Possible role of acclimatization in weather-related human mortality during the transition seasons of autumn and spring in an extreme continental climate

Statistical evidence shows that human mortality is closely related to levels of thermal environmental stress and resulting thermophysiological strain. Most climate-mortality research has focused on seasonal extremes when mortality is highest (winter) or most affected by extreme heat events (summer). Relatively little research has considered patterns of mortality, during the transitional seasons of autumn and spring. The body acclimatizes to heat in summer and cold in winter and readjusts through acclimatization during the transitions between the two. Consequently, the thermophysiological strain of re-adjustment may be expected to occur in the changeover seasons of spring and autumn. This adaptation period is not risk free, so it may be necessary to take into account the impact of the change in physiological well-being during the period of adjustment.

To better understand the influences of weather on mortality through the acclimatization process, the aim here is examine the periods that link summer and winter. The study uses the Acclimatization Thermal Strain Index (ATSI), which is a relative measure of short-term thermophysiological impact on the body. ATSI centers on heat exchange with the body's core via the respiratory system, which cannot be protected. Respiration is the body-environment heat exchange process in which the body is in closest contact with the ambient air. There are no behavioral or other adjustments to prevent the ambient air entering into the body's core area through the respiratory tract. ATSI describes the acclimatization thermal loading (ATL) on respiratory organs until full adaptation is achieved. Unlike widely used bioclimatic indices, ATSI is a relative measure, based on physiological responses known to negatively impact human wellbeing. The analysis here is based on data for Khabarovsk, a major city in the climatic region of the Russian Far East characterized by very hot summers and extremely cold winters.

The results suggest that the most severe thermal strain occurs with the adjustment shift from hot-humid to cold, in that mortality data show that the sensitivity of the body to the acclimatization process to cold during autumn is greater than the seasonal shift to heat during spring. Although mortality peaks in winter (January) and is at its lowest in summer (August), there is not a smooth rise through autumn. A secondary peak occurs in autumn (October), but nothing similar is evident in spring. This suggests the acclimatization transition from warm-to-cold produces more thermophysiological strain than the transition from cold-to-warm, which is apparent in the high coefficient of correlation between mortality and ATSI in autumn. The results show the extent to which biophysical adaptation plays a role in increased strain on the body during re-acclimatization and for this reason is a more appropriate climatic indictor than air temperature alone. The work gives useful bioclimatic information on risks involved in transitional seasons in regions characterized by climatic extremes. This could be helpful in planning and managing health services to the public and measures that might be used to help mitigate impacts.