When an Afternoon Walk in the Heat Leaves You Worn Out — Why Your Body Handles Heat Poorly During Treatment, and How to Walk in Cooler Hours

Summer heat and high humidity can be a heavier burden than expected for people undergoing treatment. During chemotherapy in particular, the body's ability to regulate its own temperature can be reduced, so you may tire and feel dizzy more easily even in the same weather that others tolerate.

When we get hot, we sweat, and as that sweat evaporates it cools the body. But when humidity is high, sweat does not dry well, and heat becomes trapped in the body. If appetite has dropped and you have not taken in enough fluids or food, dehydration can add to this, making dizziness, headache, and fatigue more likely.

These risks can be greater during treatment. Some chemotherapy drugs and targeted therapies cause fluid loss through diarrhea and vomiting, and if you also take a diuretic or blood pressure medication, dehydration can progress faster. In addition, radiotherapy and certain medications can make the skin sensitive to sunlight, so exposure to strong midday ultraviolet (UV) light may cause the skin to redden and sting easily.

For this reason, with light exercise such as walking, the timing matters. It is safer to avoid the hottest and most humid hours, roughly noon to 5 p.m., and instead choose the early morning before sunrise or the evening after sunset. On days that are extremely hot and humid, rather than forcing yourself outdoors, marching in place indoors or doing gentle stretching is a good alternative.

When you do go out, sip water often, and prepare loose light-colored clothing and a wide-brimmed hat. If you notice cold sweats, nausea, severe dizziness, a racing heart, muscle cramps, or sudden weakness while walking, stop right away to rest in shade or a cool place and replace fluids. If you do not improve with rest, or if your awareness becomes clouded, ask for help without delay and contact your medical team.

This article is for general information only and does not replace medical care. The right intensity, duration, and fluid intake differ from person to person, so please decide what suits your condition together with your treating clinicians.