Academic journal article Environmental Health Perspectives

Effect of Previous-Winter Mortality on the Association between Summer Temperature and Mortality in South Korea

Academic journal article Environmental Health Perspectives

Effect of Previous-Winter Mortality on the Association between Summer Temperature and Mortality in South Korea

Article excerpt

BACKGROUND: It has recently been postulated that low mortality levels in the previous winter may increase the proportion of vulnerable individuals in the pool of people at risk of heat-related death during the summer months.

OBJECTIVES: We explored the sensitivity of heat-related mortality in summer (June-August) to mortality in the previous winter (December-February) in Seoul, Daegu, and Incheon in South Korea, from 1992 through 2007, excluding the summer of 1994.

METHODS: Poisson regression models adapted for time-series data were used to estimate associations between a 1[degrees]C increase in average summer temperature (on the same day and the previous day) above thresholds specific for city, age, and cause of death, and daily mortality counts. Effects were estimated separately for summers preceded by winters with low and high mortality, with adjustment for secular trends.

RESULTS: Temperatures above city-specific thresholds were associated with increased mortality in all three cities. Associations were stronger in summers preceded by winters with low versus high mortality levels for all nonaccidental deaths and, to a lesser extent, among persons [greater than or equal to] 65 years of age. Effect modification by previous-winter mortality was not evident when we restricted deaths to cardiovascular disease outcomes in Seoul.

CONCLUSIONS: Our results suggest that low winter all-cause mortality leads to higher mortality during the next summer. Evidence of a relation between increased summer heat-related mortality and previous wintertime deaths has the potential to inform public health efforts to mitigate effects of hot weather.

KEY WORDS: high temperature, mortality, preventive heath services, South Korea, weather. Environ Health Perfect 119:542-546 (2011). doi:10.1289/ehp.1002080 [Online 13 January 2011]

The association between elevated temperatures and mortality has been reported since the early 20th century. For example, Gover (1938) reported excess deaths associated with elevated ambient temperature exposure in 86 U.S. cities from 1925 to 1937. Studies of army recruits published in the 1940s (Schickele 1947) and 1950s (Stallones et al. 1957) also described an association between ambient heat exposure and mortality. Heat is undeniably a natural hazard and can have a pronounced effect on human health.

Concerns about the public health threat of elevated temperatures have increased in recent years, especially considering the potential impacts of climate change and the increased heat island effects in urban settings. The Intergovernmental Panel on Climate Change (2007) reported that climate change is likely to lead to more intense and frequent extreme weather events and that human exposure to such changing weather patterns could result in increased deaths, diseases, and suffering.

To understand the public health implications of heat effects, it is important to estimate the extent to which heat-related deaths in already frail individuals are simply hastened by heat exposure, a phenomenon referred to as mortality displacement (or "harvesting") (Kovats and Hajat 2008). This process has been shown to occur on a short-term basis (within days or weeks of heat exposure) (Braga et al. 2001; Hajat et al. 2005; Kalkstein and Greene 1997; Kysely and Kriz 2008), but evidence that summer-time deaths after the Paris heat wave in 2003 may have influenced mortality the next winter suggest longer term effects as well (Valleron and Boumendil 2004). Only two previous studies have analytically assessed the association between mortality during winter and mortality during the next summer (Rocklov et al. 2008; Stafoggia et al. 2009), and both reported that low levels of winter mortality were associated with higher estimated temperature effects the next summer.

We previously reported that mortality increases with temperatures above city-specific thresholds during the hot season in six major South Korean cities (Kim et al. …

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