Academic journal article Environmental Health Perspectives

Estimated Effect of Temperature on Years of Life Lost: A Retrospective Time-Series Study of Low-, Middle-, and High-Income Regions

Academic journal article Environmental Health Perspectives

Estimated Effect of Temperature on Years of Life Lost: A Retrospective Time-Series Study of Low-, Middle-, and High-Income Regions

Article excerpt

Introduction

Ambient temperature is an important determinant of health. Many studies have reported that mortality and morbidity increase with very high and/or very low ambient temperatures (e.g., Analitis et al. 2008; Baccini et al. 2008; Basagana et al. 2015; Basu and Samet 2002; Gasparrini et al. 2015; Kenny et al. 2010). Although evidence on the temperature-health association in wealthier countries is well established, research is still emerging on this topic from low- and middle-income countries (LMICs) (Azhar et al. 2014; Burkart et al. 2014; Diboulo et al. 2012; McMichael et al. 2008; Mrema et al. 2012). Because weather-related health outcomes are dependent on local contexts, the large volume of research generated on the temperature-health association in wealthier countries may not necessarily reflect the burden from temperature exposure in LMICs. In contrast to high-income countries, the burden of disease in LMICs is often characterized by higher youth mortality rates and high prevalence of communicable diseases. Health impacts from heat and cold in LMICs may therefore occur through causal pathways other than those identified in high-income countries. For example, lower life expectancy in low-income countries might limit the pool of vulnerable elderly individuals, who suffer disproportionately from heat-health risks (Kenny et al. 2010). In LMICs, a large proportion of the population is working in physically demanding jobs on the streets, in agriculture, or on construction sites. Greater occupational exposure to environmental risk factors in these countries might lead to increased adverse health outcomes in comparison with the outcomes in wealthier countries.

A second observation of the literature regarding temperature-health associations is that most research uses the total number of daily deaths as the primary outcome (Bunker et al. 2016; Hajat and Kosatky 2010). Daily deaths are attractive to use for a number of reasons, including the accessibility of records (in some, but not all, jurisdictions), as well as high interpretability of study findings. Death counts, however, do not necessarily provide an ideal representation of the total mortality burden attributable to high and low temperatures. A major shortcoming of using daily death counts as the outcome is that this measure does not indicate the extent to which lives are shortened by exposure to heat or cold. At short time scales, such as days to weeks, many studies suggest that a fraction of the attributable mortality to heat and cold is from individuals who would have died anyway within days or weeks, a phenomenon known as mortality displacement (e.g., Hajat et al. 2005). For these decedents, it may be more appropriate instead to largely attribute their death to pre-existing ill-health condition.

Beyond short-term displacement, an additional drawback of using total death counts is that the loss of a young person's life is equally summed in the total burden as the loss of an old or elderly person' s life. Although all lives have value, the loss of a young person's life leads to a greater potential loss of societal contributions (Rocklov et al. 2009).

One approach to avoid these drawbacks is to consider years of life lost (YLL) as the outcome measure instead of death counts. YLL is an indicator of premature mortality used as a global burden of disease death metric (Lopez et al. 2006). The YLL approach accounts for the age at which death occurred by giving greater weight to deaths at a younger age. YLL is a compound measure combining the number of daily deaths with age at death; it is considered a more informative and differentiated measurement for assessing premature mortality than total or age-specific mortality rates alone. A comparison of health burdens by different exposures can facilitate risk ranking for preventive interventions, public health planning, and resource allocation (Aragon et al. 2008; Gardner and Sanborn 1990; Huang et al. …

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