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Abnormal Heat Leads to Higher Mortality
Abnormal temperatures can exacerbate many diseases and lead to increased mortality. The prolonged heat wave in the summer of 2010 in Moscow with average temperatures above 30° and air pollution from wildfires in forests and peat bogs near the city affected people with a variety of diseases, from cardiovascular problems to cancer.
Increased mortality was observed mainly in people over 65, but also affected younger age groups, according to Tatyana Kharkova, Ekaterina Kvasha and their colleagues Dmitry Shaposhnikov and Boris Revich of the RAS Institute of Economic Forecasting, Elena Lezina of MosEcoMonitoring, and Swedish experts in environmental medicine. Their collective paper 'Mortality Related to Air Pollution with the Moscow Heat Wave and Wildfire of 2010' published in Epidemiology (2014, Vol.25. No.3, p. 359-364) is based on data from Rosstat and MosEkoMonitoring and presents evidence of day-to-day variations in the number of deaths in Moscow during the 2010 heat wave.
Almost 11,000 More Deaths
Figure 1 shows variations in daily temperatures, pollution levels and the number of nonaccidental deaths in the summer of 2010 in Moscow. There were two extreme heat waves, one lasting six days in late June and the other lasting 44 days between July 6 and August 18, 2010. The researchers focused on the second, unprecedented period of abnormally high temperatures.
As seen from the graph, increases in average daily temperatures, concentrations of combustion-related particles in the air (as well as dust and dirt from smog) and day-to-day mortality show a clear correlation – their curves follow a similar pattern. As daily temperatures and air pollution started increasing in early July (aside from the peak in end-June), a similar thing happened with mortality.
In particular, the three indicators all peaked at about the same time between the second half of July and August 15, 2010, which corresponds to the period of 33 consecutive days with temperatures ranging between 30.5 C and 38-39 C, with July 28-29 and August 4-6 being the hottest days. This prolonged and intense heat wave had a drastic effect on mortality, the researchers found.
The horizontal black line shows the usual mortality in Moscow – an average of 300 deaths per day. Following the peak temperatures between late July and early August, mortality doubled and then tripled, after a period of slower, yet steady growth during the first two weeks of the disastrous heat wave.
As a result, excess deaths from natural causes totaled 10,860 between July 6 and August 18, including 8,868 deaths among people over 65 and 1,908 deaths in younger age groups.
Graph 1. Daily deaths, mean temperatures, and air pollution levels in the summer of 2010.
Source: paper by Kharkova, Kvasha et al
Note: dashed lines show expected values; solid lines show observed values.
Diseases of the Nervous System: the Main Killers
The greatest relative increases were observed for deaths from nervous system diseases (relative risk [RR] = 3.07; the higher the relative risk, the greater its contribution to mortality), followed by genitourinary (RR = 2.54), cerebrovascular (RR = 2.37), ischemic heart (RR = 2.29), and respiratory (RR = 2.05) diseases. Mortality by underlying disease is shown in Table 1.
Table 1. Observed and expected number of deaths during the abnormal heat wave in Moscow between July 6 and August 18, 2010.
Combined Effects
The estimated relative increases in daily nonaccidental mortality attributable to temperature, air pollution, and their interaction are illustrated in Figure 3. The effect of temperatures is indicated by the letter C (green line), that of pollution is indicated by E (red line), and their interaction is marked by F (blue line), while B indicates normal mortality.
The graph illustrates how a combination of different factors eventually led to a dramatic increase in mortality exceeding 900 deaths per day around August 8, 2010. The researchers are certain that both the intensity and the duration of the heat wave have contributed to this effect.
Graph 2. The effects of temperature, smog, and their combination on mortality in Moscow in July-August 2010.
Other factors may have contributed to the death toll, such as a lack of proper medical care and air conditioning, but this is a subject for another study. The authors conclude that interactions between high temperatures and wildfire air pollution should be considered in risk assessments regarding the health consequences of climate change.