Regulations, Violations, Permits, VOC, HAP and Such

RELATIONSHIP BETWEEN VAPOR PRESSURE AND VOC

Q: Why is acetone an exempt VOC (smog former) even though its vapor pressure is approximately 58 mm Hg. What is the cut off point in making this determination?

A: Vapor pressure and the smog-forming tendencies of a solvent are not related.

Years ago the EPA used a vapor pressure of 0.1 mm Hg as the threshold for determining whether or not a volatile organic compound should be considered to be a precursor to the formation of smog (ozone). The EPA assumed that if an organic compound had a vapor pressure equal to, or greater than 0.1 mm Hg, it should be considered to be a VOC, where "VOC" implied that it contributed to the formation of ozone (O3). It is true that acetone has a high vapor pressure; however, already in the 1980s, (and perhaps even before) companies such as Dow Chemical demonstrated that certain stable volatile organic compounds do not form smog with any ease. The EPA confirmed that compounds, such as 1,1,1 trichloroethane, acetone, methyl acetate, methyl siloxanes , and many fluor-chloro hydrocarbons have a negligible tendency to react with nitrogen oxides (NOx) in the presence of sunlight to form ozone. Therefore, EPA called such compounds "exempt". They do not form ozone easily and are not considered a threat to air pollution, at least as far as smog is concerned.

These organic compounds are volatile, as demonstrated by their high vapor pressures, but they are not "VOCs", "smog formers".

If you find a compound listed on an MSDS that has a vapor pressure less than 0.1 mm Hg. it is most likely that the compound is not sufficiently volatile to be considered a "VOC". While the EPA used this threshold value as the determining factor many years ago, the current requirement is to evaluate the compound in accordance with EPA Method 24 or 24a. If one of these methods provides results that show the compound to be even partially volatile, then unless it is on the list of "exempt" compounds, it is considered by default to be a smog former.

HOME | TRAINING | CONSULTING | EXPERT WITNESS | ASK THE EXPERT | LINKS