Heat health explained

Both high and low temperatures, indoors and outdoors, pose substantial risks to human health, including increases in mortality, morbidity and health service use (Ryti et al., 2016; WMO, 2015). In many countries, the health impacts of cold temperatures substantially outweigh those of heat (Gasparrini et al., 2015).

The scale and nature of the health impacts observed depends on the timing, intensity and duration of the temperature event, the level of acclimatisation and adaptation of the local population, infrastructure and institutions to the prevailing climate, as well as the definitions and methodologies used for scientific research. As such, the health effects of temperature extremes and the determinants of vulnerability are, to some extent, context specific.

Population health impacts start to be observed at winter and summer temperatures that are considered moderate for the season and then increase as temperatures become more extreme, in what is variously described as a U-, V- or J-shaped curve. The precise threshold temperatures for health impacts vary by region and country, as does the scale of the health impacts by degree change in temperature, but the overall pattern remains similar wherever it has been studied.

For both heat and cold, the impact of temperature is more marked for deaths than for hospitalisations (Hajat et al., 2016; Linares and Diaz, 2008); this may suggest that individuals die before they reach health care. Temperature extremes may also result in illness that is not sufficiently severe to require hospital attention and that has not been captured by these studies.

For heat, deaths and hospitalisations occur extremely rapidly (same day) and they may be followed by a degree of impact displacement (health impacts in the frail brought forward), which returns to normal within a matter of days (Basu, 2009). The onset of health impacts for cold are slower and persist for longer (up to 4 weeks), with short-term displacement effects not apparent (Analitis et al., 2008).

Longer heat events are associated with greater health effects because of the longer period of exposure (D’Ippoliti et al., 2010), but this has not been consistently observed for cold (Ryti et al., 2016).

Severe heat events that occur towards the beginning of a season have greater health impacts; this is likely to be partly due to loss of the most vulnerable members of the population during the first episode and partly due to population adaptation for subsequent events (Baccini et al., 2008). This pattern is less clear for severe cold, with some authors indicating that cold weather events towards the end of the season are associated with greater mortality (Montero et al., 2010a).

There is some evidence that there has been a reduction in health effects from heat extremes over recent years in some countries, which suggests that there has been some individual and institutional adaptation (Arbuth nott et al., 2016). This is less well established for cold risks.

Heat impacts

The major determinants of vulnerability of a population to temperature extremes relate to the features of the population exposed and their capacity to respond and adapt to the temperature conditions over long and short time frames. Determinants of vulnerability can be broadly categorised by demographic, health, physical, socioeconomic and institutional factors, many of which are inter-related and dynamic.

Temperature extremes rarely occur in isolation and related hazards such as snow/ice, drought/wildfires, poor air quality or other unrelated disasters may coincide in time and geography. Responses to these additional hazards may alter existing vulnerabilities and the capacity to adapt to temperature extremes.

Demographic determinants

The physiology of older people and the very young renders them more vulnerable to temperature extremes. They may also be less able to adapt their behaviours or environmental conditions and may be more dependent on others.

Health status determinants

Many physical and mental health conditions increase vulnerability to adverse temperatures through a direct effect on the body’s physiology or through the effect of certain medications

The Lancet Countdown on health and climate change: from 25 years of inaction to a global transformation for public health Watts, Nick et al. The Lancet , Volume 391 , Issue 10120 , 581 - 630

Physical determinants

People spend approximately 80 % of their time indoors, with the elderly or unwell spending longer periods indoors. Buildings (including homes, hospitals, schools and prisons) are not always adapted for temperature extremes and may have insufficient heating/energy efficiency or cooling measures.

Socioeconomic determinants

People who are socially isolated are more at risk from temperature extremes because they are less able to access community support, and may also have additional health or other vulnerabilities.

Behavioural/cultural determinants

When temperatures become more extreme, most people take some action to adapt to the conditions. However, some factors limit the ability to adapt, such as age, poor health or economic circumstances, and certain belief or value systems may also mean that appropriate action is not taken in response to the temperature conditions.

Institutional determinants

Health services need robust plans in order to manage the potential disruption and increased demand during and following temperature extremes; their ability to respond influences population vulnerability.

Case Studies

Take a look at some of the projects that are building resilience to extreme heat.

Text found on this page is derived from the following publication: McGregor, Glenn (2017) 'Meteorological risk : extreme temperatures.', in Science for disaster risk management 2017 : knowing better and losing less. Luxembourg: Publications Office of the European Union, pp. 257-270. For the full text please visit: http://dro.dur.ac.uk/23472/

Case studies included in the following publication: WHO/WMO. (2016) Climate Services for Health: Fundamentals and Case Studies for improving public health decision-making in a new climate. Eds. J.Shumake-Guillemot and L.Fernandez-Montoya. Geneva. View the publication here: https://public.wmo.int/en/resources/library/climate-services-health-case-studies