There has been much debate over this subject for many years now, but I believe that it is only a rumor. (Similar to the Mountain Dew causing impotence story) The rumor began in the 1970's and said that menthol cigarettes contained fiberglass in the filters which was why they were so "cooling". The truth is, when people smoke menthol cigarettes, this cooling sensation is caused by the menthol reacting to receptors in the throat, which usually causes them to inhale the smoke deeper into their lungs. Because the smoke from any cigarette contains many carcinogens, (cancer causing agents) it is apparent that inhaling more of the smoke will be more dangerous. It is likely that this is the reason for bleeding lungs or coughing up blood, because of the increase in smoke, and not the presence of fiberglass.