2 doctors weighed in:

What causes black stuff in phlegm?

2 doctors weighed in
Dr. Stephen Siegel
Internal Medicine - Pulmonology
1 doctor agrees

In brief: Smoke, old blood

Most common cause of black phlegm would be from smoke such as from cigarette/marijuana smoking.
Coal miners used to have black phlegm but now they wear protective gear. Another cause would be old dried up blood from a bad sinus infection or bronchitis infection. Heavily polluted air such as from a recent fire is another possibility.

In brief: Smoke, old blood

Most common cause of black phlegm would be from smoke such as from cigarette/marijuana smoking.
Coal miners used to have black phlegm but now they wear protective gear. Another cause would be old dried up blood from a bad sinus infection or bronchitis infection. Heavily polluted air such as from a recent fire is another possibility.
Dr. Stephen Siegel
Dr. Stephen Siegel
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