If SO2 is so soluble in water. How can it get to the deeper reach of lung without first dissolving in the mucus?

Absorption of SO2 in the mucus membrane of the nose and the upper respiratory tract occurs as a result of its solubility in the aqueous media.

a) Volume of water dissolved 45 volumes of SO2 at 150C. Absorption is concentration dependent & with 85% absorption in the nose at 4-6 μg/m3 and 99% at 46 μg/m3.

b) It is also pointed out that at common ambient concentration of SO2, absorption in the upper airways may be inefficient.

Increased flow rate reduce the % of inspired SO2 absorbed in the nose & upper airways and thus exercise promotes delivery to the smaller airways. NH3 is found in the mouth (a product of bacterial metabolism) and may play a role in neutralizing acid aerosols. Sulphite & bisulphite ate thought to the major ions formed on absorption of SO2. The key reactions are:

SO2 + H2O  H2SO4

H2SO3 + H2O  HSO3- + H3O+ ------------ (1)

HSO3- + H2O  SO32- + H3O+ ------------ (2)

The pKa values of reactions (1) and (2) are 1.86 and 7.2 respectively. The pH of the surface fluid in respiratory tract is 6.5 to 7.5 and thus appreciable amounts of both the bisulphite and the sulphite will be present. Absorbed bisulphate and sulphite are converted to Sulphate by molybdenum – dependent sulphite oxidase. The highest concentration of this enzyme occur in the liver and kidneys, while lower levels are found in the lungs. The importance of sulphite oxidize in removing absorbed SO2 from the respiratory tract in humans require further study

which are difficult in sulphite oxidase are reported to be more sensitive than control animals to inhaled SO2. This is how, though SO2 is soluble in water, it can get into deeper reach of lung without first dissolving in mucus membrane.