The reason more people die in winter is not just because of a drop in temperature, writes David Labanyi
The fact that there are more deaths in winter compared with other times of the year is logical and most people would assume colder temperatures are the cause.
However, when US academic Prof Joel Schwartz from the Harvard Department of Environmental Epidemiology tested if this was, in fact, the case, he came up with some surprising results.
"The number of deaths per day rises in the winter and comes down in the summer and that happens every year and everywhere, including Dublin," Schwartz told The Irish Times ahead of a lecture at the Dublin Institute of Technology.
"The question was, is this rise in deaths because of temperature alone?"
He and his team examined 7,789,655 deaths across 50 US cities from 1989 to 2000 and cross-referenced them with the daily temperatures in those cities.
In each city only the coldest three or four days (or 1 per cent of days) were used.
The cities included Detroit, where average winter temperatures fall to minus 10 degrees, and Honolulu where the temperature barely fluctuates and falls to only plus 20 degrees in the winter.
"If the increase in deaths in the winter is really due to just temperature, then we ought to see a big effect in Detroit and not that much in Honolulu," he says.
"But if you look at fluctuations in mortality in the two cities - on a percentage scale because of their different populations - it is about the same.
"In fact, the rate of extra deaths in each of the 50 cities was remarkably consistent - going up by roughly 2 per cent - regardless of whether it was as cold a winter as in Detroit, or as warm as Honolulu.
"Even though there is a 16 degree difference in the temperature that defined the coldest 1 per cent of the days across the 50 cities, the increase in mortality in all those cities was the same," he says.
"If the winter increase in mortality is the same in these cities, as a per cent, then those people aren't dying because it's cold in Honolulu. So why are they dying?"
Schwartz believes deaths increase in winter because people become acclimatised to a normal range of temperature for their city or region and it is only during unusually cold temperatures - for that area - that people start to die.
When examining the mortality rates for the different cities, the researchers also looked at data that found the highest number of deaths from heart attacks and diabetes almost always coincided with peaks in deaths for pneumonia and influenza.
"In some years the influenza epidemic comes sooner, in some years it comes later but in whichever month it does, that is when the heart attacks peak," he says.
The researchers also found that when the annual flu epidemic was milder than other years, the peak in heart attack deaths was also smaller.
"Every once in a while we get a pandemic where there is a new strain and more people die because we have less immunity to that strain. The last pandemic was 1968/1969 [ and] there was a big jump in deaths from pneumonia and influenza," says Schwartz.
"If you look at what happened with heart attack deaths that year it was exactly the same, heart attack deaths also jumped that winter.
"This suggests that it might not be the cold, it might be the influenza that is killing people in the winter."
According to Schwartz, it is because acute respiratory infections trigger fever and inflammation that they may be responsible for a higher rate of cardiovascular deaths in winter.
To understand the link, Schwartz says it is important to understand what causes heart attacks.
"Heart attacks are caused by this hardening of the arteries which is basically like these scabs forming on your blood vessels over time because we eat too much and exercise too little.
"During a heart attack those scabs break apart and pieces travel and clog the heart so we don't get any blood and we die.
"Influenza is a serious infection and for the people who don't die of it there is a lot of inflammation and fever of the body."
This inflammation makes those scabs more unstable and more likely to rupture and Schwartz says many of the heart attacks and strokes recorded during winter are side effects of influenza infections.
He also thinks that public health professionals may be underestimating the importance of influenza as a disease and says that a wider vaccination policy could save lives.
Most countries vaccinate only the elderly for influenza but many people who are having heart attacks are not 75 years of age and older, he says.
"This research raises issues such as perhaps people with diabetes and known heart conditions should also consider an influenza vaccination," he says.
Finally, this research suggests there is unlikely to be any noticeable reduction in winter time deaths due to global warming, as people will acclimatise to warmer temperatures but respiratory infections will still increase during the coldest days.
In fact, because the effect of extreme heat on mortality is higher than extreme cold, Schwartz expects a net increase in deaths as a result of global warming.