It's not the heat, it's the humidity. But why does humidity make you hotter?
Humidity is the measure of how much moisture is in the air. The hotter the air, the more moisture it can hold. For example, a high relative humidity of 100 percent means that the air is saturated with moisture and can't hold any more. Our bodies rely on the evaporation of moisture from our skin to keep cool. When relative humidity is low, sweat evaporates from the skin very quickly. But when it is high, body sweat does not evaporate as easily, and we feel sticky and hot.
Overheating can be very dangerous. Heat exhaustion occurs when the body loses fluid and salt through perspiration faster than they can be replaced, causing dizziness. Exercising in hot weather can cause muscle cramps, especially in the legs, because of brief imbalances in body salts. People not used to exercising in heat may experience a quick drop in blood pressure that can lead to fainting. And in some cases, extreme heat can cause body temperature to rise to 105 degrees or higher, causing a heat stroke, with confusion and unconsciousness.
Low humidity makes us feel cooler than the actual temperature because our sweat evaporates quickly. But low humidity can cause dry, itchy skin and chapped lips. In the winter, if it is 70 degrees inside your home and humidity is low (around 10 percent), it will feel like 65 degrees. Investing in a humidifier to bring the humidity up to 70 percent will make it feel 5 degrees warmer. And since it costs less to humidify the air than it does to heat it, you can save money. A relative humidity of about 45 percent is considered ideal for indoor environments.