Heat and cold as cardiovascular health hazards

Both hot and cold environments trigger a stress response in the human body and can lead to cardiovascular problems.

Physiologists have recently investigated both factors in scientific studies. The results, which were published in the journals Scientific Reports and Experimental Physiology, are especially interesting in light of the current multiple global crises.

The climate and energy crises are currently among the greatest challenges of our time and are having a direct physical effect on people's health. 

How heat affects health: ( of industrial workers) Seven male participants spent nine consecutive regular workdays in a controlled laboratory setting.

On the first and last three days, normal summer temperatures for Central European conditions ranged from 25.1 to 25.7 degrees during work and 21.8 to 22.8 degrees during rest periods. Days four through six represented the heat wave; during this period, researchers created ambient temperatures between 35.2 and 35.8 degrees during work periods and 25.5 to 27.1 degrees during rest periods including while sleeping at night. During the entire study, participants completed daily tasks to simulate typical industrial work.

The results show that even relatively mild heat waves cause an increase in core and skin temperatures and an increase in skin blood flow. While these physiological reactions help the body from overheating at rest, during standing the body must now defend both internal temperature and maintain blood pressure to prevent fainting, which puts an extra strain on the cardiovascular system.

Interestingly, many of these responses persisted after the heat wave was over, suggesting a residual effect of the heat wave. These responses reflect the stress on the cardiovascular system that industrial workers face during heat waves, which can lead to heat illness, fainting and even potentially death due to accidents or serious medical complications in persons with underlying cardiovascular disease.

How cold affects health: In another study, researchers examined the impact of mild cold exposure on the cardiovascular system, with a specific focus on what mechanisms are responsible for the increase in blood pressure.

Since preventing a rise in blood pressure in the cold is important, the study aimed to examine if the rise in vessel resistance (i.e. vasoconstriction) is due to a change in blood flow in the skeletal muscles or simply the skin. In a laboratory  the researchers cooled the skin temperature of 34 test subjects from a normal 32 to 34 degrees to about 27 degrees with ten degrees cold air—on one occasion the entire body was cooled, on another only the face was cooled.

They observed that when the entire surface of the body is cooled, blood pressure increases mainly due to an increase in vascular resistance of the skin, although there was also a slight reflex increase in resistance of the blood vessels inside the skeletal muscle. However, importantly, when only the face was cooled, they saw a very similar increase in blood pressure that was due to a reflex increase in vascular resistance of the skin throughout the whole body.

Thus, the team was able to show that the mechanism(s) responsible for the rise in blood pressure during cold exposure depends on which parts of the body are cold. These data are important to educate the population about preventing the potential negative consequences of cold exposure because contrary to the perception of many, cold is even more dangerous to the body than heat.

It doesn't take sub-zero temperatures—as you might think—to cause serious reactions in the body, which will become common for many people unable to heat their homes during the energy crisis. While people typically know to wear warm clothing to protect the skin of their body, arms and legs, they were able to show that protecting the face is equally important even in a mild ambient temperature of ten degrees.

Extreme effects: Both studies show that climatic conditions can have extreme effects on our cardiovascular system. While negative health aspects triggered by heat waves will increase due to the climate crisis, it is particularly surprising that even cold temperatures around 10 degrees can have significant negative effects on our cardiovascular system even in young people who were part of these studies.

Future studies extending these findings to our aging population and those with pre-existing medical conditions will certainly help mitigate the risk of these new environmental challenges.

Jason T. Fisher et al, Cardiovascular responses to orthostasis during a simulated 3-day heatwave, Scientific Reports (2022). DOI: 10.1038/s41598-022-24216-3

Hendrik Mugele et al, Control of blood pressure in the cold: differentiation of skin and skeletal muscle vascular resistance, Experimental Physiology (2022). DOI: 10.1113/EP090563