The post The Deadliest U.S. Natural Hazard: Extreme Cold has generated a number of questions. Mr.. Goklany has graciously supplied a followup which I have posted below. – Anthony
by Indur Goklany
A few readers have raised a number of questions including what deaths are covered under the categories of “extreme cold” and “extreme heat” in my blog. The estimates I provided are only supposed to include deaths that are directly due to exposure to extreme cold or heat, that is, hypothermia and hyperthermia. They do not include deaths that may be associated with cold or hot weather but are attributed to other primary causes, e.g., the flu, heart attacks (e.g., coronary thrombosis), strokes (e.g., cerebral thrombosis), accidents caused by ice, etc. Professor William Keatinge has a very readable piece here (see pp.47-52) that briefly discusses the different mechanisms by which heat or cold may kill directly and indirectly.
Had I had accounted for the deaths associated with hot and cold weather, the numbers would have been have been much higher (particularly for cold weather). The UK Office of National Statistics (ONS) probably has the most easily accessible and up to date information on excess winter mortality. The following figure from the UKONS shows that there were 25,300 excess winter deaths in England and Wales from December 2007 to March 2008.
Excess winter mortality (EWM) is calculated as “winter deaths occurring in December to March minus the average of non-winter deaths (April to July of the current year and August to November of the previous year).” Honestly, UKONS could have been clearer about how precisely EWM was computed: Did they compute EWM using aggregate deaths over the 4-month winter period and subtracting from that aggregate deaths during the remaining 8-months of the year, or did they compute the difference between the average monthly difference in mortality for the 4-month winter period and the 8-month non-winter period and multiply by 4? In any case, either way, more people die in winter than during other seasons]
UKONS notes that “the number of extra deaths occurring in winter varies depending on temperature and the level of disease in the population, as well as other factors.”
Considering that global temps were higher in 1998-1999 than in subsequent years, offhand I don’t see any correlation with global temps and EWM, nor should it be expected. First, any correlation would be with local temps (no teleconnections). Moreover trends would be confounded by changes (improvements mainly) in adaptive capacity, housing stock, insulating characteristics of clothing, greater income and affordability for energy, etc.
Having more deaths in winter is not just a UK phenomenon
Healy (2003) looked at excess winter deaths for EU-14 from 1988-97. His results, summarized in the table below, show:
- Portugal has the highest seasonal variation in mortality in Europe, with a winter increase of some 28% above the average mortality rate, equivalent to 8,800 premature winter deaths each year.
- Ireland has an increase of some 21%, or 2,000 excess winter deaths annually.
- Spain: 21%, 19,000 excess annual deaths.
- The UK: 18%, or 37,000 annual excess winter deaths.
- Greece: 18%, or 5,700 premature winter deaths annually.
- Italy: 16%, or 27,000 excess deaths.
Healy’s results indicate that:
- Mean winter environmental temperature and mean winter precipitation are positively associated with levels of relative excess winter mortality in Europe. “A highly significant regression coefficient of 0.27 is found (p<0.001) with regard to environmental temperature.”
- Overall level of relative humidity is also positively associated with excess mortality across Europe: “a significant regression coefficient of 0.23 (p=0.02) is reported. The relationships between mean winter rainfall and excess deaths is also found to be significant (a regression coefficient of 0.54, p<0.001).
- There is a “paradox of excess winter mortality”, namely, “higher mortality rates are generally found in less severe, milder winter climates where, all else equal, there should be less potential for cold strain and cold related mortality. This result indicates that the typical, inverse relation normally found between cold exposure and rates of (all year) mortality does not hold for excess winter mortality”, perhaps due to differences in housing standards, noting that “countries with comparatively warm all year climates tend to have poor domestic thermal efficiency. Because of this, these countries find it hardest to keep their homes warm when winter arrives. This is especially the case in Portugal, Spain, and Ireland, where winter temperatures are comparatively mild and excess mortality rates in winter are very high. Conversely, countries with severe climates-such as those in Scandinavia-have to maintain high levels of thermal efficiency, as temperatures demand that houses must retain warmth.” I would also speculate that physiological acclimation, and differences in wealth during the 1988-97 period may also have played a role.
What fraction of the problem is due to influenza?
Donaldson and Keatinge (2002), estimate this to be around 2-3% based on a daily record of deaths in southeast England from 1970 to 1999 for all causes and for influenza. However, others believe it could be higher, perhaps 19% [see Fleming et al. (2002), and Donaldson/Keatinge reply). Whatever the number is, it’s going to fluctuate wildly from year to year and depend on, among other things, the accuracy of official forecasts about which strain each year’s flu vaccine should primarily guard against.
Donaldson, GC; Keatinge, WR. 2002. Excess winter mortality: influenza or cold stress? Observational study. British Medical Journal. January 12; 324(7329): 89-90.
Healy, JD. 2003. Excess winter mortality in Europe: a cross country analysis identifying key risk factors. Journal of Epidemiology and Community Health 57:784-789
Fleming, DM et al. and Donaldson, GC; Keatinge, WR 2002. Excess winter mortality: Method of calculating mortality attributed to influenza is disputed. British Medical Journal. June 1; 324(7349): 1337.