Background
Noxious air pollution is bad enough under temperature regimes to which people are accustomed; but when the stress of extreme heat or extreme cold is added to the mix, it could logically be expected that the negative effects of air pollution on peoples' health may be amplified. And the question thus arises: which is the more deadly in this regard, extreme heat or extreme cold?

What was done
In a study designed to answer this question for the inhabitants of Shanghai, China (the country's largest city), the authors employed a generalized additive model with penalized splines to analyze mortality, air pollution, temperature and covariate data over the period 1 January 2001 through 31 December 2004, focusing on particulate matter of diameter 10 µm or less (which is commonly referred to as PM10) and ozone (O3).

What was learned
Cheng and Kan report that they "did not find a significant interaction between air pollution and higher temperature [>85th percentile days]," but they say that "the interaction between PM10 and extreme low temperature [<15th percentile days] was statistically significant for both total and cause-specific mortality." More specifically, they found that compared to normal temperature (15th-85th percentile days), a 10-µg/m³ increase in PM10 on extreme low temperature days led to all-cause mortality rising from 0.17% to 0.40%. And they add that "the interaction pattern of O3 with low temperature was similar," noting that their finding of "a stronger association between air pollution and daily mortality on extremely cold days confirms those of three earlier seasonal analyses in Hong Kong, Shanghai and Athens," citing the studies of Touloumi et al. (1996), Wong et al. (1999, 2001) and Zhang et al. (2006).

What it means
Not only is extreme coldness in and of itself a greater killer of people than extreme warmness - see Health Effects (Temperature - Hot vs. Cold Weather) in our Subject Index - but it would appear from this study and the others it sites that extreme cold also enhances the killing power of the noxious air pollutants PM10 and ozone.

References
Touloumi, G., Samoli, E. and Katsouyanni, K. 1996. Daily mortality and "winter type" air pollution in Athens, Greece - a time series analysis within the APHEA project. Journal of Epidemiology and Community Health 50, Supplement 1: 47-51.

Wong, C.M., Ma, S., Hedley, A.J. and Lam, T.H. 1999. Does ozone have any effect on daily hospital admissions for circulatory diseases? Journal of Epidemiology and Community Health 53: 580-581.

Wong, C.M., Ma, S., Hedley, A.J. and Lam, T.H. 2001. Effect of air pollution on daily mortality in Hong Kong. Environmental Health Perspectives 109: 335-340.

Zhang, Y., Huang, W., London, S.J., Song, G., Chen, G., Jiang, L., Zhao, N., Chen, B. and Kan, H. 2006. Ozone and daily mortality in Shanghai, China. Environmental Health Perspectives 114: 1227-1232.