Chronic coughing of white phlegm is a symptom of several different diseases that could be potentially causing your discomfort. The most important distinction to make with this type of condition is determining the duration of how long this coughing has occurred.
If you have noticed this white phlegm only recently and it has been a short-term nuance, the chances are that this is just mucous that had accumulated during a recent upper respiratory tract infection and will self-resolve within a few days. You can hasten the disappearance of these symptoms if you are using some cough lavage or drink a few more glasses of water every day to help dissolve the mucus and make it easier to cough out.
If this has been a more chronic condition, this is when you will need to seek further medical attention. Just a chronic cough associated with white phlegm can point to a disease like chronic bronchitis. This is a long-term infection of the lower respiratory tract and leads patients to suffer from a chronic cough. Generally, patients will also have a history of smoking or asthma if they are with this condition.
If the symptoms are more cumbersome than just a simple cough, the severity of the lung infection can increase. Although rare in North America compared to Europe, tuberculosis is still a very real possibility in modern society. There has actually been an increase in prevalence of tuberculosis infections in North America in recent years due to not only an increase in the number of immigrants coming into the country who have the disease, but because of the decision by a growing number of parents to not vaccinate their children due to concerns about the development of autism.
With both of the previous conditions I have mentioned, depending on the stage of the infection, it is possible to have either white or colored phlegm. A simple white content throughout the whole duration of the infection more likely points to a viral respiratory infection. Consider yourself a prime candidate for this type of infection if you have a young child or routinely spend time with young children. They are most likely to contract these viral respiratory infections and pass it on to you.
What you can do if you find yourself suffering from this condition on a long-term basis is go to your local pulmonologist to get your symptoms checked out. You will need to provide a sample of your phlegm in all likelihood and the doctor may ask about recent trips you have taken and your history of vaccination against tuberculosis. If the origin of your infection is still not clear, the next step that will probably follow is that he will ask to perform a bronchoscopy on you. This entails passing a table into your throat to visualize the lower respiratory tract to visualize what exactly you are dealing with. Treatment can range from anything between symptomatic management to long-term antibiotic use to try to rid yourself of the infection. 
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