Last year for the first time in My life I coughed up a ball of white, thick, dry, sticky sputum that appeared to have dark grey specs, like lung tissue. I'm 44, gave up smoking 23 years ago, and have had three more incidents in 10 months, the latest being today. If I unravel the sputum it resembles the shape of a bronchial tube. My doctor cultured a sample and said it came up negative. Why do I keep coughing this stuff up?