Joggers in Tennessee are running out the clock ahead of winter wind chill warnings and plummeting temperatures expected Friday.
"If it's really cold like single digits, it hurts when you breathe," one jogger said. "That's the hardest part about running in the cold."
Cold temperatures stress the human body in many ways.
Breathing in cold air can hurt a person's airways, and people with chronic lung conditions like COPD and asthma are more at risk of deep coughs or difficulty breathing.
In the cold, the risk of heart attack and stroke goes up. The blood vessels contract to keep humans warm, raising blood pressure. That constriction can also cause more urination, leading to dehydration.
Cold also hurts our immune system.
"Some recent evidence shows that even the colder it is, the easier it is to be infected with some respiratory viruses," Dr. Peter Chin-Hong said. "Staying as warm as possible is always going to be helpful and might actually save you from getting some infections."
Frost nip, that feeling when the skin is numb and tingly and is pink or red in color, is also common. It's uncomfortable but not permanent. Frostbite, which is more severe, can cause more damage.
Prolonged cold exposure can also cause hypothermia — when the body is losing heat faster than it can make it.
A 2015 study shows more than 7% of global deaths were tied to cold exposure, meaning staying warm isn't just about staying safe; it's about staying healthy too.