Hot Temperature Has Highest Mortality Risk, But Moderately Cold Temperature Contributed To The Most Deaths In India

Extreme temperatures, including cold spells and heat waves, have detrimental effects on human health. Most climate change projections foresee extreme weather becoming more prevalent. Indeed, various parts of the world have already begun to experience the impact of extremely hot weather. Much less attention has been given to the health effects of cold temperature ranges.

To tackle this knowledge gap, a recent Indian study estimated the association between extremely and moderately hot and cold temperatures with human mortality [1]. The study found that while extremely hot temperature had the highest mortality risk, moderately cold temperature contributed to the most deaths in India.

The study linked a nationally representative mortality dataset of 400,000 deaths with daily temperature data from 2001-13. The study examined all medical causes of death and deaths from ischaemic heart disease, stroke, and respiratory diseases among adults aged 30-69 years. Daily mean temperature (i.e., an average of the daily minimum and maximum temperatures) was used to quantify temperature–mortality associations. Temperatures were separately into extremely cold (0.4˚C to 13.8˚C), moderately cold (13.8˚C to roughly 30˚C), moderately hot (roughly 30˚C to 34.2˚C), and extremely hot ranges (34.2˚C to 39.7˚C). The study used a novel “case-crossover” method to examine the transient effect of temperature on mortality and applied non-linear models to quantify mortality risks.

The study found that at extremely hot temperature, excess mortality peaked within 0-1 day, but no significant excess risks were found beyond 4 days. By contrast, excess mortality from extremely cold temperature lasted for up to two weeks. Both moderately cold and extremely hot temperature was associated with significantly higher mortality risks for medical causes of death (Figure). Strokes had significantly higher mortality risk at extremely hot temperature. Ischemic heart disease, stroke, and respiratory diseases had significantly higher mortality risks at moderately cold temperature.

From 1981 to 2010, the proportion of days that were extremely cold or moderately cold fell by 7% and 2%, respectively, and the proportion of days that were moderately hot or extremely hot rose by 11% and 17%, respectively (Table). Combined with population growth, the numbers of Indians exposed to moderately hot and extremely hot days rose substantially by 67% and 76%, respectively, over the 30 years. However, most of the population exposure (and deaths) occur in the moderately cold range (Figure). Thus, 197,000 deaths from medical conditions were attributable to moderately cold temperature in 2015. Fewer deaths were attributable to extremely cold temperature (n=17,100) and to extremely hot temperature (n=4,700).

The study identified 30˚C as the daily mean temperature with the lowest mortality risk for all medical deaths (Figure). This temperature may seem quite high for people in the West who are acclimatized to milder climates, but it is common for most Indians who, over generations, have adapted to hotter climates. The mean daily temperature of 30˚C typically corresponds to the range of 25˚C (minimum) to 35˚C (maximum) within a 24-hour period. Thus, it is possible that night-time cooling effects can offset day-time heat effects.

A highlight of this study is its use of a nationally representative mortality dataset, the Million Death Study (MDS) [2]. The MDS is one of the largest studies of premature mortality in the world and monitors all births and deaths from 2001-2023 in over 3.7 million households over three specific time periods (2001-3 and each of 10 years thereafter) in India. The MDS employs a “verbal autopsy” method to capture deaths from both urban and rural areas, including many deaths that occur at home without receiving medical attention.  A verbal autopsy works by sending non-medical interviewers to households to ask living relatives key demographic and health questions about the deceased, including a half-page narrative of the symptoms before the death occurred. These data are then randomly assigned to two of 404 trained physicians (based on their ability to read the local language) who assign an underlying cause of death.

Initial disagreements in classification undergo anonymous reconciliation by both physicians. Further disagreements are resolved by one of 40 senior physicians. This study included a total of 411,613 deaths from the MDS, which is far greater than the sum of all earlier focal studies in India. Thus, the MDS provides a more complete picture of temperature effects on mortality in the sub-continent.

The study has implications for public health and clinical practice. Interventions in India should focus on the effects of moderately cold temperature, which contribute to the most deaths. Adults with existing vascular or respiratory disease are at higher risk, and strategies to adjust their medication or avoid cold exposure during cold time periods could reduce deaths. Attention should also be paid to extremely hot temperature, which has relatively higher mortality risk and is expected to become more severe with climate change. The study also points out the need for more nationally representative mortality studies like the MDS from low- and middle-income countries so as to directly document the effects of climate change on human health. Finally, the novel methodology in this study is now being considered to quantify the effects of air pollution in Canada.

These findings are described in the article entitled Mortality attributable to hot and cold ambient temperatures in India: nationally representative case-crossover study, recently published in the open access journal PLOS Medicine. This work was conducted by Sze Hang Fu, Peter S. Rodriguez, and Prabhat Jha from the Centre for Global Health Research, University of Toronto, and Antonio Gasparrini from the London School of Hygiene & Tropical Medicine.

References

1. Fu SH, Gasparrini A, Rodriguez PS, Jha P. Mortality attributable to hot and cold ambient temperatures in India: a nationally representative case-crossover study. PLoS Medicine. 2018;15(7):e1002619. doi: 10.1371/journal.pmed.1002619. PubMed PMID: 30040816; PubMed Central PMCID: PMC6057641.
2. Aleksandrowicz L, Malhotra V, Dikshit R, Gupta PC, Kumar R, Sheth J, et al. Performance criteria for verbal autopsy-based systems to estimate national causes of death: development and application to the Indian Million Death Study. BMC Medicine. 2014;12(1):1-14. doi: 10.1186/1741-7015-12-21.
3. Office of the Registrar General & Census Commissioner, India. Census of India 2011: Provisional Population Totals. New Delhi: Office of the Registrar General & Census Commissioner, India; 2011.