As a result of these risks the US-EPA has issued Acute Exposure Guidelines for PAA and there are three AEGL levels: “AEGL-1 is the airborne concentration, expressed as parts per million or milligrams per cubic meter (ppm or mg/m3) of a substance above which it is predicted that the general population, including susceptible individuals, could experience notable discomfort, irritation, or certain asymptomatic nonsensory effects. However, the effects are not disabling and are transient and reversible upon cessation of exposure.” AEGL 2 is the level where those exposed may experience “irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape.” And AEGL 3 is the level where those exposed may experience “life-threatening health effects or death.”
PAA is normally found as an equilibrium mixture with acetic acid and hydrogen peroxide:
CH3COOH + H2O2 <=> CH3COOOH + H2O
Therefore, whenever PAA solution is used, in addition to PAA vapor there is also hydrogen peroxide vapor and acetic acid vapor, both of which have OSHA permissible exposure limits (PELs) of 1 ppm and 10 ppm respectively calculated as an eight hour time weighted average, both significantly higher than the AEGL 1 for PAA of 0.17 ppm. Currently there is no OSHA PEL for PAA or ACGIH TLV, though the ACGIH has proposed a 15 minute short term exposure limit (STEL) for PAA of 0.2 ppm. The analysis below will use the AEGL 1 but it is simple to adjust the numbers if the new ACGIH STEL is adopted.
ChemDAQ recently launched a PAA monitor as part of its Steri-Trac® gas monitoring system that also includes monitors for hydrogen peroxide. Prior to the launch of this product, there were no monitors for PAA and very few analytical methods available, despite the widespread use of PAA. Employers seeking to protect their workers would often rely on detecting only the hydrogen peroxide and acetic acid components, but this approach is flawed in that PAA vapor is more hazardous that either of the other two vapors and in mixtures with a high PAA content, it is the dominant vapor present.
The best strategy to designing a gas detection system is to assess which vapor presents the greater hazard and detect that one. As discussed above, PAA is usually used as an equilibrium mixture with hydrogen peroxide and acetic acid and it is supplied in a variety of blends, some with a high PAA/H2O2 ratio and some with a low PAA/H2O2 ratio. Ratios in commercial blends typically vary from 10:1 to 1:5 PAA:H2O2.
The last piece to the puzzle is the vapor pressures of hydrogen peroxide and PAA solutions. At room temperature the vapor pressures of PAA and hydrogen peroxide are 1.93 and 0.26 kPa respectively at 25 oC [CRC Handbook of Chemistry & Physics 76th Ed, Lange’s Handbook of Chemistry, 12th Ed). Combining the vapor pressure and the PEL/AEGL 1 for the two compounds gives the relative hazard (43:1 PAA/H2O2). Multiply this number by the ratio of PAA/H2O2 in the composition gives the risk factor for that PAA blend.
If we assume that the risk of a minority vapor can be ignored if the risk is less than 20 % for the combination of both PAA and hydrogen peroxide, then if the risk factor is less than 0.2, the predominant risk is hydrogen peroxide and a hydrogen peroxide monitor will suffice. If the risk factor is greater than 5, then the predominant risk is PAA and a PAA monitor only is sufficient. If the risk factor is between 0.2 and 5, then both PAA and hydrogen peroxide vapors pose a risk and both types of monitor should be employed.
The risk factors for several common blends are shown below. The risk factors for other blends may be readily calculated as described above.
|PAA (wt %)||H2O2(wt %)||Risk Factor||Monitor|
|0.23||7||1.4||H2O2 and PAA|
|2||22||4||H2O and PAA|
In Conclusion, for all of the brands and blends that we currently have data on (~ 70), none of them would be adequately monitored using a hydrogen peroxide monitor alone and those facilities using only a hydrogen peroxide monitor maybe seriously underestimating the exposure risk of their employees. The majority of PAA blends would require PAA monitors only and a few blends, with low PAA/H2O2 ratios, need both hydrogen peroxide and PAA monitors in order to adequately monitor the vapor.