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The Metric

Jan 21, 2014

Alternative measures of environmental health impacts

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About a quarter of the total global burden of disease is attributable to remediable environmental causes. Typically measured as an aggregate of years of life lost among national populations, the causes of this composite social cost vary from air-pollution-related respiratory illness, to waterborne diarrhea, to the effects of toxins and some infectious diseases. In past iterations of the EPI, the indicator that tracked environmentally caused burden of disease — quantified by the World Health Organization (WHO) as disability-adjusted life years (DALYs) — was our sole measure in the issue category Environmental Health.

For many reasons, the EPI replaced DALYs with a measure of child mortality in 2012 EPI. Primary among them is that DALYs did not offer reliable time series data by which to chart national performance progress. Also, discrepancies that make accurately correlating exposure and risk from country to country called into question the project of sufficiently modeling the relationship between years lost and the environment. Starting in 2012, the EPI began using the WHO’s measure of child mortality. Probably most influential in this decision was the much greater correlation between environmental factors and the rate of death of children. While less than a quarter of overall premature mortality is the result of preventable environmental factors, deaths among children up to age 14 were environmentally caused up to 36 percent of the time.

There are a number of problems with too directly equating child mortality and environmental health, however. As DALYs also make clear, it is difficult to tease apart causes and solutions that are environmental in nature from those belonging to the realm of public health. For instance, many of the gains in pursuing the Millennium Development Goal (MDG) of lower rates of child mortality have been achieved through improving access to healthcare, vaccinations, and the distribution of clean cookstoves. Second, many environmental causes of disease occur in conjunction with other causes, a phenomenon known as comorbidity. Malnutrition, for instance, can exacerbate or be exacerbated by diarrhea. Third, because of the extreme difference in child mortality rates between industrialized countries and developing countries, child mortality can be seen as a proxy for level of economic development.

The relationship between the environment and human health is complicated by linkages to other causes and effects, including genetics, access to healthcare, level of economic development, and varying degrees of exposure. The latter complication is particularly cogent to understanding risk: two areas may contain identical levels of a pollutant, yet demographic, ecological, or infrastructural variations may lead to wildly different exposure rates among populations. It can also be difficult to unbundle some environmental effects—indoor and outdoor air pollution, for instance.1 For those reasons, the EPI prefers strict performance-based measures, like rates of child mortality, over modeled data. Still, it is quite possible that additional measurements can be developed.

For example, a spate of recent studies have investigated the benefits of green space, an open, usable commons, and parks on people’s physical and psychological well-being. This work has been supplemented by other research that explores the importance of local access to nutritional food to the health of populations. While these studies offer a different, and promising, lens for comprehending the effects of the environment and place on human health, until now the research has predominantly been applied to urban areas in the industrialized world. It is also more relevant to health impacts that are widely considered lifestyle-based, like obesity and heart disease.2

Some promise also lies in constructing aggregate indicators that can account for all the various effects of the environment on human health. Researchers from the Netherlands combined data on exposure levels, mortality and illness, and Dutch environmental reports to provide a picture of the relative effects of environmental conditions on national public health. They found that the overwhelming majority of environment-related health loss was due to outdoor air pollution. While their methods were permitted by specific conditions in the Netherlands, particularly as they relate to the consistency of data and demography, the research may serve as a model for future indicators of environmental health.3 Of course, the kind of data-collection required to construct a normalized global aggregate indicator is still a long way off.

For now, child mortality is the best global measure for the effects of the environment on human health. It may not be perfect, but it speaks with extreme clarity about the importance of a healthy environment to the most vulnerable among us.

1 Knol, A., Petersen, A. C., van der Sluijs, J. et. al. (2009) Dealing with uncertainties in environmental burden of disease assessment. Environmental Health 8:1-13.

2 Mitchell, R., Popham, F. (2008) Effect of exposure to natural environment on health inequalities: an observational population study. The Lancet 372:1655-1660.

3 De Hollander, A. E. M., Melse, J. M., Lebret, E. et al. (1999) An aggregate public health indicator to represent the impact of multiple environmental exposures. Epidemiology 10:606-617.