Update on ACGIH STEL for Peracetic Acid

Peracetic acid (PAA) is a strong oxidizing agent and an effective biocide that upon reaction degrades to safe byproducts (acetic acid, oxygen and water) and so PAA has found widespread use from disinfection in food production, sterilization in healthcare, aseptic packaging in food processing among others. As with any broadly biocidal disinfectant, exposure to workers poses health risks. There is increasing awareness of the inhalation risks of PAA vapor and recently the EPA issued Acute Exposure Guidelines for PAA. The AEGL 1 for PAA is about 0.17 ppm for a ten minute to 8 hour exposure. The AEGLs are not workplace exposure limits since 'AEGLs represent threshold exposure limits for the general public and are applicable to emergency exposure periods ranging from 10 minutes to 8 hours.'

A workplace exposure level is typically lower than the AEGL because a workplace exposure limit refers to the exposure a worker may receive on a repetitive basis. There are currently no workplace exposure limits for PAA, but the ACGIH, one of the pre-eminent workplace safety organizations in the world is considering a 15 minute short term exposure limit threshold limit value (STEL TLV) for PAA. The ACGIH had previously issued a notice for change (NIC) for a STEL of 0.2 ppm, but the NIC has been revised to a STEL of 0.4 ppm (15 min). The ACGIH is currently accepting comments on this proposed STEL. The full documentation for the NIC is available for purchase for $60. ChemDAQ encourages all interested parties to submit their comments directly to the ACGIH.

A workplace exposure limit is only useful if there is a way to measure the gas or vapor concentration. Fortunately, ChemDAQ now offers a continuous monitor for PAA designed for workplace safety. The range is 0 to 3 ppm, with a minimum detection limit of 0.04 ppm.

Until recently there were few practical methods to measure PAA and many industrial hygienists would settle for measuring hydrogen peroxide and acetic acid, which are equilibrium components with PAA in solution and for which both analytical methods and workplace exposure limits are readily available (OSHA, NIOSH and ACGIH) This approach has its limitations since PAA is a more irritating agent that hydrogen peroxide, and means optimized to detect hydrogen peroxide may not detect PAA very well. It is likely therefore that the workplace exposure to PAA has been greatly underestimated.

Peracetic Acid: Parts Per Million in Water and in Air

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. In both cases ppm stands for Parts Per Million.

Using Peracetic Acid (PAA) as an example, PAA is commonly used in dilute solution e.g. ~0.2% by weight solution. This means that for every 1000g or solution, 2g of it is PAA, the rest is water, hydrogen peroxide, acetic acid, surfactants etc. The ppm is just a fractional weight, similar to a percentage (parts per hundred).

For 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). Instead we normally deal with gas pressures and volumes. 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.

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. It is possible to estimate the gas or vapor ppm from a liquid ppm, but it is a little involved (To avoid details, skip to last two paragraphs).

A gas and vapor are very similar, in that both relate to chemicals in the gas state, but a vapor is a chemical whose liquid is below its boiling point. Thus if water evaporates from a glass of water, that is water vapor. If the water and air is heated above the boiling point of water (100 ^oC), then the water would be present as a gas.

Some liquids evaporate easily (alcohol for example), others barely evaporate at all (olive oil). There is an equilibrium between the liquid state and the vapor state known as the vapor pressure and as the temperature of a liquid rises towards the boiling point the vapor pressure increases until at the boiling point the vapor pressure equals atmospheric pressure. Vapor pressures for many compounds have been measured and tabulated in chemical handbooks and so values are readily available.

If the liquid is a mixture, such as PAA solution, then the vapor pressure of a component is proportional to the mole fraction of that component (fractional number of molecules of the component compared to all molecules, see Raoult’s Law).

If we know the vapor pressure of our component (Vapor Pressure of PAA = 1.93 kPa at 25^oC, CRC Handbook of Chemistry and Physics, 76th Ed, p 6-80), we can calculate the PAA vapor pressure of various PAA solutions. Similar calculations can be be done for the other components such as hydrogen peroxide as well.

PAA Concentrations

        Liquid (%, ppm)                     Vapor(ppm)

• 0.05%   500 ppm                   2.5 ppm
• 0.1%       1,000 ppm               4.9 ppm
• 0.2%      2,000 ppm               9.9 ppm
• 0.5%       5,000 ppm              25 ppm
• 1%          10,000 ppm              50 ppm
• 5%>        50,000 ppm            260 ppm
• 10%         100,000 ppm         540 ppm

These PAA vapor concentrations are in ppm. The estimates also depend on the concentrations of other components in the mixture such as acetic acid and hydrogen peroxide (always found in PAA solutions) since they affect the mole fraction calculation discussed above. For the estimates in the table above the acetic acid and hydrogen peroxide concentrations were 5 and 10% w/w respectively.

These estimates though are only approximations. In particular, they represent equilibrium values which are not found in most applications where PAA is used. For example if the PAA vapor is being swept away by a ventilation system then the vapor concentrations will not reach the levels in the table above. These vapor pressure calculations are useful estimating of the potential concentration of PAA vapor and this allow a rational basis for determining the risk of exposure to PAA and what means should be employed to keep workers safe.

Note: if the PAA is being sprayed, then the concentrations included aerosols or the complete evaporation of droplets in which case the vapor pressure calculations do not apply.

If the PAA vapor concentration has the potential to exceed safe levels, then the PAA vapor should be monitored. Continuous monitors for PAA and many other compounds with potentially hazardous vapors are readily available. Even if the PAA is controlled with ventilation or is used within dedicated equipment, the potential exists for it escape into the environment. Any equipment can fail from wear and tear, mechanical failure or user error. Even though there is no OSHA PEL for PAA, the EPA has issued Acute Exposure Guidelines for PAA as discussed above, the ACGIH is considering a 15 minute short term exposure limit of 0.4 ppm and even manufacturers of PAA such as Solvay recommend a TWA exposure limit of 0.2 ppm.

In summary, there is a lot of confusion between ppm vapor concentrations and liquid concentrations for compounds like PAA. The two are different but are related by a somewhat involved vapor pressure calculation. The calculated vapor pressures though are only estimates but are useful in determining if there is a risk of over exposure. If there is a significant risk of over exposure, then continuous monitors for PAA should be employed.

Hydrogen Peroxide Emission Problems from Sterilizers

The previous ChemDAQ blog discussed continued off-gassing of plastic parts sterilized in a hydrogen peroxide sterilizer as reported by Rika Yoshida, Hiroyoshi Kobayashi at a recent conference of the World Formum for Sterile Hospital Supply. This presentation also discussed hydrogen peroxide vapor emissions from hydrogen peroxide sterilizers. The sterilizers included several models of Sterrad^® sterilizer from Advanced Sterilization Products, and the V-Pro1^TM from Steris. The authors have also published some of these results in the Japanese Journal of Environmental Infections, and a full text copy of their paper is available.

The investigators measured the hydrogen peroxide concentration inside a sterilizer immediately after the end of the cycle, when people would be reaching in to remove the load, and found very high concentrations (34 ppm Sterrad 100S; 60 ppm Sterrad 200; and 13 ppm V-Pro1) in some cases close to the NIOSH Immediately Dangerous to Life and Health value of 75 ppm. Since the time to unload a sterilizer is fairly short, a single exposure will probably not exceed the OSHA PEL of 1 ppm calculated as a time weighted average over 8 hours, but it would probably exceed the 3 ppm 15 minute short term exposure limit found in some states (Washington and Hawaii) and even the OSHA PEL may be reached in a busy facility for someone running multiple loads a day.

ChemDAQ has received many reports from users of hydrogen peroxide emissions when the sterilizer door is opened at the end of a cycle. In a typical ChemDAQ installation the sensor is placed on top of the sterilizer and so the concentration measured will be much lower than would measured from a sensor placed inside the sterilizer chamber because the vapor gets diluted by the time it reaches the sensor.

Users often see small increases in hydrogen peroxide concentration, usually less than 1 ppm, though one model of sterilizer was found to emit much higher concentrations (~30 ppm) each time the door was opened. In this case, the hospital had four sterilizers, all new, all showing the same behavior. The manufacturer was unable to solve the problem and the hospital now instructs their technicians to open the sterilizer door at the end of the cycle and leave the area until the ChemDAQ monitor shows that it is safe to return.

Rika Yoshida, Hiroyoshi Kobayashi began their investigation in response to complaints of eye and respiratory system irritation from healthcare workers using hydrogen peroxide sterilizers. There are many similar reports of eye and respiratory system irritation from hydrogen peroxide sterilizers in the FDA’s MAUDE database and so it is likely that the results reported in their paper are not unique.

For many years ChemDAQ has been pointing out that all sterilant chemicals are potentially hazardous since they are designed to kill all microorganisms and therefore we recommend that all sterilant gases and vapors should be monitored.