On Heatwaves, Health, and Vulnerability

11/17/20223 min read

"Given the tremendous size and heat capacity of the global oceans, it takes a massive amount of heat energy to raise Earth’s average yearly surface temperature even a small amount. The roughly 2-degree Fahrenheit (1 degrees Celsius) increase in global average surface temperature that has occurred since the pre-industrial era (1880-1900) might seem small, but it means a significant increase in accumulated heat." (Climate.gov, 2022)

As global average temperatures continue their steep ascent, several areas all over the world have been experiencing increasingly intense, frequent, and long periods of extreme heat, often known as heatwaves.

A heatwave is generally considered to have occurred when a city or region experiences a period of unusually high temperature by the standards of that city or region.

According to the Bureau of Meteorology of Australia, “[a] heatwave occurs when the maximum and the minimum temperatures are unusually hot over a three-day period at a location," "in relation to the local climate and past weather at the location” (BOM). In the USA, a heatwave is defined as “a period of two or more consecutive days when the daily minimum apparent temperature (the actual temperature adjusted for humidity) in a particular city exceeds the 85th percentile of historical July and August temperatures (1981–2010) for that city” (EPA, 2022). The Met Office considers a UK heatwave to have occurred when “a location records a period of at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold”, which varies from county to county (Met Office).

Heatwaves have significant implications on human health on the physical and mental level.

According to Wolf and McGregor (2013), “there is clear evidence for a climatological relationship between heat and mortality”. Heat-related physical health impacts include heat exhaustion, heat stroke, “dry, warm skin, due to the inability of the body to control high temperatures”, “swelling in the lower limbs, heat rash on the neck, cramps, headache, irritability, lethargy and weakness,” “severe dehydration, acute cerebrovascular accidents”, and increased risk of thrombogenesis (WHO), resulting in “high death and hospitalization rates” (Wolf and McGregor, 2013). Heatwaves have also been known to have adverse effects on mental health. A 2021 study in New York identified “positive associations between short-term exposure to extreme heat (27.07∘C) and increased ER visits for total mental disorders, as well as substance abuse, mood and anxiety disorders, schizophrenia, and dementia” (Yoo et al).

In a 2013 study setting out a heat vulnerability index developed for London, heat was conceptualized as “a function of exposure to heat and the sensitivity of people” (Wolf and McGregor, 2013).

Exposure to heat is determined by indoor and outdoor temperatures, which in turn are influenced by “radiation, elevation, wind, land use,” “orientation of houses and windows, ventilation, and heat protection measures” (Wolf and McGregor, 2013). Heat levels are also noted to increase where there is higher population density, in more built-up areas because of the urban heat island effect, where there are fewer “cool spots associated with large parks”. The urban heat island effect is likely to play a role in London, being “a large city characterized by high urban densities, buildings and construction materials with a high thermal mass and increasingly important anthropogenic heat inputs into the urban atmosphere” (Wolf and McGregor, 2013).

Sensitivity depends on heat risk factors generally encompass “demographic characteristics (age, gender, family status), health status (pre-existing illness), access to resources, support and information (e.g. with regard to heat protection measures) and mobility” (Wolf and McGregor, 2013). Specifically, the clustering of high vulnerability areas in central London can be explained by high density housing in that area, while in east London the clustering is likely due to “poor health status and welfare dependency” (Wolf and McGregor, 2013).

The usefulness of heat vulnerability indices lie in the identification of individuals, groups, or regions with the highest vulnerability to heatwaves or other high heat conditions. While undoubtedly no index can be perfect, they can function as good springboards for further studies and deeper analysis.

References: