The source of a white phlegm can be from conditions from the nose, lungs and stomach.
Common causes include:
1. Nose & sinuses - Common cold, Allergic Rhinitis (sensitive nose), Sinusitis (sinus infection) etc
2. Lungs - bronchitis (inflammation of the lungs), pneumonia (infection of the lungs), smoker's lungs etc
3. Stomach - acid reflux from the stomach
4. Dehydration
I would suggest a visit to a General Practitioner for an initial assessment if your condition has been persistent for a while.
Hope this helps and all the best!
