As temperatures drop, the heat stress that the human body must handle also decreases. According to a recent study synthesizing heat exposure data from about 400 experiments, higher temperatures increase cardiovascular burdens such as heart rate and cardiac output. When exercising, muscles generate a lot of heat, and in hot environments, the body increases blood flow to the skin and sweats more to release this heat outside. At the same time, the heart must circulate more blood to supply oxygen and nutrients to active muscles. During high-intensity exercise, cardiac output can increase to about five times that of resting levels.

However, when autumn arrives and temperatures drop, it becomes much easier to release body heat through the skin, and the cardiovascular burden required for thermoregulation decreases. This is why running at the same pace feels easier in autumn than in summer. Interestingly, our bodies' adaptation to summer does not immediately disappear just because the weather has cooled. When people consistently exercise in the heat, their bodies change. Even at the same speed, heart rate and temperature rise less, and the ability to dissipate heat using sweat improves. This process of the body adapting to handle heat efficiently is called "heat adaptation."

Such adaptation does not vanish instantly when the heat disappears. A meta-analysis analyzing the loss of heat adaptation estimated that the adaptation effects observed in heart rate and core body temperature at the end of exercise gradually decay at a rate of about 2.5 percent per day after stopping heat exposure. However, the rate of loss varies depending on physiological responses such as sweating. In other words, even though external heat stress decreases quickly, the physiological adaptations acquired during the summer remain for a certain period. Our bodies experience autumn a little more slowly than the calendar does.

Then what about the respiratory system? Autumn is not necessarily a comfortable season for the respiratory tract. As exercise intensity rises, minute ventilation increases and mouth breathing becomes more frequent. The nose warms and humidifies external air before sending it to the airways, but inhaling a large volume of air rapidly through the mouth introduces relatively cold and dry air into the airways. This process increases heat and moisture loss from the airway surface, which can trigger exercise-induced bronchoconstriction in individuals with asthma or airway hyperresponsiveness.

Pollen is also a variable. While tree pollen is primarily problematic in spring, the impact of grass and weed pollen can grow during late summer and autumn. Because ventilation increases during exercise compared to everyday conditions, exposure to airborne allergens also rises. Ultimately, early autumn is a fascinating transitional period in exercise physiology. While an environment is created where heat stress decreases and running performance can improve, the respiratory system is exposed to new stimuli such as cold, dry air and seasonal allergens.

Rather than suddenly increasing exercise intensity just because the weather has cooled, it is wise to monitor both respiratory symptoms and temperature changes together. The same autumn breeze carries different physiological meanings for the heart, muscles, and respiratory system.