Phlegm often occurs due to an infection of the lungs or sinuses, such as strep throat, the flu, sinusitis or the common cold, according to Healthgrades. Allergies, inflammation, tonsillitis, reactions to some medications, exposure to secondhand smoke, trauma, asthma, gastric reflux, bronchitis or laryngitis can also cause phlegm. Additionally, pneumonia, cystic fibrosis, tuberculosis, airway obstructions, cancer and congestive heart failure are some other, more serious, underlying causes of the condition.
Everyone produces phlegm on a daily basis, and it is a natural body process that is not noticeable until some type of illness causes phlegm production to increase to a perceptible level. As explained by Healthgrades, some infections cause phlegm to amass to such a level in the air passageways that it becomes difficult to breathe properly. If this is a concern, or if phlegm turns to an unusual color, contains trace amounts of blood, and does not clear up over time, it is important to seek medical attention and obtain a doctor's diagnosis of the condition.
Common causes of phlegm often produce additional symptoms, such as fever, sneezing, cough, watery eyes, nausea, fatigue, sore throat, headache and muscle aches, notes Healthgrades. Phlegm that is accompanied by throat swelling, chest pains, shortness of breath or facial inflammation constitutes a medical emergency. Additional causes for concern include rapid heartbeat, personality changes, difficulty swallowing and speech alterations.