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Chemical Reaction: Down the Drain
By: Steve Herman
Posted: August 26, 2008, from the October 2007 issue of GCI Magazine.
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The persistence in water, soil, sediment and air as the half-life in days is shown. The results are color-coded like a traffic light: green is good, orange is a little bad and red is very bad. With propylene glycol the sediment half-life is bad, but 0% of the propylene glycol is in the sediment, so it really doesn’t matter. The bioaccumulation number and fish toxicity are green, which is good. The overall safety profile is excellent. To be a PBT—which, from an environmental perspective, is a really bad chemical—P, B, and T must all be red.
The PBT profiler is easy indeed when compared to EPI Suite (EPI = Estimation Programs Interface). The components of EPI Suite are tabulated in Table 1. Input can be either chemical name or SMILES (Simplified Molecular Input Lines Entry System) notation, a standard method of entering a chemical structure into a computer using a standard keyboard.1 A few basic rules and notations are shown in Table 2. Propylene glycol has a simple structure, with SMILES notation OCC(O)C.
Popping the SMILES notation into EPI Suite and hitting “calculate” yields copious physical data. An important result for environmental effects comes from the Level III Fugacity Model.
Fugacity is a measure of the partitioning of a chemical into the environment. Imagine spraying a few grams of a chemical next to a pond in a park with a gravel shoreline. The material will have different affinities and lifetimes for water, gravel, air and soil. The fugacity calculation shows that almost 98% of propylene glycol will end up in the water and soil, with negligible presence in air and sediment.
Combining fugacity with other data offers an informed guess—and a guess is all it really is—of the environmental consequences of a material. If a chemical is toxic to fish but has almost no partitioning into water, the toxicity becomes irrelevant. A trained regulatory person is essential to intelligently interpret the mass of data available in data bases and modeling programs.