Whether due to allergies or illness, we've all had to deal with Phlegm at some point in our lives. A child's runny nose, sinus congestion and coughing up "gunk" are just a few common types of Phlegm. According to Chinese Medicine there are two main causes: diet and the environment. Diet is often the cause of chronic presentations, like with allergies, whereas, according to Chinese Medicine, the elements in your environment cause illness (you can read more on that here).
The Spleen is in charge of Transforming and Transporting fluids in the body. When it is working properly, it provides moisture to the skin, hair, eyes, blood, etc. When it isn't working properly, the fluid can accumulate and become Dampness, which can present as foggy thinking, low energy with a heavy, draggy feeling to it, achy muscles, joints and general pain that is worse with humid or damp, rainy weather. With long standing accumulation and Spleen weakness, the Dampness can congeal and become Phlegm which is stickier and more tangible. Phlegm is what congests your nose and chest or becomes nodules such as lipomas, cysts and other non-blood related "lumps and bumps."
Whether the Phlegm or Dampness is due to illness or allergies, diet is critical. What you eat can either improve or worsen your symptoms. Phlegm is a Yin substance which means it is slow moving, sticky, sluggish and Cold. You can have Phlegm Heat conditions, but that usually occurs when the Phlegm has been stagnant for long periods of time, and like friction in a car engine, Heat results. In and of itself, Phlegm is Cold and improves with Heat as it starts to dissolve and move. Eating warm Sweet foods will support the Spleen and will help your body prevent further Phlegm from accumulating, or prevent Phlegm in general. Unfortunately, this kind of Sweet does not include refined sugar, candy or brownies (sorry); I'm talking about foods like sweet potato, beetroot, squash (summer and winter), rice, millet, grapes, coconuts, Wild Atlantic salmon and organic chicken; these foods are sweet in terms of the 5 flavors--Sweet, Salty, Pungent, Bitter and Sour (there is technically a 6th flavor, Bland, associated with the Spleen, but it is a secondary flavor). Bitter foods like alfalfa, celery, radish (especially Daikon), green tea and lemon (technically sour, but helps with Phlegm), can help drain Damp and Phlegm that is already there.
Foods to avoid when you have Phlegm are foods that are hard to digest: wheat and other gluten grains (rye, barley, some oats), dairy (especially from a cow--sheep and goat dairy are less problematic, but should still be avoided), nuts and nut butters, cold or iced food or beverages, sugar and artificial sweeteners. The worst food to have is cold dairy, like ice cream. All of these foods encourage inflammation and mucus production by damaging the Spleen; when your system is already weakened (shown by illness or allergies) these foods will only encourage your body to make that much more Phlegm. For people with healthy Spleens some of these foods are fine to eat in moderation, but when your Spleen is compromised, they are best to be avoided. If you are unsure about the state of your Spleen, you can ask your local acupuncturist, or look for symptoms like: fatigue, low appetite, loose stools or constipation, achy muscles, symptoms that are worse with rain or high humidity, foggy thinking or lots of phlegm (allergies, needing to blow your nose every morning, chronic throat clearing, post-nasal drip).
Start with your diet. If that is not enough, Acupuncture and Chinese Herbal Medicine can greatly improve your health by strengthening your Spleen and reducing Phlegm. First food, then medicine.