An article published in the prestigious UK medical journal The Lancet in July this year has challenged the commonly held belief that extremely warm weather is linked to higher death rates than cooler weather. A research team led by Antonio Gasparrini, from the Department of Medical Statistics at the London School of Hygiene and Tropical Medicine, collected data from more than 380 locations in 13 countries including Australia, Brazil, Canada, the United Kingdom, and the United States. Following an analysis of more than 74 million deaths between 1985 and 2012, colder weather was attributed to more deaths than warmer weather by approximately 20 times. The study was based on the largest dataset ever collected to assess associations between temperature and mortality risk. Such a large dataset also provided evidence from a wide range of climates and populations with different demographic, socioeconomic and infrastructure characteristics.
Gasparrini and his team reviewed numerous epidemiological studies that reported evidence of a link between both ambient temperatures and mortality or morbidity. They discussed the consensus among researchers that both extremely cold and hot temperatures affect health, and the debate around the importance of both extremes (particularly the optimum temperatures that correspond to minimum effects for a number of health outcomes). The aim of the research was to quantify the total mortality burden attributable to non-optimum ambient temperature and the relative contributions of hot / cold / moderate / extreme temperatures.
Time series daily data including mortality, weather variables and air pollution concentrations were collected from each location. Mean daily temperatures from central monitoring stations were chosen as the exposure index and calculated as an average between the maximum and minimum values or the 24 hour average. A sensitivity analysis was conducted, with both air pollution and humidity controlled. The data was then subjected to a series of statistical tests.
Risk increased slowly and in a linear fashion for cold temperatures below the minimum mortality temperature
The results showed a large range of temperatures from 6.5°C to 27.6°C in Canada that were typical of cold, temperate, tropical and sub-tropical climates. Risk increased slowly and in a linear fashion for cold temperatures below the minimum mortality temperature. In the cases of London and Madrid, there were higher increases in mortality under extreme cold conditions than in the other locations. The highest risks of mortality in cold temperatures were reported from Italy, China and Japan. The lowest risks were reported from Thailand, Brazil and Sweden. Less than 1% of the deaths were attributed to extreme hot or cold temperatures. In contrast, 7.3% of the deaths were attributed to colder temperatures.
In previous studies, a range of underlying mechanisms have been linked to the increased mortality risk associated with exposure to high and low ambient temperatures. Physiological effects leading to heat related deaths are not yet well understood and may be highly variable for different causes of mortality. In the case of heat and cardiovascular mortality, acute events appear to be triggered once the body exceeds its thermoregulatory threshold. Heat has also been reported to increase the risk of mortality for other causes. One mechanism for this could be the alteration of fluid and electrolytic balance in people affected by chronic disease.
The biological processes underlying mortality in cold temperatures have been reported to have predominantly cardiovascular and respiratory effects. Exposure to cold has been associated with cardiovascular stress by affecting blood pressure, blood viscosity and vasoconstriction. Cold temperatures have also been reported to suppress mucociliary defences and other immunological reactions, which increases the risk of respiratory infections. Such responses can persist longer than for the effects attributed to heat.
The authors also noted that research into the association between human health and ambient temperature has predominantly focused on extreme heat, and public health plans have implemented policies and interventions designed for extreme heatwave conditions. The authors recommend that public health policies and adaptation measures should take into account the full range of effects associated with temperature. Further research is also needed to clarify the extent to which excess mortality related to temperature is preventable. In conclusion, the authors also recommend that an estimate of the health consequences of a range of a climate change scenarios needs to be addressed.