|MadSci Network: Medicine|
First, some information about the lungs. Lungs serve the very important purpose of bringing air into the body and exchanging oxygen for carbon dioxide. Since long before people started smoking, there has existed the problem of breathing in small particles - like spores, dust, pollen, and ash - which get trapped in the lungs and potentially damage the lungs' lining. Air-breathing animals have evolved two ways to deal with this problem. Coughing provides a quick fix by violently expelling air to hopefully dislodge any offending particles. But more importantly, the lining of the lungs is covered with cilia which continuously beat generating a net flow of material up and out to the throat. It is the cilial movement that ultimately clears foreign particles out of the lungs.
The most problematic effects of smoking have to do with these two mechanisms of clearance. First, smoking reduces coughing in response to smoke, which is why they can smoke without continuously coughing, and second, smoking effects the rate at which the lungs' cilia beat. In acute (short term) cases of exposure to smoke, i.e. first time smoking, or infrequent exposure, coughing is induced and cilial beating increases to clear the smoke out of the lungs. The opposite is true for chonic (long term) exposure. Smokers' cilia beat slower than normal, and continue to slow down the long the person smokes. Needless to say this makes exposure to smoke ever more dangerous, since it takes longer and longer to clear the particles out of the lungs the longer one smokes.
On the positive side, quitting smoking reverses this effect, and the cilia gradually recover their beating rates over time. This recovery of the cilia also increases the clearance from the lungs, such that the smoke particles are eventually expelled. This is true for smokers who quit before they develop lung disease even after smoking over 40 pack-years (1 pack-year = smoke equivalent to 1 pack of cigarettes per day per year, e.g. 40 pack-years could be 1 pack a day for 40 years, or 2 packs a day for 20 years, etc.). However, the risk of developing lung disease, in the form of chronic obstructive pulmonary disease (COPD, a catchall term which includes emphysema and chronic bronchitis), appears after only 20 pack-years, by which time the smoking has also greatly increased the risk of heart disease. So waiting 40 years to quit does not guarantee a clean bill of health. Similarly, since lung cancer is the result of cummulative exposure to the carcinogens in smoke, quitting after decades of smoking doesn't remove the risk of developing cancer.
Regarding secondhand smoke, the major consideration for secondhand smoke is the same as for firsthand smoke: exposure. Acute exposure to secondhand smoke, e.g. sitting near a smoker at a restaurant, is only hazardous to people with asthma or allergies to smoke. While this is a real problem for these individuals, it is no more hazardous than more common allergens like cat dander, dust, mold, or pollen. However, chronic exposure, e.g. living with someone who smokes a pack or more a day, can have much more specific and serious effects. At the low end of chronic exposure, the problems of allergies and asthma are compounded by an increased succeptibility to respiratory diseases, like influenza and pneumonia, caused by the decreased cilial clearance of the lungs. This is especially dangerous to infants and most pediatricians advise against smoking around small children. At the high end of chronic exposure to secondhand smoke, there are several cases of non-smoking relatives of smokers getting COPD, usually chronic bronchitis, which was partially reversed by ending exposure. One point that should be made is that secondhand exposure to smoke is much less dangerous than smoking, which makes sense, since smokers are exposed to more secondhand smoke than anyone.
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