7 Ways High Humidity Can Affect Your Health

Medically reviewed by Jane Kim, MD

Humidity is the amount of water vapor in the air that can make an already hot day feel unbearably sweaty and sticky. When it's hot, your body sweats and air evaporates moisture off your skin to help you cool down. But hot temperatures feel even hotter in high humidity because the air can't help your sweat evaporate as effectively. 

The temperature the human body actually feels is called the heat index, which factors in the humidity and air temperature. Measuring the heat index is how a 90-degree Fahrenheit day (32 degrees Celsius) with 75% humidity feels like 109 degrees Fahrenheit (42 degrees Celsius). Heat indexes starting at 103 degrees Fahrenheit (39 degrees Celsius) create dangerously hot temperatures for the human body.

In the summer, humidity feels bad because warm air naturally has more water vapor than cold air. If you live in a climate near warm oceans or bodies of water, that water evaporates into the air and makes humidity worse. Because high humidity makes it difficult for the body to stay cool, people are more likely to experience heat-related illnesses that make them feel sick and exhausted.

Here are some specific ways humidity can affect the human body.

Dehydration

Humidity can cause excessive sweating and make you dehydrated. Dehydration is when you lose more water than your body can replenish. Since high humidity (high humidity usually means over 75% relative humidity) makes sweat linger on your skin, your body can't effectively cool down. As a result, you'll keep sweating as your body tries to regulate its temperature, causing you to lose water. Without enough fluids, your body can't function properly and can start to shut down.

If you become dehydrated in humid conditions, you can experience dizziness, fatigue, headache, muscle cramps, dry mouth, dark-colored urine, and less urinating and sweating. These are symptoms related to general dehydration, and not specifically dehydration in humid climates.

Drinking more water can usually treat mild dehydration. You may also need to drink fluids that replace water and electrolytes—like a sports drink. If you don't replenish your fluids in humid conditions, dehydration can become life-threatening and require medical attention. Signs you're severely dehydrated include loss of consciousness, confusion, lack of urination, heart palpitations, and rapid breathing.

Muscle Cramps

As humidity makes you overheat and sweat profusely, you can lose too many electrolytes and start cramping up. Electrolytes are electrically charged minerals in sweat that help your body regulate hydration, muscle function, the nervous system, and blood pH. Low electrolyte levels make it difficult for your body to regulate muscle function, causing muscle cramps.

Also called heat cramps, heat-related muscle cramps or spasms indicate your body is overheating and losing electrolytes. Heat cramps usually occur in your arms, legs, or abdomen. Cramping is especially common if you're doing any physical activities. If you are already dehydrated from extremely humid conditions, you can also experience heat cramps, which are a sign that your body is overheating. Like other heat illnesses, replenishing your fluids and electrolytes is the first step in treating muscle cramps.

Fatigue

Feel exhausted after a day in sticky heat? Humidity paired with hot temperatures can make you feel lethargic and sleepy. When it's hot and humid, your body works overtime to cool down. This constant effort to regulate your temperature is a lot of work and uses up precious energy. 

When your body increases sweating to cool down, you also lose a lot of water and electrolytes. You can feel fatigued if you become mildly dehydrated from water loss. Low electrolyte levels like potassium, calcium, and magnesium can also make you feel tired. You may also experience cramps, nausea, and vomiting when electrolytes are imbalanced. Resting out of the heat and drinking plenty of water and fluids with electrolytes can help you recover.

Fainting

When the body gets too hot in high humidity, it also dilates blood vessels to help release heat. When blood vessels dilate too much and there is some dehydration, blood pressure drops, and blood flow to the brain is reduced, causing someone to pass out.

Fainting due to heat is technically called heat syncope. Heat syncope also includes episodes of dizziness or light-headedness. Heat-related fainting usually happens when someone becomes dehydrated or isn't acclimated to humid environments. Fainting is common during physical exertion in heat, especially after standing for a long time or suddenly standing after sitting or lying down. If you faint in hot and humid temperatures, rehydrate and rest in a cool place.

Heat Exhaustion

Since humidity can make hot temperatures feel worse, prolonged periods in heat and excessive sweating can lead to heat exhaustion. Heat exhaustion is when your body can't regulate its temperature and starts to overheat. Heat exhaustion symptoms include excessive sweating, dizziness, weakness, headache, nausea, and elevated heart rate. 

Insufficient fluid intake, spending several days in hot and humid weather, and strenuous physical activity are common causes of heat exhaustion. Heat exhaustion is often a milder heat-related illness that can be managed by moving into a cooler environment and replenishing fluids and electrolytes. But if your symptoms worsen and you start vomiting, heat exhaustion can become heat stroke.

Heat Stroke

Heat stroke is a life-threatening illness that happens when sweating fails to cool your body down. As a result, your body loses control of temperature regulation, and your core body temperature rises quickly. A body temperature above 103 degrees Fahrenheit (39 degrees Celsius) is a warning sign of heat stroke. But it only takes 10 to 15 minutes for your body temperature to rise to 106 degrees Fahrenheit (41 degrees Celsius) or higher. At this temperature, your body can no longer regulate its temperature, and your organs can begin to become damaged and fail.

Heat stroke can be fatal if you don't get medical attention quickly, and the heat illness can cause loss of consciousness and seizures. Other warning signs of heat stroke include confusion, slurred speech, excessive sweating, hot and dry skin, nausea, vomiting, and low blood pressure. Always call 911 or seek immediate emergency care if someone has signs of heat stroke.

Heat Rash

When it's hot and humid, excessive sweating can irritate your skin. Heat rash—aka prickly heat, miliaria, or sweat rash—happens when your sweat ducts and glands become blocked due to excess sweat. Sweat can become trapped under your skin when you keep sweating in humid conditions. Trapped sweat then blocks sweat ducts and causes inflammation that leads to a skin rash. Heat rash symptoms can vary, but the rash usually looks like small clusters of tiny blisters or red bumps. This skin rash may also feel itchy or sting.

Heat rash usually appears on areas where skin touches the skin and causes friction, like your neck, groin, armpits, breasts, or elbow creases. You may also experience heat rash where clothing touches your skin. Heat rash is more common in children—especially babies under three weeks old. Adults can experience heat rash, too, especially if they live in a hot, humid climate. Reducing sweating, keeping skin dry, and getting out of the heat can usually treat heat rash. If the rash doesn't resolve on its own, you may need to see a healthcare provider for corticosteroids or antibiotics.

A Quick Review

Humidity is the measurement of water vapor in the air and can make high temperatures more dangerous. Humidity makes your body feel even hotter and disrupts how your body sweats and cools down. When it's humid, sweat evaporates slowly from your skin, making your body work overtime to stay cool.

Excessive sweating can make you lose too much water and electrolytes, causing dehydration, muscle cramps, and fatigue. Extreme heat can also cause dangerous heat-related illnesses like heat exhaustion and heat stroke. 

If you'll be outside in high heat, be mindful of how the humidity can cause dangerously hot temperatures. To help prevent heat-related illnesses, make sure to hydrate, limit strenuous activities, take breaks inside, and wear breathable clothing to help your body cool down.

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