How is the global climate changing?
Heat kills. And under climate change it is doing so with ever greater frequency and intensity. Heat-related deaths worldwide increased by 74% between 1990 and 2016, with the most extreme single heatwaves claiming tens of thousands of lives and orders of magnitude more requiring medical treatment or hospitalization.
Extreme temperatures already claim more lives around the world than any other natural hazard and under climate change this risk is increasing. This makes heat stress a key priority under climate change. Nevertheless, a key obstacle to understanding heat stress is the complexity of measuring it. The thermal experience of climate change is shaped not only by a person's physical location, but also their social position. The jobs we do, the roles we play in society, the conditions we work in, and our freedom within those roles, all shape our exposure to the changing climate.
Average global land temperature
Average Cambodia land temperature
Source: ERA5, C3S/ECMWF | WBG Climate Change Knowledge Portal (CCKP, 2021). Climate Data: Historical. https://climateknowledgeportal.worldbank.org/ country/cambodia/climate-data-historical
Uneven occupation distribution
Understanding how these effects are intensifying under climate change is challenging but vital. The health impacts of prolonged heat exposure range from acute impacts such as heatstroke, to less visible, chronic disease vulnerability, especially when high temperatures meet high humidity and even more so when combined with heavy work. Those who are relatively socially disadvantaged are therefore likely to find themselves disproportionately affected by temperature extremes.
Left to right: Sunday Market, Phnom Penh, Cambodia. Photograph: Victor Wong | A farmer harvests mung beans in Cambodia's northern province. Photograph: World Bank Photo Collection | Construction workers on a building site in Siem Reap, Cambodia. Photograph: Alamy Stock Photo
Social heatwaves
Whilst the scale of the problem is increasingly recognised, understanding the lived experience of excess heat is a major research challenge. A key issue facing such efforts is that heat stress is socially as well as geographically determined. The thermal experience of climate change is thus determined both by one’s position in space, and one’s position in society. The jobs we do, the roles we play in society, the conditions we work in, and our freedom within those roles, all shape our exposure to the changing climate.
Garment factory, Sangkat Chaom Chao, Cambodia. Photograph: UN Women Cambodia/Charles Fox
What is heat Strain?
Heat strain occurs when our core temperature rises above this and our body cannot get rid of excess heat. Heat strain can occur when an individual is doing vigorous work or exercise, or because they are in an excessively hot environment. Often, it is a combination of the two. When this happens, the body's core temperature rises and the heart rate increases. If heat strain occurs for a short period, it may result in headaches, nausea and loss of concentration. If it is allowed to continue for a longer period, then it may result in kidney damage, lung damage, heart disease, and even death.
Research methods
‘Oppressive Heat’ will undertake a novel, rapid, and high-impact approach to this pressing global problem. Using newly developed CORE body-worn thermal sensors at the centre of a novel suite of conceptual and methodological tools, it will provide an entirely new humanscale lens on and directly inform heat stress policy.
A farmer sprays his crop, Prey Veng, Cambodia. Photograph: Thomas Christopholetti
HEAT STRESS ACROSS SPACE
In 2023, Cambodia, like much of Southeast Asia, experienced an unprecedented heatwave. Yet although the whole region was sweltering, workers in different parts of the country experienced a very different risk of experiencing heat stress. Cambodia is broadly divisible into four climatic zones: the Tonle Sap Lake Zone, the Platteau and Mountainous Zone, the Coastal Zone, and the Mekong delta. Workers in each of these zones had a different propensity to heat stress. In any given week, 80% of workers in the Tonle Sap Lake Zone experienced unsafe core temperatures at work at least once. By contrast, this figure was 72% in the coastal region, and 52% in the Platteau and Mountainous Zone and the Mekong Delta.
Heat stress across four climatic regions of Cambodia
Tonle Sap Lake zone
Kampong Chhnang
Platteau and Montainous zone
Mondulkiri
Coastal zone
Kampot
Plain zone
Svay Rieng
Location of data collection in all four of Cambodia’s climatic zones
HEAT STRESS ACROSS OCCUPATIONS
Heat stress is not only influenced by where we are, but also by what we are doing. Strenuous work raises our core temperature, which our body needs to release through sweating and increased blood flow to the skin. When the external environment is too hot and humid, it harder for heat to escape through our skin and sweat, which means workers doing strenuous work can experience dangerous core temperatures. In our study period, construction workers - who do strenuous work – spent 11.7% of working minutes at unsafe core temperatures, compared with 2.7% of working minutes for garment workers and 1.2% of working minutes for informal workers.
Risk of heat stress across four Cambodian cities
Comparison of heat stress amongst different occupations
HEAT STRESS WITHIN A WORKPLACE
So, heat stress depends on what work we do - but not everyone in a given workplace is doing the same thing. In any given sector, certain roles have a much higher risk of heat stress than others. The physicality of your work, and the specific conditions you have to do it in, play a key role in defining your experience of the environment. In the garment sector, for example, workers who are ironing spend 46% of their time at unsafe core temperatures, compared with only 2.5% of working time amongst workers who are sewing. In the construction sector, building a wall outside, beneath the sun, means 14.7% of working time at unsafe core temperatures, almost ten times more than building a wall inside, where only 1.8% of time is spent at unsafe temperatures. Informal sellers had the lowest propensity to heat stress overall, but selling fish, likely due to heat from the grill, saw 10% of working time at unsafe temperatures.
Which activities have the highest percentage of working time at unsafe temperatures?
Percentage of working time at unsafe temperatures. The size of the circle reflects the proportion of time at unsafe temperatures. The colour of the circle reflects the industry.
What is Wet Bulb Global Temperature (WBGT)?
The way that the body experiences heat depends on three factors: Temperature, humidity, and wind speed. So, to capture all of these together, we use a single metric called Wet Bulb Global Temperature, or WBGT. The WetBulb Globe Temperature (WBGT) is a measure of the heat stress in direct sunlight, which takes into account all of these factors. When WBGT goes up, then the proportion of time people spend at unsafe core temperatures goes up, unless they are able to take steps to avoid those rises. As shown below, when WBGT is at low levels, workers spend on average only 5% of working minutes with a core temperature over 38˚C, whereas when WBGT reaches severe levels, this rises to 27% of working time across all working groups.
Percentage of participants heat stressed in 7 days
Percentage of participants heat stressed in 7 days – overview and ‘by sector’
However, this effect varies a lot by sector. Garment workers are less sensitive to rising temperatures, increasing their proportion of minutes at unsafe temperatures only about 40% on severe WBGT days. Construction and informal workers, by contrast, roughly triple their proportion of working minutes at unsafe core temperatures between on days where WBGT reaches severe levels.
So what does 2050 look like?
As temperatures rise in Cambodia and globally, these issues will intensify. Our data suggest that by 2050 Cambodia will each year see an additional 227,818,146 hours of unsafe temperatures amongst its workers. As a result of the additional 135.7 days which reach “severe” WBGT, garment workers will see an additional 7.8% of working minutes at unsafe temperatures, whilst the construction sector will see a 20% rise and informal workers - from a low base - will see a 58% rise.
Using a brand new suite of methods, closely linked to policy and advocacy, Oppressive Heat aims not only to shed light on the vulnerabilities faced by workers, but to find ways to ameliorate them, in order to protect workers from the worst effects of our warming world.
