On June 19, 2023, on the Mexico City subway carrying Juan to his appointment, the temperature was 42 °C (107.6 °F). As soon as he arrived at the clinic, the first patient was an elderly woman who reported dizziness and nausea for the past two days. During June, the heat wave affecting the Mexican capital, one of the largest metropolises in the world, was already having consequences on the health of its inhabitants.

That day, as reported by the National Meteorological Service, an extreme maximum temperature of 41 °C (105.8 °F) was recorded for Mexico City, while the Ministry of Health reported 1,072 cases associated with extreme temperatures and 100 deaths nationwide attributable to the summer third heat wave, which occurred between June 1 and June 22.

Recently, this phenomenon has become increasingly frequent and scientists suggest that due to global warming, we are observing an increase in temperature, in particular, heat waves during the summer. This is causing an increase in diseases associated with extreme heat, mainly in areas where the phenomenon known as urban heat islands occurs.

What are heat islands?

The term heat island refers to those sites or areas within cities in which the surface temperature of streets and buildings is significantly higher than that recorded on the periphery of urbanized areas, with differences that can reach up to 10 °C (50 °F). These heat islands are the result of the growth and development of urban areas, the construction of new buildings, and the increase in streets and avenues covered by materials such as asphalt or concrete that allow heat to accumulate during the day.

In addition, as part of the activity of cities, there are other sources of heat such as automobiles, public and cargo vehicles, or refrigeration equipment such as air conditioners  — increasingly used — whose temperature is also stored on the surfaces of facades and streets. All the heat accumulated during daylight hours is slowly released at night.

The consequences of extreme heat

Rising temperatures have impacts on the health and well-being of the population manifested in different ways, ranging from loss of comfort, which can be associated with irritability and lack of concentration, to dehydration, respiratory problems, sunstroke, and fatigue. Heat stroke can even cause death, since, facing high temperatures, the body’s capacity to dissipate heat can be overcome, with children, the elderly, and people with respiratory diseases being the most vulnerable sectors of the population.

Heat stroke, according to the Mexican Institute of Social Security (IMSS), occurs when the temperature rises above 40 °C (104 °F), exceeding the body temperature, which is considered normal between 36.5 °C (97.7 °F) and 37.2 °C (99 °F). This occurs mainly when the person loses the ability to self-reduce the body’s temperature through sweating due to dehydration.

The effects associated with urban heat islands and heat waves are a problem that has consequences for the world’s population. In its 2022 report, The Lancet Countdown published the results of a study conducted in 43 countries in which it highlights that heat-related mortality increased by about 68% in people over 65 years of age between the periods 2000-2004 and 2017-2021. For the same period, the document reports the percentages of change and number of deaths per country. In the case of Latin America, Ecuador stands out, with a percentage change of 1477% (300 deaths); Honduras, with 547% (190); Brazil, 191% (3,920); Mexico, with 123% (2,070), and Argentina with 85% (1,300).

Another effect is the difficulty in preserving food, which tends to spoil easily if exposed to higher temperatures, making it necessary to use refrigeration systems for preservation.

The case of Mexico City

But before taking any action to prevent or reduce this phenomenon, it is necessary to know in detail the areas where the highest temperatures occur, what is the amount of population exposed, and the conditions of each place. Therefore, from the National Autonomous University of Mexico, a group of researchers from the Seminar on Climate Change, Biodiversity, and Health are conducting a study to identify the areas of greatest vulnerability, as well as the perception of the population in two municipalities, to propose measures to mitigate the effects of the heat island on the population.

According to the preliminary results of the survey, 10% of people say that during heat waves they usually suffer from headaches, while almost half of them suffer from headaches only sometimes. In addition, almost one-third of the respondents indicated that they sometimes suffer from dizziness, and 15% that on hot days they regularly tend to suffer from nausea or vomiting.

The information obtained in this study allows us to refine the lines of action to identify the most appropriate and pertinent actions to prevent and care for the population exposed to these heat islands. Studies conducted by various researchers indicate that in areas where there is little or no vegetation, temperatures tend to be higher than in areas with urban trees.

How can this be solved?

The impact of the heat islands effect is so important that various national and international, governmental, academic, and civil society organizations (NGOs) have proposed various initiatives and actions to reduce the concentration of heat in cities and, therefore, the risk of affecting the population. These include increasing the number of urban trees and green areas, painting roofs and facades with light colors, and installing green rooftops, which consist of covering these surfaces with natural vegetation. These actions generate greater reflectance, thus reducing heat storage. Other measures include encouraging natural ventilation and installing low-energy and low-heat equipment in buildings.

Attracting attention to minimize or address the effects associated with urban heat islands depends on society as a whole. Thus, actions must be carried out from different trenches and at different scales, from the various spheres of government, academia, organized civil society (NGOs), and from within the family at the personal level. All these spheres must work in a coordinated and harmonious manner. From the individual and family level, we can do a lot to minimize energy consumption, from replacing energy-saving equipment to planting trees on our properties or including vegetation on our rooftops. Governments and NGOs should promote mechanisms to adapt the legal framework to favor urban tree planting and architectural designs following the climate of each city, to re-naturalize urban areas. There are various options, so the first issue we must have is the civil and political will to address this problem, which can increase significantly if ignored, as a result of global warming.

_{Leticia Gómez-Mendoza is a full-time professor at the Colegio de Geografía, Facultad de Filosofía, UNAM. She holds a PhD and a Master’s degree in Geography from UNAM. She is a specialist in meteorology, variability, and climate change.}

_{*Translated by Janaína Ruviaro da Silva from the original in Spanish.}