The Association between Two Windchill Indices and Daily Mortality Variation in the Netherlands Anton

Exposure to low ambient air temperatures not only is uncomfortable but creates stress on the cardiovascular and respiratory system. People exposed during a short time to moderate cold show increases in heart rate, blood pressure, and blood viscosity.'-5 Inhalation of cold air causes bronchoconstriction, especially among those with chronic obstructive lung disease.6'7 There is some evidence that exposure to low ambient air temperatures even creates a threat to human survival. For various countries, it has been shown that national death rates increase substantially during spells of cold.8'3 This association can be attributed only to a small extent to influenza epidemics or to seasonal mortality variation.'4 Nearly all studies on the association between exposure to cold and mortality have applied a simple exposure measure: outdoor air temperature. However, the effect of weather on the thermal balance of the human body is determined not only by ambient air temperature but also by a number of other meteorological factors such as solar radiation and wind speed."2 Especially significant is the chilling effect of strong wind. The combined effect of temperature and wind speed on heat loss of the human body has been expressed by a number of windchill indices.'5"6 Such indices may serve as indicators of weather conditions involving increased heat loss and, as a consequence, an increased risk of death. Therefore, when these indices exceed a preestablished threshold value, preventive actions can be initiated to protect the population groups most at risk. The purpose of the present study was to assess the association between windchill indices and daily variation in mortality. The central question was whether daily variation in mortality is more strongly associated with windchill indices than with temperature. Two windchill indices were considered, that of Siple and Passel and that of Steadman. Their indices were derived in entirely distinct ways. Siple and Passel estimated the heat loss corresponding to a specific combination of temperature and wind speed as the amount of time required for water in a cylinder of synthetic material to freeze when exposed to that combination of temperature and wind speed.15"6 They derived windchill indices from these time intervals and from extrapolation of their results to temperatures above 0°C. It has been acknowledged that a cylinder of water probably cannot adequately represent the thermophysiology of a clothed and physically active human person. Nevertheless, mainly because of its ease of calculation, the Siple and Passel index is used by the meteorological offices of most countries. Steadman constructed an elaborate theoretical model involving the clothing thickness needed to maintain the thermal balance of the human body in different situations.'7 His model includes a large number of parameters such as physical activity, solar radiation, wind speed, and ambient air temperature. Steadman derived simplified formulas for calculating windchill values corresponding to specific combinations of temperature and wind