Academic journal article Environmental Health Perspectives

Heat Wave and Mortality: A Multicountry, Multicommunity Study

Academic journal article Environmental Health Perspectives

Heat Wave and Mortality: A Multicountry, Multicommunity Study

Article excerpt

Introduction

Globally, there is increasing interest in assessing heat wave-related health effects after major heat waves in the United States and Europe, for example, Chicago in 1995 and Europe in 2003 (Le Tertre et al. 2006; Semenza et al. 1996). Several recent studies have linked heat waves with significant impacts on human health, including mortality (Kent et al. 2014; Li et al. 2015; Wang et al. 2015). The frequency, intensity, and duration of heat waves will increase in the future, which has the potential to greatly exacerbate the health impacts of heat (Field et al. 2014). Thus, understanding the relationship between heat waves and health is crucial for better adaptation and mitigation strategies (Meehl and Tebaldi 2004).

Recently, a few studies have suggested decomposing the health impacts of heat waves into a) effects of high temperature through modeling daily temperature as a numeric explanatory variable, possibly allowing for lagged effects; and b) added effects of heat waves through modeling heat wave days as a categorical variable (Anderson and Bell 2009, 2011; Gasparrini and Armstrong 2011; Hajat et al. 2006). The rationale of this method assumes that the impacts of heat should be modeled as the sum of two items: a) daily temperature has independent impact on health; and b) heat waves lead to an additional risk due to several consecutive days. However, some studies reported that heat waves had added effects on mortality (Anderson and Bell 2009; Hajat et al. 2006; Rocklov et al. 2012), while others suggested that there were no added heat wave effects (Barnett et al. 2012; Gasparrini and Armstrong 2011). Therefore, it is not conclusive for existence and size of the added heat wave effects on mortality.

Studies have shown that associations between heat waves and health vary substantially by location (Anderson and Bell 2009, 2011; Guo et al. 2012). However, there are several challenges in comparison of the heat wave impacts on health from different climate zones or cities/regions/countries. For example, various studies used different heat wave definitions regarding the temperature threshold (e.g., 95th or 97.5th percentiles of temperature), temperature metric (e.g., minimum, mean, or maximum temperatures, or apparent temperature), and duration of heat wave in days (e.g., [greater than or equal to] 2, [greater than or equal to] 3, or [greater than or equal to] 4 d). Use of different time periods (e.g., summer, warm season, or whole year of data) also hinders comparison and synthesis of results across studies. Some studies used time-series analyses, while others used case-crossover designs or case-only studies to assess the health effects of heat waves. Also importantly, some studies considered lag periods of heat wave effects, while others did not, which makes it difficult to compare the cumulative effects of heat waves, because, indeed, heat waves could have a significant lag effect.

Therefore, it is important and necessary to conduct research in different regions/countries using the same statistical design to assess and compare the associations between heat waves and health events. Large studies have investigated the impact of heat waves on mortality in Asia (Son et al. 2012), the United States (Anderson and Bell 2011), and Europe (Baccini et al. 2008), but to the best of our knowledge, there are no previous studies of heat waves across the globe. We have recently established a multicountry, multicommunity (MCC) collaborative network to assess impacts of weather on mortality internationally (Gasparrini et al. 2015a, 2015b; Guo et al. 2014, 2016). This setting provides a unique opportunity to examine: a) the effects of heat waves on mortality in 400 communities in 18 countries/regions; b) effect modification by heat wave characteristics (intensity and duration); c) whether heat waves had added effects on mortality; d) regional differences in the heat wave effects; e) potential factors contributing to the heterogeneity of heat wave effects on mortality; and f comparison for heat waves defined by daily mean, minimum, and maximum temperatures. …

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