Background
The authors write that "the human body responds to cold weather by reducing the blood flow to the peripheral parts of the body," and that "this process leads to an increase of blood viscosity and concentration which may cause greater clotting and, therefore, a higher incidence of thrombosis," citing the studies of Keatinge et al. (1984), Schneider et al. (2008) and Wilson et al. (2010). Moreover, they state that "the influence of cold on health can be felt not only during extreme cold spells but, more importantly, during longer exposure periods such as winter [italics added]," as has been found to be the case by Davie et al. (2007) and Donaldson and Keatinge (2002).

What was done
In exploring this subject for themselves, Vasconcelos et al. studied the effect of a daily human-biometeorological index known as the Physiologically Equivalent Temperature or PET, which is based on the input parameters of air temperature, humidity, mean radiant temperature and wind speed, as employed by Burkart et al. (2011), Grigorieva and Matzarakis (2011) and Cohen et al. (2012), focusing their attention on Lisbon and Oporto Counties in Portugal over the period 2003-2007.

What was learned
The five Portuguese researchers report finding "a linear relationship between daily mean PET, during winter, and the risk of myocardial infarction, after adjustment for confounding factors," thus confirming that "the thermal environment, during winter, is inversely associated with acute myocardial infarction morbidity in Portugal," where they observed "an increase of 2.2% of daily hospitalizations per degree fall of PET, during winter, for all ages."

What it means
In Portugal, as in many other countries where, in the words of Vasconcelos et al., low winter temperatures "are generally under-rated compared to high temperatures during summer periods," cold weather is demonstrated to be "an important environmental hazard" that is much more deadly than the heat of summer.

References
Burkart, K., Khan, M., Kramer, A., Breitner, S., Schneider, A. and Endlicher, W. 2011. Seasonal variations of all-cause and cause-specific mortality by age, gender, and socioeconomic condition in urban and rural areas of Bangladesh. International Journal for Equity in Health 10: 10.1186/1475-9276-10-32.

Cohen, P., Potchter, O. and Matzarakis, A. 2012. Daily and seasonal climatic conditions of green urban open spaces in the Mediterranean climate and their impact on human comfort. Building and Environment 51: 285-295.

Davie, G.S., Baer, M.G., Hales, S. and Carlin, J.B. 2007. Trends and determinants of excess winter mortality in New Zealand: 1980 to 2000. BMC Public Health 7: 263.

Donaldson, G.C. and Keatinge, W.R. 2002. Excess winter mortality: influenza or cold stress: Observational study. British Medical Journal 324: 89-90.

Grigorieva, E. and Matzarakis, A. 2011. Physiologically equivalent temperature as a factor for tourism in extreme climate regions in the Russian far east: preliminary results. European Journal of Tourism, Hospitality and Recreation 2: 127-142.

Keatinge, W.R., Coleshaw, S.R.K., Cotter, F., Mattock, M., Murphy, M. and Chelliah, R. 1984. Increases in platelet and red cell counts, blood viscosity, and arterial pressure during mild surface cooling: factors in mortality from coronary and cerebral thrombosis in winter. British Medical Journal 289: 1404-1408.

Schneider, A., Panagiotakos, D., Picciotto, S., Katsouyanni, K., Lowel, H., Jacquemin, B., Lanki, T., Stafoggia, M., Bellander, T., Koenig, W., Peters, A. and AIRGENE Study Group. 2008. Air temperature and inflammatory responses in myocardial infarction survivors. Epidemiology 19: 391-400.

Wilson, T.E., Gao, Z., Hess, K.L. and Monahan, K.D. 2010. Effect of aging on cardiac function during cold stress in humans. American Journal of Physiology - Regulatory, Integrative and Comparative Physiology 298: R1627-R1633.