Cold spells

Deaths during heat-waves have received much media attention in recent years, and yet cold weather is even more lethal. Heart/respiratory diseases and strokes claim more lives in cold weather.

What are dangerously low temperatures? The EU has no official meteorological definition of cold and extreme cold. A Dutch studypdf defines a cold spell as a period of at least 9 consecutive days in which the lowest temperature reaches –5°C or lower, including at least 6 days in which the lowest temperature touches –10°C or lower.

High-risk groups needing special attention are:
  • the elderly,
  • children (who lose heat faster due to their higher body surface/weight ratio),
  • people with ischemic diseases (mainly heart disease and stroke),
  • people with chronic respiratory diseases or asthma

Coronary heart disease, strokes and respiratory diseases are responsible for most part of excess winter deaths. Other contributing factors are influenza, social class and per capita gross national product.

Severe weather has had dramatic consequences in Europe in recent years. Institutions at all levels are now developing strategies, by learning from recent events, to mitigate the future impact of extreme weather events on health.

See Public health response to extreme weather and climate events - Fourth Ministerial Conference on Environment and Health - Budapest, 2004pdf

Extreme cold – health-related effects

The human body protects itself against the cold by a series of thermoregulatory mechanisms. A deficient thermoregulatory system or an experience of thermal (heat and/or cold) stress can have serious health consequences. About half of winter deaths in the EU are due to coronary thrombosis. The impact of a cold spell on the number of deaths due to cardiovascular problems can be felt anything from 7 to 14 days later.

Other ways in which cold weather can affect health are:

  • Arterial hypertension, hyper viscosity and thrombosis (possible leading to stroke),
  • respiratory diseases (respiratory infections represent about 25% of additional winter deaths). The impact of a cold spell on deaths due to respiratory problems can be felt anything from 15 to 30 days later.
  • hypothermia (of special concern for the homeless, alcoholics and drug-addicts).
  • peripheral vascular diseases,
  • endocrinal diseases, and
  • asthma.


The main indirect threat in cold weather is carbon monoxide poisoning.

See Froid et Santé. Eléments de synthèse bibliographique et perspectives – Institut de veille sanitaire, France

Wind chill

Wind chill describes how fast the body loses heat under the combined effects of low temperature and wind: a 90km/h wind with an ambient temperature of -10°C gives the same sensation of cold as an ambient temperature of -30°C with no wind. Exposure to even low wind chills can be life threatening to both humans and animals. There are different ways of measuring wind chill. Environment Canada offers a useful wind chill calculator, based on the loss of heat from the face – the part of the body most exposed to severe winter weather. The factors contributing to excess winter deaths (including air pollution and the psychological and sociological impact of winter) are complex, so that it is difficult to quantify the impact of any one factor. Further studies will determine if the higher death rate associated with cold is preventable.

Ischemic heart disease

Ischemic heart disease is the biggest single cause of death in winter, accounting for about half of all such deaths. Many take place hours or a day or two after exposure to cold, suggesting thrombosis starting during or shortly after exposure to cold. Rapid death from thrombosis can be explained by the fact that the composition of the blood changes in the cold: the red cell count, plasma cholesterol and plasma fibrinogen all increase – all factors known to contribute to thrombosis.

The 1997 EU Eurowinter study found that cold housing contributes to deaths from respiratory problems in winter, and exposure to outdoor cold contributes to deaths from arterial thrombosis. It also showed that a 1°C fall in temperature below 18°C had a greater impact on death rates in warmer regions than in colder regions (e.g. Athens 2.15% vs. south Finland 0.27%). The study showed the death-rate rose more sharply at a given fall of temperature in regions with warm winters, in populations with cooler homes, and among people who wore fewer clothes and were less active outdoors.

See Temperature and cardiovascular mortality

The Eurowinter Group: Cold exposure and winter mortality from ischemic heart disease, cerebrovascular disease, respiratory disease, and all causes in warm and cold regions of Europe. Lancet 349:1341-1346, 1997 (not electronically available)

Coronary events

Coronary events (anything from slight angina pectoris to a serious heart attack) increase during comparatively cold periods, especially in warm climates. In colder climates there is less increase, suggesting that some events in warmer climates are preventable. People living in warm climates, particularly women, should keep warm on cold days. Coronary events are more common in cold weather but additional deaths are no greater in people with a history of heart disease.

See Cold periods and coronary events: an analysis of populations worldwide


Hypothermia

Hypothermia-related deaths are rare in the EU – probably around 1% of the total number of deaths attributable to cold in mild EU countries – and most are preventable.
In moderately cold environments, the body adapts to keep the core temperature within a degree or two of the normal 37°C. However, in intense cold without adequate clothing, the body is unable to compensate for the heat loss and the body's core temperature starts to fall. The sensation of cold followed by pain in exposed parts of the body is one the first signs of mild hypothermia.

StageCore temperature
Signs & Symptoms
Mild hypothermia
37.2-36.1ºC
Normal, shivering may begin.
36.1-35ºC
Cold sensation, goose bumps, unable to perform complex tasks with hands, shivering can be mild to severe, hands numb.
Moderate Hypothermia
35-33.9ºC
Shivering intense, lack of muscle coordination, movements slow and laboured, stumbling, mild confusion, but may appear alert. Use sobriety test: if unable to walk 10m in a straight line, the person is hypothermic.
33.9-32.2ºC
Violent shivering, difficulty speaking, sluggish thinking, amnesia starts to appear, gross muscle movements sluggish, unable to use hands, stumbles frequently, signs of depression, withdrawn.
Severe Hypothermia
32.2-30ºC
Shivering stops, exposed skin blue or puffy, muscle coordination very poor, unable to walk, confused, incoherent/irrational behaviour, but may be able to maintain posture and appearance of awareness.
30-27.8ºC
Muscle rigidity, semiconscious, stupor, loss of awareness of others, pulse and respiration rate decrease, possible heart fibrillation.
27.8-25.6ºC
Unconscious, heart beat and breathing erratic, there may be no pulse.
25.6-23.9ºC
Pulmonary oedema, cardiac and respiratory failure, death. Death may occur before this temperature is reached.


Source: Canadian Centre for Occupational Health and Safety (CCOHS)

Studies on excess winter deaths in the EU

Winter mortality is a well-reported phenomenon throughout the world, and most countries have a winter death toll which is 5% to 30% higher than usual (the Eurowinter Group, 1997). Half the additional deaths are due to cerebrovascular diseases and ischemic heart disease, and the other half to respiratory disorders. The seasonal fluctuation seems to be mostly due to the cold, with some deaths attributable to influenza A and other risk factors (study on vulnerability to winter mortalitypdf)

There are various mechanisms by which cold affects human health:
  • sympathetic tone ("fight or flight" response) increases,
  • blood pressure rises,
  • myocardial oxygen consumption increases,
  • the red blood cell and platelet count increases,
  • plasma beta-thromboglobulin, platelet factor 4 and plasma fibrinogen levels increase,
  • levels of antithrombin III (an important inhibitor of blood clot formation) drop.

Respiratory and other viral and bacterial infections, which mostly occur in winter, may trigger coronary heart disease or stroke, as they affect blood coagulation factors, cause damage to vessel walls and may promote atherosclerosis.

The most extensive study on Excess winter mortality in Europe: a cross country analysis identifying key risk factors was published in 2002. The results show a positive link between premature winter deaths, mean winter environmental temperature and mean winter precipitation. In other words, the premature death toll is higher in countries with a warmer winter climate.

Housing standards are a potential factor behind this paradox. Houses in countries with comparatively warm climates all year round tend to lose heat easily, so people find it hard to heat their homes when winter arrives. This is especially true in Portugal, Spain, and Ireland, where winter temperatures are comparatively mild and excess mortality rates in winter very high. Conversely, houses in countries with severe climates – such as Scandinavia – have to be thermally efficient to retain warmth.

Results from this study for 1988–1997:

Country
Approx. % increase in premature deaths
Number of premature winter deaths
Portugal28%8 800
Ireland21%2 000
Spain21%19 000
England19%31 000
Wales17%1 800
Northern Ireland17%800
Scotland16%3 100
UK in total18%37 000
Greece18%5 700
Italy16%27 000
Finland, Germany, and the Netherlands appear to suffer far less from excess winter mortality.

References:

Climate warming and health adaptation in Finland. Hassi, J. and Rytkönen, M. 2005. FINADAPT Working Paper 7, Finnish Environment Institute Mimeographs

Excess winter mortality in Europe: a cross country analysis identifying key risk factors

Vulnerability to winter mortality in elderly people in Britain: population based studypdf

Winter Excess Mortality: A Comparison between Norway and England plus Wales

Housing standards and excess winter mortality

The impact of Heat Waves and Cold Spells on Mortality Rates in the Dutch Populationpdf

Heat related mortality in warm and cold regions of Europe: observational studypdf

Hypothermia-Related Deaths – United States, 1999–2002 and 2005

Monitoring excess winter deaths

England and Wales

An annual figure is provided for additional winter deaths (excess winter mortality). This figure is calculated as winter deaths (those occurring in December to March) minus average non-winter deaths (occurring in April to July of the current year and August to November of the previous year).

See Excess winter mortality - National Statistics - United Kingdom