The central impression from the review of the research literature on indoor air pollution as a cause of ARI among children in developing countries is the imbalance between the number of articles reporting on original, empirical research, on the one hand, and the number of calls for research, reviews and projections based on those articles, on the other. Figure 2 illustrates the discrepancy between the research that is done and the global needs for research, where the smallest proportion of the research is done where the results could potentially have the greatest impact on the health of populations. This is an illustration of what has been labeled the 10/90 gap (ie, that 10% of health research resources are said to go to 90% of the disease burden).
A majority of the empirical studies indicate that increased ARI risk is associated with indicators— measurement or proxy—of individual exposure to indoor air pollution. Not all of the studies are reported in such a way that it is possible to draw clear conclusions, even allowing for small samples, but results from only one study are negative. One study is inconclusive because of homogeneity of exposure and small sample size. Otherwise, the tendency is consistently toward an association. Recurring problems are small samples and the fact that studies were not primarily designed to address the ARI/air pollution question, but included data on air pollution indicators for at least a subset of the study subjects. However, several have a well-planned design and use plausible methods for diagnosis and exposure classification.
A PubMed search including combinations of terms such as indoor air pollution, biofuel smoke vs terms pneumonia, ARI and child health, and developing country yields a total of 160 articles published from 1985 to 2007. Weeding out those that do not actually address the association between indoor air pollution and ARI, the total number comes down to 44. Figure 2 illustrates the yields using the terms indoor air pollution, child ARI, and developing country singly and the meager overlap.
As the search question is narrowed, the proportion of original research articles comes down and the proportion of reviews goes up. Of the articles that address indoor air pollution in developing countries in relation to ARI, nearly half (19 of 44 articles) are review articles or other nonempirical publications. Among the empirical articles, 16 report epidemiologic studies of ARI in relation to air pollution and 9 are descriptive studies addressing exposure conditions or measurements of concentrations of specific pollutants.
Besides diarrheal diseases, acute respiratory infection (ARI) constitutes one of the major groups of causes of death among children in developing countries and therefore globally. According to estimates for 2000 to 2003 presented in the 2005 World Health Report, acute respiratory disease accounts for 19% of total deaths in children < 5 years of age, making ARI the second-most-common cause of death in that age group, after neonatal causes, and slightly ahead of diarrheal diseases. While ARI contributes 2 to 4% of deaths in children < 5 years of age in the low-mortality member states, these causes contribute 19 to 21% of child deaths in the Eastern Mediterranean, Africa, and South East Asia regions, and 12 to 14% in the high-mortality countries of Europe, the Americas, and the Western Pacific region.