If you live in a hot and humid climate, you may know what it’s like to feel as if you’re gasping for air on an especially hot day. For people with asthma or other chronic lung conditions, heat waves—which often coincide with dangerous ozone levels and humidity—may cause asthma flare-ups and breathing difficulties. This is because hot air traps more moisture than cold air does. Humid air cannot absorb more water vapor, which makes cooling by sweating challenging. This is why you may perceive humid heat as unbearable compared to dry heat of the same temperature.

To understand how breathing works when it’s hot, it’s important to learn about the relationship between oxygen and the molecule responsible for transporting oxygen, called hemoglobin.

What is hemoglobin?

Hemoglobin is an iron-containing protein that binds to oxygen molecules in your lungs. Red blood cells transport oxygen to all cells in your body. Hemoglobin makes up one-third of our red blood cell volume, which makes sense as almost all the oxygen circulating in your body —about 98.5%—is bound to this molecule.

The strength of oxygen binding to hemoglobin is temperature-dependent. Lower temperatures help oxygen and hemoglobin to bind together, while higher temperatures make it easier to break the bond between oxygen and hemoglobin.

How does heat affect hemoglobin-oxygen bonding?

Everyone knows how warm your muscles and skin become when you exercise or have a high fever. Having a higher temperature in a particular organ (such as your working biceps) lowers the strength of oxygen bonding with hemoglobin. This means there is more effective oxygen delivery to organs that require lots of it. Usually, the lungs are cooler, which is perfect for picking up oxygen and loading it onto hemoglobin.

How does heat affect the respiratory system?

Exposure to excessive heat relaxes many blood vessels in the skin in your body’s attempt to lower its temperature. Usually, you do this by sweating, but in extreme cases, this response may lead to fainting due to a drop in blood pressure.

Our respiratory system responds to overheating by increasing the rate and depth of our breathing (hyperventilation), which can make it feel like it’s hard to breathe.

Hyperventilation can be dangerous to people with asthma and chronic lung conditions because the larynx may get irritated by heat and humidity. That irritation can lead to a narrowing of the lower airways (called bronchoconstriction) and cough. Using an inhaler—prescribed by your doctor—can help prevent asthma attacks and uncontrolled coughing that are caused by a combination of bronchoconstriction and hot weather.

Climate change has been found to change the air quality around us and makes extreme fluctuations in weather more likely, which can affect more than your breathing. Everyone should know how to stay safe during the hottest summer days:

• Stay indoors in air-conditioned space as much as possible.
• Drink plenty of fluids, especially when you’re exercising.
• Wear light, loose-fitting clothes—you may even want to wet your T-shirt.
• Avoid strenuous activities during the hottest part of the day.
• Seek medical help if you have symptoms of heat-related illness.

As we head into summer, stay cool, be safe and breathe easy.