When external threats enter your nose, your nasal cells quickly respond by releasing tiny, fluid-filled sacs called extracellular vesicles (EVs), which go straight into your mucus. EVs contain molecules that can kill pathogens before they can even start to spread inside your body. But when you breathe in cold winter air continuously, the temperature of your nasal cells drops, impairing their ability to respond to pathogens efficiently. The study found that nasal cells exposed to cold temperatures release fewer EVs that are also of poor qu