Background
The authors write that "with the increasing concern directed at climate change, temperature-related environment variables have been studied as risk factors for cardiovascular and respiratory diseases, especially in European, Australian, and US cities," noting that "in addition to heat or cold waves, diurnal temperature range (DTR) has been suggested as a predictor of mortality (Curriero et al., 2002) and was, in fact, found to be a risk factor for human health."

What was done
Focusing on the four largest cities of Korea - the populations of which ranged from 2.5 to 9.8 million (Seoul, Incheon, Daegu and Busan) - Lim et al. were able to obtain daily hospital admission information pertaining to them from the countrywide database of the Korea National Health Insurance Corporation, which covers 97% of Korea's population. And based on these data, they assessed the effects of increasing DTR on hospital admissions for the most common cardiovascular and respiratory diseases in those four cities for the period 2003-2006, employing two different statistical approaches: "a Poisson generalized linear model (GLM) and a temperature-matched case-crossover (CC) design (Basu et al., 2005)."

What was learned
The three South Korean researchers report that "the area-combined effects of DTR on cardiac failure and asthma were statistically significant," and that the DTR effects on asthma admissions were greater for the elderly (75 years or older) than for the non-elderly group. "In particular," as they describe it, "the effects on cardiac failure and asthma were significant with the percentage change of hospital admissions per 1°C increment of DTR at 3.0% and 1.1%, respectively."

What it means
Since it has long been known that the DTR has declined significantly over many parts of the world as mean global air temperature has risen over the past several decades (Easterling et al., 1997), it can be appreciated that the global warming with which the DTR decrease is associated (which is driven by the fact that global warming is predominantly caused by an increase in daily minimum temperature) has likely helped to significantly reduce hospital admissions for cardiac failure and asthma within the larger cities of Korea. In fact, it has likely done so for major cities all around the world, as Lim et al. note that DTR effects on non-accidental, cardiovascular, and respiratory mortality or emergency admissions have been studied in a number of Asian countries, citing the work of Cao et al. (2009), Chen et al. (2007), Kan et al. (2007), Liang et al. (2008), Liang et al. (2009), Lim et al. (2012), Shinkawa et al. (1990), Song et al. (2008) and Tam et al. (2009). And they say that each of these studies "reported a 1 to 2% increase in mortality risk per 1°C increase of DTR."

References
Basu, R., Dominici, F. and Samet, J.M. 2005. Temperature and mortality among the elderly in the United States: a comparison of epidemiologic methods. Epidemiology 16: 58-66.

Cao, J., Cheng, Y., Zhao, N., Song, W., Jiang, C., Chen, R. and Kan, H. 2009. Diurnal temperature range is a risk factor for coronary heart disease death. Journal of Epidemiology 19: 328-332.

Chen, G., Zhang, Y., Song, G., Jiang, L., Zhao, N., Chen, B. and Kan H. 2007. Is diurnal temperature range a risk factor for acute stroke death? International Journal of Cardiology 116: 408-409.

Curriero, F.C., Heiner, K.S., Samet, J.M., Zeger, S.L., Strug, L. and Patz, J.A. 2002. Temperature and mortality in 11 cities of the Eastern United States. American Journal of Epidemiology 155: 80-87.

Easterling, D.R., Horton, B., Jones, P.D., Peterson, T.C., Karl, T.R., Parker, D.E., Salinger, M.J., Razuvayev, V., Plummer, N., Jamason, P. and Folland, C.K. 1997. Maximum and minimum temperature trends for the globe. Science 277: 364-367.

Kan, H., London, S.J., Chen, H., Song, G., Chen, G., Jiang, L., Zhao, N., Zhang, Y. and Chen, B. 2007. Diurnal temperature range and daily mortality in shanghai, China. Environmental Research 103: 424-431.

Liang, W.M., Liu, W.P., Chou, S.Y. and Kuo, H.W. 2008. Ambient temperature and emergency room admissions for acute coronary syndrome in Taiwan. International Journal of Biometeorology 52: 223-229.

Liang, W.M., Liu, W.P. and Kuo, H.W. 2009. Diurnal temperature range and emergency room admissions for chronic obstructive pulmonary disease in Taiwan. International Journal of Biometeorology 53: 17-23.

Lim, Y., Park, A., Hajat, S. and Kim, H. 2012. Modifiers of diurnal temperature range and mortality association in six Korean cities. International Journal of Biometeorology 56: 33-42.

Shinkawa, A., Ueda, K., Hasuo, Y., Kiyohara, Y. and Fujishima, M. 1990. Seasonal variation in stroke incidence in Hisayama, Japan. Stroke 21: 1262-1267.

Song, G., Chen, G., Jiang, L., Zhang, Y., Zhao, N., Chen, B. and Kan, H. 2008. Diurnal temperature range as a novel risk factor for COPD death. Respirology 13: 1066-1069.

Tam, W.W., Wong, T.W., Chair, S.Y. and Wong, A.H. 2009. Diurnal temperature range and daily cardiovascular mortalities among the elderly in Hong Kong. Archives of Environmental and Occupational Health 64: 202-206.