Heat-related mortality during the summer of 2022 may have exceeded 70,000 deaths, according to a study published in the scientific journal The Lancet Regional Health. The team from the Barcelona Institute for Global Health (ISGlobal) revised previous estimates associated with record temperatures during 2022.
In an earlier study, the same team used weekly temperature and mortality data in 823 regions in 35 European countries to estimate around 62,000 heat-related deaths in 2022. The group acknowledged that using weekly data would likely underestimate heat-related mortality and pointed out that daily data would be more accurate to estimate the impact of high temperatures on mortality. However, this data is not always available.
To address this problem, the team developed a theoretical framework to quantify errors using aggregated data, including weekly and monthly temperature data. Models based on aggregated data are helpful because this data is easily and quickly available from real-time institutions such as Eurostat, for example. To develop the new framework, the research team aggregated data from 147 regions in 16 European countries and then compared the estimates of heat- and cold-related mortality by different levels of aggregation: daily, weekly, 2-weekly, and monthly.
The results detected significant differences according to the time scale used. Using weekly, 2-weekly, and monthly models underestimated the effects of temperature (heat and cold) compared to a daily model. What’s more, the degree of underestimation was higher with increasing length of the aggregation period. For example, from 1998 to 2004, the daily model estimated 290,140 deaths due to cold and 39,434 deaths due to heat, while the weekly model showed about 10-20% lower results.
The researchers then used this new framework to revise their earlier study’s mortality rates attributed to record temperatures in 2022. The results showed that previous values underestimated heat-related mortality by about 10%, which means the actual values were closer to 70,000 deaths caused by high temperatures.
“In general, we do not find models based on monthly aggregated data useful for estimating the short-term effects of ambient temperatures,” explains Joan Ballester Claramunt, the ISGlobal researcher who leads the European Research Council’s EARLY-ADAPT project. “However, models based on weekly data do offer sufficient precision in mortality estimates to be useful in real-time practice in epidemiological surveillance and to inform public policies such as, for example, the activation of emergency plans for reducing the impact of heat waves and cold spells.”
Nevertheless, there are situations where researchers need to use weekly data because it’s often challenging to design large-scale studies based on daily data. In this situation, weekly data offers a good approximation, but it’s essential to understand its limitations.
Ballester J, van Daalen K, Chen Z et al (2023) The effect of temporal data aggregation to assess the impact of changing temperatures in Europe: an epidemiological modelling study. The Lancet Regional Health, https://doi.org/10.1016/j.lanepe.2023.100779