The key message therefore is that a ppm gas is not the same as a ppm in liquid and so again using PAA as an example, while the EPA Acute Exposure Guide Line for PAA vapor is 0.17 ppm ( AEGL 1, 10 min to 8 hr time weighted average) it does NOT mean that 2,000 ppm solution (0.2%) solution is immediately deadly. The pressure of a gas mixture is the sum of the pressure of its components, at least to a reasonable approximation ( Ideal Gas Law) and so it is more convenient to work in parts per million volume than weight. Instead we normally deal with gas pressures and volumes. The ppm is just a fractional weight, similar to a percentage (parts per hundred).įor a gas or vapor, ppm is still parts per million, but now it is parts per million by volume because we do not normally deal with the weights of gases (except perhaps atmospheric pressure, ~14 psi, is the weight of the atmosphere on us and a liter of air weights about 1.2g). This means that for every 1000g or solution, 2g of it is PAA, the rest is water, hydrogen peroxide, acetic acid, surfactants etc. Using Peracetic Acid (PAA) as an example, PAA is commonly used in dilute solution e.g. In both cases ppm stands for Parts Per Million. One of the most common sources of confusion when talking to people about workplace exposure limits is the difference between liquid ppm and gas phase ppm.
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