Mucous membranes are the moist linings of the respiratory system (nasal passages, throat, bronchial tubes, lungs). Water soluble gases and particles can dissolve in the mucus (whose purpose is to protect the tissues from the drying effects of the inhaled air) and damage the underlying tissue. Materials that irritate or inflame mucuous membranes in this manner are lachrymators.

Bronchitis does not need to be associated with an infection (which is what your doctor treats with antibiotics). Usually, acute bronchitis will be due to some irritant in the air, while chronic bronchitis will be associated with recurrent infections. Pulmonary emphysema usually results from long-term exposure to gases that inflame the walls of the alveolar sacs, scarring them and reducing the ability of the lungs to exchange blood gasses (the sacs rupture because they lack elasticity, leaving voids in the lung tissue---see picture below). Many cases of silicosis or coniosis (the latter is the more general name for this kind of disease) are reversible, since the respiratory system has a mechanism for cleaning out solid debris (via mucus), though excessive and repeated exposure to aerosol particles can cause permanent damage in the form of emphysema.

A healthy lung (left) and lung tissue with cancer (top right) and emphysema from "black lung" (coal dust coniosis).