What Causes Mucous And Phlegm In Throat?

I have continuous mucous and phlegm in my throat and am constantly hacking it up and spitting it out. Sometimes I can t talk because it gets caught in my throat on my vocal cords. It is very thick and clear. I have been diagnosed with asthma since age 3 (I am age 72 now) but am taking Qvar for that and don t experience asthma attacks at all. Breathing cold air is getting harder. Thanks, Don