Stop sign before entering the desert


Human health and social development under threat by extreme heat and air pollution

Extreme heat and high levels of air pollution create a major and immediate threat to human health. How should climate research respond?

It is ‘very likely’ that future heat waves will be of higher intensity and frequency, according to the latest report of the International Panel on Climate Change (IPCC). Surely, the heat wave that struck Europe this summer, nicknamed "Lucifer", has exposed thousands of people to heat stress.

Due to such incidents, health impacts of climate change are gaining more attention on the world political agenda: The new Director General of the World Health Organization (WHO), Dr. Tedros Adhanom, has made analysis and action on health impacts of environmental and climate change one of his five priorities. A key issue will be the serious health impacts of air pollution in many urban areas around the world, and the health co-benefits of climate change mitigation actions that reduce the use of polluting fossil fuels.

In the absence of substantial reductions in air pollution, many people will suffer a double burden of heat and air pollution.

Jana Sillmann

The reasons for concern are plenty, as extreme heat may lead to a range of health effects, including heat stroke, heat exhaustion, dehydration and chronic kidney diseases, and indirectly also to increased chemical exposure and poisoning. Moreover, heat stress can lead to a worsening of cardiovascular and respiratory diseases, mental health degradation, and cognitive impairment. These health problems are already important health concerns in many tropical and sub-tropical areas.

Wide-spread air pollution also causes cardiovascular and respiratory health problems and several million deaths each year according to the WHO. The combination of high heat levels and significant air pollution raises the health risks. Children, elderly and pregnant women are particularly vulnerable to both extreme heat and air pollution.

How should climate research respond to this reality?

Studies indicate that there are synergistic effects between extreme heat and air pollution on human health and that combined extremes produce disproportionally greater impacts, beyond the sum of their individual effects.

The direct impacts of heat and air pollution on human health and the resulting indirect socio-economic impacts are largely rooted in health conditions where the cardiovascular system is being impaired. Occupational heat stress may lead to substantial loss in productivity in many parts of the world affected by extreme heat, and particularly in regions where the need for economic growth is the highest.

Research shows that tropical latitudes will be most impacted by extreme heat in the future. Large urban areas in this part of the planet are also at risk of severe air pollution. This region is also where the lowest socio-economic quintile of the world population lives; many million people carry out their daily work and other activities in very hot conditions.

Heavy labour activities create internal heat stress adding to the external heat exposures, which is a major reason for the special occupational health problems with high environmental heat levels. With high population densities and little incentives for improvement in the work environment, there are millions of people working in factories with poor or non-existent ventilation systems or in agriculture where they are fully exposed to heat and poor air quality.

Bilde mangler beskrivelse

Figure 1. Average of max daily temperature relative to humidity’ (WBGT) during the locally hottest month, 30 years mean, 1981-2010, CRU data (Kjellstrom et al. 2017)

Measuring heat stress

Humidity plays an important role in determining the heat stress threshold for people. The human body cools down through evaporation of sweat. If air humidity is high, the evaporation process is hindered and thus our bodies overheat leading to heat stress.

Wet bulb globe temperature (WBGT) is one way to quantify the heat stress that can affect health and productivity. The map above shows the current monthly average WBGT levels during the hottest month in each of 67,420 grid cells around the world. The levels are for the afternoon in the shade, so afternoon levels in the sun can be 3 degrees higher. From the colour scheme we see that, already now, most highly populated tropical and sub-tropical areas have Moderate or High heat risks.

What happens if the climate warms by two degrees?

In the light of increased frequency and intensity of heat waves coupled with future development of population, it is estimated that, by the end of the century, about 111 million people will be exposed to extreme heat of WGBT 30-31 degrees under an average global temperature increase of two degrees. 11 million will experience WGBT 31-32 degrees, and 300 000 people with be exposed to temperatures over WGBT 32 degrees in the hottest monthly mean.

Adding to the distress, the projected heat exposed regions often overlap with regions that currently have the highest levels of air pollution. Thus, in the absence of substantial reductions in air pollution, many people will suffer a double burden of heat and air pollution.

Studies on occupational health show an increased loss of productivity due to heat from less than two percent today to over six percent by the end of the century (UNDP report on “Climate change and Labour”, 2016). Construction, agriculture and manufacturing are only a few of the affected activities where outdoor and indoor exposure to heat create socio-economic losses yearly, and for which losses are expected to increase rapidly. An increase in non-fatal diseases resulting from heat stress is expected to put pressure on the health systems.

Furthermore, the expansion of ‘super-hot’ areas in tropical latitudes is expected to trigger migration to cooler regions. According to International Organization for Migration (IOM), even in the best-case scenario of the lowest increase in temperature, millions will potentially be unable to maintain their daily life activities during hot periods of the year in low and middle income countries but also in developed countries (IOM report "Extreme heat and migration", 2017).

Therefore, even if we do manage to keep global average temperatures from rising above the two degree threshold, global warming will have severe impacts on millions of people.

Future research needs

A growing number of toxicological and epidemiological studies have demonstrated that heat is a major global health hazard, and the same can be said for air pollution, e.g. in terms of fine particulate matter and surface ozone. The impact of heat and air pollution is expected to increase together with increased frequency and magnitude of heat waves due to synergistic co-occurrence of these extremes.

Potential physiological and toxicological synergistic effects are however poorly understood. An important research challenge is disentangling the impacts of air pollution and heat and their potential synergistic effects.

Health effects will potentially affect the demand for health services, reduce the productivity of the labor force, and lead to migration. The social and economic impacts may be huge, but we know little about them. Most assessments of health effects focus on mortality under heat waves, and there are studies of how heat affects the productivity of labor, which have been generalized to broader scales.

However, behind the number of deaths, there is larger amount of illness among people who need health care. Moreover, loss of labor productivity implies that employers will have to replace it. This is costly, but perhaps less costly if people migrate from more exposed areas. To address the overall consequences, one therefore needs to integrate knowledge from many different perspectives.


CICERO leads an interdisciplinary working group consisting of climate change researchers, occupational health experts and medical experts from Ullevål Hospital and the Norwegian Institute for Public Health that aims to address the issues of synergistic effects of heat and air pollution on human health in the light of future climate change (the EXHAUSTION project). This extends current research done in context of combined effects of climate change and air pollution on agriculture within the current CiXPAG project. Two seminars in August at CICERO and the Medical Faculty of the University in Oslo by Professor Tord Kjellstrom described some of these health threats and the future trends under different climate change pathways. It is clear that further research on these two major health threats heat and air pollution will help guide future prevention policies at global and national levels.