Dehydration: The most important reason for phlegm not coming out is the lack of water in the body. When you do not drink enough water, the mucus in the lungs becomes drier and thicker like glue (Thick and sticky mucus). It sticks to the walls of the airways and does not come out easily even when you cough.
Chronic Bronchitis / COPD: Smokers and those who inhale excessive dust and smoke often have chronic inflammation in the airways of the lungs. As a result, the glands in the airways always produce thick mucus. This blocks the airways and causes the mucus to build up inside.
Weakness of the respiratory nerves and weak cough reflex: You have to cough hard to get rid of the phlegm. However, in the elderly, those with severe muscle weakness, and those with chronic respiratory diseases, the ability to cough (cough reflex) is very low. Since the cough is not strong, the body is unable to bring the mucus deep in the airways up.
Damaged Cilia: There are millions of small hair-like cells called ‘cilia’ inside the airways of our lungs. These always move upwards and help push mucus and dust out of the throat. However, when these cilia are damaged or their movement stops due to smoking or severe respiratory infections, the mucus accumulates inside instead of being expelled.
Asthma & Allergies: In people with allergies and asthma, the airways narrow rapidly (airway constriction) and a large amount of mucus is produced. As the airways become narrowed, the way for this thick mucus to come out is blocked and it gets stuck in the chest.