Mucus Congestion refers to a build up of mucus in your lungs and throat, causing congestion and — in severe cases — leading to difficulty breathing or swallowing. Mucus, also known as sputum, is a sticky, gelatinous material that lines your lungs, throat, mouth, nose, and sinuses. It's produced by membranes in the nose and sinuses known as the mucous membranes. Mucus production is normal and serves many purposes, even when you're healthy. It protects the tissue that lines your lungs, throat, and nasal and sinus passages, keeping it from drying out. It also works to trap unwanted bacteria and allergens (such as dust or pollen), preventing them from spreading through your body and making you sick. Mucus even contains antibodies, or enzymes, designed to kill or neutralize these harmful materials. The body usually produces as much as 1.5 liters of mucus a day, even when you're not feeling sick.