I have a stressful job, so I smoke ten a day to keep myself calm. However, I now find I'm coughing each morning and bringing up some rather mucky-looking stuff. Why?

It sounds as if you may now be developing what's called "smoker's lung". This is an inflammation, caused by long-term smoking. To be frank, if you don't stop now, things may progress towards more serious lung disease. Please go and see your GP right away because she can arrange free help in giving up ciggies. Good luck.