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We read with great interest the recent paper by Martins et al1 concerning the influence of socioeconomic conditions on air pollution adverse health effects in elderly people in Sao Paulo, Brazil. These results are very interesting and may promote understandings of which social category of people are most sensitive to air pollution. The authors suggest that socioeconomic deprivation represents an effect modifier of the association between air pollution and respiratory deaths in elderly people for an increase of 10 µg/m3. They conclude that poverty represents an important risk factor that should be taken into account when determining the health consequences of environmental contamination. We agree with these conclusions. Nevertheless, the question is to know if poor people died because they are more ill, or because of inaccessibility (geographical and economic considerations) to the healthcare system, or because they were more exposed to air pollution?
We know that people with lower socioeconomic status are more sensitive to a large number of risk factors according to different life habits, or to addictive behaviours, such as smoking habits.2 When air pollution is considered, socioeconomic characteristics as an effect modifier can take two aspects. Firstly, people with low socioeconomic status may be more sensitive in terms of health effect because they have associated diseases and people with certain diseases had a greater risk of dying during an episode of increased of air pollution than did members of the general population.3 Furthermore, people living in underprivileged sectors would have both more limited access to health care4 and greater exposure to air pollution. Exposure condition is the second aspect of the interpretation of the effect modifier. Jerrett et al5 argue, low socioeconomic conditions may be associated with manufacturing and so with a higher workplace exposures, but also with a lower mobility. In addition, persons with lower socioeconomic characteristics may be exposed to a complex mix of pollution from indoor sources, as well as outdoor pollution because of traffic, industry, and waste burning in developing countries. It seems necessary to explore the link between individual exposure and socioeconomic characteristics because these two factors are strongly correlated.
More studies are needed to investigate this effect modifier and particularly the signification of this effect. To understand this effect we will need individual data on risk factor but also data on individual exposure to have a good interpretation of the results and to have policy implications.
The comments of Filleul and Harrabi on our paper reflect the major concerns about the role of socioeconomic conditions in the association between air pollution and health effects and keep the topic on discussion. Low socioeconomic status evolves different and complementary aspects that can act synergistically to aggravate health conditions. For instance, being more vulnerable to diseases and having less access to health (geographical and economic considerations) are factors that, in general, are concurrent among poor people and can contribute to death. In addition, if they also are more exposed to air pollution, and probably not only air pollution (indoor and outdoor) but soil and water pollution, we have the whole picture of what it is to be in the lowest socioeconomic levels of any society. If the discussion focuses only on levels of exposure it reflects the concept of linear dose-response relation between PM10 and respiratory diseases that is well known and accepted.1 In our paper we showed that the different effect size of PM10 on respiratory deaths for each studied region, measured as a percentage increase in mortality for each 10 μg/m3 of PM10, is more correlated with socioeconomic conditions than with the different levels of PM10 concentration in each region. However, we agree with Filleul and Harrabi that different study designs can provide information on individual exposure status to clarify the contribution of socioeconomic status on the association between air pollution and health effects. We are developing additional studies focusing on the analysed populations looking for answers to the questions our study brought up. Nevertheless, the results already presented are enough to suppose that “different people” may react differently to the same risk factor.
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