pleural thickening

I'm 24 years old, and my yearly chest x-ray at work revealed that I have some pleural thickening at the top of my lungs.  I was told by the doctor that although normally she doesnt see pleural thickening in someone my age that it's probably nothing to worry about and made a note to monitor it at my next physical.  She also said that although some pleural thickening is caused by exposure to asbestos, the signature of that exposure is pleural thickening in the bottom of the lungs.  We dont use asbestos in my line of work but we do use flame retardant man made vitrious fiber blankets such as ominsil and refrasil.  I did used to smoke, although i've quit and I now run 15+ miles a week.  I also had a very bad case of bronchitis last year.  Could that have been the cause?  Should i be worried and should I get another opinion from a doctor outside my work?