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Deaths Associated with Unseasonable Cold and Warmth in Monterrey and Mexico City, Mexico
Reference
O'Neill, M.S., Hajat, S., Zanobetti, A., Ramierz-Aguilar, M. and Schwartz, J. 2005. Impact of control for air pollution and respiratory epidemics on the estimated associations of temperature and daily mortality. International Journal of Biometeorology 50: 121-129.

What was done
The authors assessed the influence of air pollution and respiratory epidemics on empirical associations between apparent temperature, which "represents an individual's perceived air temperature," and daily mortality in Mexico's largest and third largest cities: Mexico City and Monterrey, respectively.

What was learned
O'Neill et al. report that "the effects of cold weather on all-age mortality were similar in Monterrey and Mexico City." When considering the entire temperature spectrum, however, they found that "in Mexico City, the 7-day temperature mortality association has a hockey stick shape with essentially no effect of higher temperatures [our italics]," whereas in Monterrey, the function they fit to the data "shows a U-shape," with "a higher mortality risk at both ends of the distribution," although the effect is much weaker at the high-temperature end of the plot than at the low-temperature end, and the absolute value of the slope of the mortality vs. temperature relationship is smaller across the high-temperature range of the data.

Most interesting of all, perhaps, was the researchers' finding that "failure to control for respiratory epidemics and air pollution resulted in an overestimate of the impact of hot days by 50%," whereas "control for these factors had little impact on the estimates of effect of cold days," plus their acknowledgement that "most previous assessments of effects of heat waves on hot days have not controlled for air pollution or epidemics."

What it means
Once again (for more examples, see the reviews archived under the several regional subheadings of Health Effects (Temperature - Hot vs. Cold Weather) in our Subject Index), it is demonstrated that cold temperatures are much more deadly than are hot temperatures. In addition, it is demonstrated that the death-dealing effects of heat waves are typically not adjusted for concurrent effects of air pollution and respiratory epidemics, which often account for as much as half of the deaths attributed to hot temperatures, whereas these two conflicting factors do not appear to impact assessments of the effect of cold temperatures on deaths.

A prime example of the failure to account for concurrent air pollution effects on mortality occurred in the aftermath of the European heat wave of 2003. In analyzing the impact of air pollutants present during that episode in the United Kingdom, Stedman (2004) found that 21-38% of the total excess deaths claimed to be due to high temperatures were actually the result of elevated concentrations of ozone and PM10 (particulate matter of diameter less than 10µm). Likewise, Fischer et al. (2004) determined that 33-50% of the deaths attributed to the same heat wave in the Netherlands was also caused by concurrent high ozone and PM10 concentrations. This little complexity is something that is often conveniently forgotten by climate alarmists when they rail on the evils of global warming.

References
Fischer, P.H., Brunekreef, B. and Lebret, E. 2004. Air pollution related deaths during the 2003 heat wave in the Netherlands. Atmospheric Environment 38: 1083-1085.

Stedman, J.R. 2004. The predicted number of air pollution related deaths in the UK during the August 2003 heatwave. Atmospheric Environment 38: 1087-1090.

Reviewed 21 June 2006