5.13 Environmental factors and the global burden of disease

In different ways and to different degrees, each of the disease and disability categories discussed above owe their presence to one environmental factor or another. A summary portrayal of the environmental portion of the global burden of disease is given in Table 5.14. Applying these percentages across the diseases and conditions indicated leads to an estimate of the total DALY burden that is associated with environmental factors. This is 320470 or 23% of the world's total DALY burden.

The percentages shown reflect how important these factors are in causing the burden and indicate how much their control would contribute to its reduction. Thus, even though many diseases (diarrhoeal diseases, malaria, measles, polio and tetanus) might be arguably 100% environmental, a lesser percent age is indicated because of the availability of effective preventive and curative care, including vaccines in some instances. On the other hand, no one would deny that it would be preferable, even for diseases for which effective vaccines are available, to improve the environment in order to reduce their transmission. However, for environmentally-mediated infectious diseases for which effective vaccines are not available, we attribute a high proportion to environment (e.g. ARI and malaria).

In our analysis of the approximate environmental contribution of the health conditions described in this chapter, we emphasize long-term sustainable prevention rather than curative measures. We also assume that environmental preventive actions are taken first. The environmental fraction is considered as the fraction of disease occurrence that could be averted through feasible environmental interventions, before application of other interventions. Using this approach, it was estimated that approximately 60% of the current global ARI disease burden could be avoided in a sustainable manner if environmental improvements were made to eliminate exposures to severe indoor air pollution in developing countries (see Section 5.2). Less crowding and improved housing conditions would also contribute to ARI reduction. The estimate is based on the very large potential for prevention shown in Fig. 5.1, the high level of air pollution exposures in populations in developing countries, the major ARI health risks calculated in Section 4.2 and the historical trends of these diseases (see Section 5.2). To achieve maximum impact, improvement of environmental conditions should be combined with improved nutrition and effective treatment of childhood pneumonia.

In the case of diarrhoeal diseases, our estimate of a 90% environmental contribution (Table 5.14) is based on the remarkable variation between different regions in DALYs per capita in (see Section 5.3), and the known environmental transmission pathways for these diseases. Although diarrhoeal deaths are averted effectively through ORT in the immediate term, the longer-term solution is to improve basic sanitation, water supply and food safety.

Table 5.14
Proportion of global DALYs associated with environmental exposures - 1990

N.E.: not estimated

Source: DALY data from Murray & Lopez, 1996b.

The vaccine-preventable infectious diseases and tuberculosis are all transmitted more rapidly in conditions of crowding and poor housing (see Section 5.4). Environmental improvements are of value, but are not as effective as vaccines. The environmental fraction was set at 10% (Table 5.14). Using similar considerations approximate environmental fractions were assigned to most of the disease groups described in this chapter.

When combined, these estimated environmental contributions add up to 23% of the global burden of disease (Table 5.14), but this figure does not take into account that "other diseases" and "intentional injuries" have some environmental associations. The proportion of all DALYs contributed by environmental DALYs to each disease (Table 5.14) creates numbers that can be compared with a previous estimate of the importance of different risk factors for the global burden of disease (WHO, 1996d). Few environmental "risk factors" were assessed, but estimates were made for "water and sanitation" (6.8%), "occupation" (2.7%) and "outdoor air pollution" (0.5%), among other "risk factors" such as "tobacco smoking" (2.6%). Only 40% of all DALYs were associated with the risk factors analysed, and several of the major disease problems listed in Table 5.14 (e.g. malaria and injuries) were not referred to at all. It is reassuring to note that the approach taken by WHO (1996d) resulted in very similar estimates to our own for diarrhoeal diseases (6.8% for "water and sanitation" in WHO, 1996d, versus 6.5% for diarrhoeal diseases in Table 5.14).

The diseases which contribute the most environmental DALYs (ARI, diarrhoeal diseases, malaria and unintentional injury) are also the diseases which particularly affect children. Using the percentage of all DALYs contributed by children under age 15 (Table 5.3), estimates of the "child component" of environmental DALYs can be made (Table 5.14). As seen in Table 5.14, this child component adds up to 15% of all DALYs or about two-thirds of environmental DALYs. Taking action to reduce environmental DALYs could thus make a major contribution to child health.

Ideally at this point in our analysis we would explore alternative future scenarios to better understand the importance of controlling the various environmental factors discussed. Unfortunately, we are too handicapped by the lack of suitable comparative data to be able to generate convincing scenarios. Instead, we turn to recently developed scenarios which project the global DALY burden to the year 2020 to see to what degree environmental factors have been taken into account.

As noted in Section 2.2, the world's population grew from 5300 million in 1990, and is expected to reach 6100 million by the year 2000, and 7700 million by 2020. In 2020, nearly 10% of the population will be over the age of 65. Cities are developing and growing fast. At present about half of humanity lives in urban areas; this is expected to rise to almost two-thirds by 2020.

The impact of these changes alone on the relative importance of different diseases and disabilities upon the total disease burden is expected to be profound. These changes are expected to contribute to new disease patterns, whereby non-communicable diseases come to the forefront in developing countries. The decline of infectious diseases is "implicitly based on the presumption that socioeconomic development will decrease disease incidence and severity, and/or that research and development will guarantee the availability of antibiotics effective against resistant strains of major pathogens" (Murray & Lopez, 1996b). This assumption may be considered very optimistic in light of the severe constraints outlined in earlier chapters; e.g. increased waste production and pollution (Section 3.2), water scarcity (Section 3.3), land limitation (Section 3.4), and global environmental change (Section 4.9). A major commitment to socioeconomic development for the poor, incorporating environmental improvements and major investments in water, sanitation and other infrastructure is needed to achieve the assumed improvements in health.

In projecting DALYs for the year 2020, Murray and Lopez 1996b developed base-line, optimistic and pessimistic scenarios. Underlying mortality rates were derived from regression equations relating age-, sex- and cause-specific mortality rates to four determinants of mortality: income per capita, human capital (average number of years of schooling of the population), smoking intensity and time. There is a wide range of difference between the optimistic future and the pessimistic future for the infectious disease groups. Road traffic accidents (a sub-group of the injuries groups) are higher in number in the "optimistic" scenario than they are in the "pessimistic" scenario. The underlying statistical approach used in the making of these projections forces this outcome; wealth means more cars and more cars means more accidents. Similarly, where development and technology lag, incidence of infectious diseases will increase proportionately. None of these projections takes into account the nature of specific interventions that might be undertaken to reduce risks or hazards (with the exception of smoking which is built into the model directly).

While specific environmental interventions have not been factored into these scenarios, the DALY projections nevertheless provide an interesting starting point for generating alternative scenarios which do incorporate environmental factors, both in terms of the prevalence and incidence of various conditions as well as in terms of the potential for interventions to modify these factors. The potential detrimental consequences of global environmental change on human health worldwide makes it even more imperative that future scenarios take these consequences explicitly into account.