current symptoms of asthma and low PM10. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

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Spearman's correlation (rho) was employed to determine associations between current symptoms of asthma, rhinoconjunctivitis and eczema, with centre level altitude, latitude, longitude and PM10, and with country level GNI, in 6–7-year and 13–14-year age groups.

This is the largest study of asthma, rhinoconjunctivitis and eczema ever conducted, and the first time that the relationship between the three diseases, and various geospatial and geo-climatic phenomena has been examined. ISAAC Phase Three was performed in all WHO regions thus the main continents and ethnic areas of the world were represented in the study.

The large variability of prevalence between regions, continents, countries, centres in the same country, and even between centres in the same city (as in Mexico City) is probably the main and most consistent finding of ISAAC.6–8 The marked difference of prevalence between centres in the same city, as occurred in Mexico City, has also been found amongst New York neighbourhoods, with a 3–19% difference in asthma prevalence across the city associated with variations in allergen exposures.13 This highlights the heightened impact of still undefined local environmental or ecologic factors present at each centre, which may determine varying epidemiological and clinical expressions of diseases in populations at similar settings.14–16 The latter, i.e. the influence of local variation in exposures on prevalence, seems to be supported by the large variability found amongst centres in the prevalence of allergen sensitisation 17 and airway hyperresponsiveness.18

Although the whole study found a subtle relationship between GNI and overall symptom prevalence, in large regions such as Latin America (56 centres in 17 countries), which accounted for about 25% of all the children surveyed by ISAAC Phase Three in the world,19 there was not a significant correlation between GNI and prevalence. Using the GNI or other country-average index to study the effect of socioeconomic status on the prevalence of current symptoms, especially when including centres from affluent and non-affluent countries, may be misleading, since the income distribution in developing countries is highly unequal. Therefore, assigning the GNI of a developing country to all centres in that country would be inadequate because the risk of losing the large differences in income distribution between centres in developing countries, and regions.20 Until socioeconomic data at an individual level are available for all participating centres, the effect of socioeconomic development on the prevalence and severity of asthma, rhinoconjunctivitis and eczema will be bedevilled by this methodological issue. The results of studies looking at the relationship between socioeconomic status and prevalence of symptoms vary from an inverse association,21–24 to no association25 to a positive association26; apart from methodological considerations which may reflect the lack of proper and standardised instruments to assess such associations at centre or individual level.

Higher or lower prevalence of current symptoms of asthma, rhinoconjunctivitis and eczema occurred regardless of whether centres were located in developing or developed countries or regions,1,2,5–8 and this was consistent with the findings of ISAAC Phase One.2 For instance, in Africa the 13–14-year age group had the same mean prevalence of current wheezing as Western Europe (14%); similar prevalence of rhinoconjunctivitis and eczema also occurred between developing and developed countries, as shown in Tables 1 and 2. In general, most centres with a high or low prevalence in Phase Three tended to remain close to the level of prevalence they had in Phase One; although there were some centres showing important increases or decreases between the two phases. In areas sharing the same language but different socioeconomic development, the prevalence of current asthma symptoms was higher in developing than developed countries (i.e., Iberian Peninsula vs. Latin America).19

The inclusion of many more centres from developing countries in Phase Three than in Phase One is an important achievement of ISAAC as the proportion of the world population that lives in developing areas is about 80%. This is important because several theories on asthma causation are related to the protective effect of factors usually associated with low socioeconomic status. Considering that conditions of poverty have been persistently affecting populations in developing areas for a long time, without dramatic changes in the last 20 years, a lower prevalence of asthma, rhinoconjunctivitis, and eczema would have been expected in children and adolescents from those low-resourced countries. The participation of many more centres from developing countries certainly provides a more realistic representation of the world prevalence of asthma, rhinoconjunctivitis and eczema in childhood. At present, and thanks to the information gathered by ISAAC, the old statement that asthma was just a disease of industrialised and developed countries can be considered simply as an exclusion bias.

As in Phase One, most symptomatic children had symptoms of only one disorder in the previous year, which indicates that many different risk factors may be required for the clinical expression of these related disorders or may involve different time of occurrence, latency periods and time trends.

Although the proportion of subjects having the three diseases simultaneously was relatively low, those children probably represent a highly demanding group for specialised health care. Furthermore, it is likely that as seen in daily practice, coexistence of these diseases may be associated with greater functional impairment in terms of activity limitation and lower quality of life as compared to children who have one condition alone. Co-morbidities are important for clinicians treating asthma as they may be markers of patients at risk of poor outcomes, they may point to specific effective treatment options and they are important to researchers as possible confounding factors in clinical trials.

The effect of geo-climatic factors on the current symptom prevalence should be interpreted cautiously because of the dynamic nature of climatic factors that sometimes act independently of latitude and longitude, as is apparently occurring in this last decade. Consequently, geo-climatic factors should be considered as part of a much more complex model interacting with several other ecological conditions which also could be altered by seasonal or yearly changes. As other authors previously found,27,28 ISAAC Phase Three found correlations between some geo-climatic factors and the prevalence of asthma, rhinoconjunctivitis and eczema; however, the interpretation and time-dependency of such effects remains difficult to understand.

The complex relationships between air pollution and symptoms of asthma, rhinoconjunctivitis and eczema probably cannot be elucidated from data as those obtained by ISAAC. Further information on the type, magnitude and temporality of exposures, which also consider variations caused by geo-climatic (i.e. seasonal) factors, should help to a better understanding of the true effects of air pollution on the prevalence of asthma, rhinoconjunctivitis and eczema in children.

Protection of human and animal subjects. The authors declare that no experiments were performed on humans or animals for this investigation.

Right to privacy and informed consent. The authors declare that no patient data appears in this article.

Confidentiality of data. The authors declare that no patient data appears in this article.

The named authors declare that they have no conflict of interest.

Currently the main source of funding for the ISAAC International Data Centre (IIDC) is the New Zealand Lottery Board. Many New Zealand funding bodies have contributed support for the IIDC during the periods of fieldwork and data compilation (the Health Research Council of New Zealand, the Asthma and Respiratory Foundation of New Zealand, the Child Health Research Foundation, the Hawke's Bay Medical Research Foundation, the Waikato Medical Research Foundation, Glaxo Wellcome New Zealand, the Auckland Medical Research Foundation and Astra Zeneca New Zealand). Glaxo Wellcome International Medical Affairs supported the regional coordination for Phase Three and the IIDC as well as The BUPA Foundation. Without help from all of the above, ISAAC would not have been such a global success.