The main difficulty is that the ppm measuring unit is unaffected by temperature or pressure. Some may object to the blanket statement that a ppm concentration is unaffected by temperature and pressure. While the parts-per-million, which rigorously is the mole fraction of the analyte in the balance gas does not change with temperature and pressure, the partial pressure will change. Unfortunately, most gas detection instruments do not measure mole fraction, but respond—directly or indirectly— to the partial pressure of the analyte.
This confusion is caused by the classical error of comparing apples and oranges. Another substance with potentially hazardous health effects, the element beryllium, has an even lower MCL than does arsenic, 0.
Another way to think of parts per million: If this were the concentration of space aliens in New York City, there would only be about eight of them secreted throughout all five boroughs and plus square miles!
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs. More about Kevin and links to his professional work can be found at www.
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