From the CO2Science Archive: Writing as background for their study, Sharafkhani et al. (2019) say that “the impact of Diurnal Temperature Range [DTR] on human health is not well understood.” And thus they set out to investigate the effect of DTR [the variation between the high and low air temperature during one day] on mortality in Tabriz, Iran, which is the capital of the East Azerbaijan Province and fifth largest Iranian city with a population of more than 1.6 million persons.
Paper reviewed: Sharafkhani, R., Khanjani, N., Bakhtiari, B., Jahani, Y., Tabrizi, J.S. and Tabrizi, F.M. 2019. Diurnal temperature range and mortality in Tabriz (the northwest of Iran). Urban Climate 27: 204-211.
The data utilized in the study included death classification codes for non-accidental, respiratory and cardiovascular deaths in Tabriz over the 6-year period 2010-2016, and meteorological data on minimum and maximum daily temperature (from which the DTR was calculated). To assess the effects of DTR on cause, age and gender specific mortality, Sharafkhani et al. employed Distributed Lag Non-Linear Models combined with a quasi-Poisson regression, while controlling for potential confounding effects from long-term daily mortality trends, day of week effect, holidays, mean temperature, humidity, wind speed and air pollutants.
The results of the analysis revealed there was no significant impact of DTR on mortality risk when examined over the whole year. When re-running the analysis for the hot (May-October) and cold (November-April) seasons of the year Sharafkhani et al. report, once again, that there was no significant impact of DTR on mortality risk during the warm season. However, in the cold season, the cumulative relative risk of non-accidental (NAD), respiratory (RD) and cardiovascular (CVD) death all increased at higher DTR values (95th and 99th percentiles). Consequently, the authors conclude that “the risk of NAD, RD and CVD increases in the cold season.”
The significance of this work is two-fold. First, the findings demonstrate that the risk of death increases when the weather is cold. Thus, a warming of temperature would likely result in a decline in the relative risk of death and produce a net saving of lives. Second, such mortality reductions are likely already occurring, as Sharafkhani et al. correctly note that “in most urban areas of the world, the DTR index is declining because of [recent increases in] minimum daily temperature [that are] mainly due to the urban heat island effect.” Thus, as societies continue to urbanize, the urbanization-caused reduction of DTR will likely be magnified, further reducing temperature-related mortality.