Research Article: Mortality attributable to hot and cold ambient temperatures in India: a nationally representative case-crossover study

Date Published: July 24, 2018

Publisher: Public Library of Science

Author(s): Sze Hang Fu, Antonio Gasparrini, Peter S. Rodriguez, Prabhat Jha, Madeleine Thomson

Abstract: BackgroundMost of the epidemiological studies that have examined the detrimental effects of ambient hot and cold temperatures on human health have been conducted in high-income countries. In India, the limited evidence on temperature and health risks has focused mostly on the effects of heat waves and has mostly been from small scale studies. Here, we quantify heat and cold effects on mortality in India using a nationally representative study of the causes of death and daily temperature data for 2001–2013.Methods and findingsWe applied distributed-lag nonlinear models with case-crossover models to assess the effects of heat and cold on all medical causes of death for all ages from birth (n = 411,613) as well as on stroke (n = 19,753), ischaemic heart disease (IHD) (n = 40,003), and respiratory diseases (n = 23,595) among adults aged 30–69. We calculated the attributable risk fractions by mortality cause for extremely cold (0.4 to 13.8°C), moderately cold (13.8°C to cause-specific minimum mortality temperatures), moderately hot (cause-specific minimum mortality temperatures to 34.2°C), and extremely hot temperatures (34.2 to 39.7°C). We further calculated the temperature-attributable deaths using the United Nations’ death estimates for India in 2015. Mortality from all medical causes, stroke, and respiratory diseases showed excess risks at moderately cold temperature and hot temperature. For all examined causes, moderately cold temperature was estimated to have higher attributable risks (6.3% [95% empirical confidence interval (eCI) 1.1 to 11.1] for all medical deaths, 27.2% [11.4 to 40.2] for stroke, 9.7% [3.7 to 15.3] for IHD, and 6.5% [3.5 to 9.2] for respiratory diseases) than extremely cold, moderately hot, and extremely hot temperatures. In 2015, 197,000 (121,000 to 259,000) deaths from stroke, IHD, and respiratory diseases at ages 30–69 years were attributable to moderately cold temperature, which was 12- and 42-fold higher than totals from extremely cold and extremely hot temperature, respectively. The main limitation of this study was the coarse spatial resolution of the temperature data, which may mask microclimate effects.ConclusionsPublic health interventions to mitigate temperature effects need to focus not only on extremely hot temperatures but also moderately cold temperatures. Future absolute totals of temperature-related deaths are likely to depend on the large absolute numbers of people exposed to both extremely hot and moderately cold temperatures. Similar large-scale and nationally representative studies are required in other low- and middle-income countries to better understand the impact of future temperature changes on cause-specific mortality.

Partial Text: Most of the epidemiological studies that have examined the detrimental effects of ambient hot and cold temperatures on human health have been conducted in high-income countries [1,2]. A surprising conclusion has been that much of the excess mortality from temperature effects arises due to moderately cold temperatures [2]. Populations in low- and middle-income countries (LMICs) such as India are particularly vulnerable to adverse climate conditions due to having fewer physical adaptive measures such as air conditioning or heating, and less access to medical treatment for temperature-related health sequelae.

The MDS recorded a total of 591,121 deaths from 2001–2013 from all causes, of which we geocoded 546,360 with complete death information and assigned them to the six climate regions. The main analyses focused on 411,613 medical deaths at all ages from 2001–2013 from six climate regions (Fig 1). The first-stage regional estimates showed distinctive temperature–mortality association by climate region for medical deaths (Fig A-D in S1 Appendix). Regions with lower death counts (e.g., Am, Bwh, Cwb) had higher uncertainty in their estimates. For the pooled estimates, most of the relevant medical conditions affected by temperature occurred above age 30 years. Mortality from all medical causes at ages 30–69 years showed excess risks at moderately cold temperature and hot temperature (Fig 2, middle panel). The lowest mortality risk was at 30°C and rose sharply at hot temperatures, averaging about a 9% excess risk with every degree temperature increase from 35°C to 40°C. Similarly, the all-medical mortality risk rose with lower temperature, averaging about a 3% excess risk with every degree drop in temperature from 30°C to 16°C, at which point the risks inflected and decreased. This decreasing risk at extremely cold temperature might be an artifact of the spline parameterization and due to the small number of deaths occurring on extremely cold days (Fig P in S1 Appendix). Mortality risks were similar for all ages (Fig 2, top panel) and ages 70 years and above (bottom panel), but with significant and steeper risks for hot temperature for ages 70 years and above.

We demonstrate that cold temperatures contributed to higher attributable risks of mortality than hot temperatures in India during 2001–2013, consistent with previous findings based on nonlinear temperature–mortality associations drawn from mostly high-income countries [2]. These previous findings reported an overall attributable risk of 7.29% (eCI 7.02 to 7.49) for cold temperature and 0.42% (0.39 to 0.44) for hot temperature, which are similar to our results for all ages (Table C in S1 Appendix). Among countries, our results are most comparable to those of Australia.



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