What to Look for in a PAA Vapor Monitor, in Light of the New ACGIH STEL for PAA?

Article discusses features to look for in a gas monitor for peracetic acid, in light of the recent adoption of a TLV STEL by the ACGIH.

The ACGIH recently settled on a value for a short term exposure limit (STEL) for peracetic acid of 0.4 ppm calculated as a 15 minute time weighted average. The ACGIH is one of the most respected industrial hygiene organizations and they have been issuing occupational exposure limits called threshold limit values since 1941 and many of the world’s legal exposure limits either directly reference the ACGIH or, started life as ACGIH TLVs, the latter including most of OSHA’s permissible exposure limits (PELs) .

Biocidal chemicals play an essential role in industries from healthcare, food production and processing to waste water treatment. In all these applications, operators need a chemical that is very efficient (i.e. kills the bugs very well), but does not leave residues or otherwise harm the environment.

It is no wonder that PAA is becoming so popular. PAA is a strong oxidizing agent and so is great for destroying pathogens, and the chemical also rapidly breaks down to harmless byproducts (oxygen, water and acetic acid/vinegar). While great for the environment, peracetic acid does pose a risk for anyone contacting the liquid or exposed to the vapor. The difficulty up to now was that employers did not know what was a safe limit. Too little ventilation risks exposing workers to excessive peracetic acid vapor, but too much ventilation is very wasteful and expensive.

Since peracetic acid has a strong odor, many employers were veering on the excess ventilation side. The new STEL for peracetic acid will allow facilities to keep their people safe and limit their energy costs for the air handling system to what is actually needed. However, a STEL is of little value without a means for users to be able to monitor the PAA vapor concentration in real time in order to be achieve both worker safety and cost control.

A continuous gas monitor must of course detect the target gas (PAA), over the desired concentration with sufficient resolution. For example, the ChemDAQ Steri-Trac® PAA monitor has a minimum detection limit (mdl) of 0.04 ppm and a range of 0 to 6 ppm and so with a 10:1 STEL/mdl, it is well suited to detect PAA around the ACGIH STEL of 0.4 ppm.

The monitor should have rapid response and recovery times, and good repeatability, linear response etc. etc. The ChemDAQ monitors responds well to PAA. Contact ChemDAQ if you need any of these numbers.

Every facility is different and so the system should be modular and scalable so that it can be customized for each application and changed as the need arises. It is also beneficial to have each detector point a stand-alone instrument so that there is no single point of failure that will turn off all protection. Each ChemDAQ can be used on its own or connected to a central computer.

It is very useful to be able to be able to track the PAA concentration over time – a rising concentration may indicate an issue somewhere that can be rectified before it becomes a problem. Up to 32 of the ChemDAQ Steri-Trac monitors can be connected to the ChemDAQ DAQ® computer. The DAQ not only logs the data, but it also provides graphs, reports and can export the data as a text (.csv) file or to an SQL compatible database. The latter output is primarily for larger commercial organizations that want to be able to tie the ChemDAQ monitors into their building’s air handler system. The monitors can also individually control air handlers etc, by means on relay contacts which works well for uses with smaller numbers of monitors.

A gas monitor is only as accurate as the calibration and many monitors for reactive gases fail because of calibration errors. ChemDAQ takes a different approach. All ChemDAQ sensors are factory calibrated thus eliminating the errors of on-site calibration. ChemDAQ provides a sensor exchange / calibration service called SXP® in which freshly calibrated sensors are periodically sent to customers, who then return the expired sensors back to ChemDAQ.

There are a couple of quirks that make peracetic acid even more difficult to monitor than some other reactive gases. The first PAA quirk concerns the calibration gas. Most gas monitors are calibrated with test gas from a compressed gas cylinder, but to calibrate with peracetic acid requires a two process. The first step is generate a consistent test stream of peracetic acid and the second is to calibrate this test gas against traceable standards. Only then are we able to calibrate the ChemDAQ monitor. The SXP program works especially well for difficult calibration gases such as PAA.

It is possible to bump test a sensor with a surrogate gas, but bump testing is not a reliable method for calibrating reactive gases such as peracetic acid. Bump testing is useful if one needs a quick basic function test of a gas monitor, but it lacks the accuracy needed for calibration. Therefore, If you are looking for a peracetic acid monitor, make sure that it is calibrated with peracetic acid.

The second quirk involves the chemistry of PAA. PAA is used as an equilibrium mixture with acetic acid and hydrogen peroxide. Therefore, there is always some acetic acid and hydrogen peroxide vapor present along with the peracetic acid vapor. Hydrogen peroxide will often give a cross sensitivity on the peracetic acid sensor and vice versa. Since peracetic acid is more hazardous than hydrogen peroxide it is important to ensure that the gas monitor is only reading peracetic acid and not reading reading some combination of the two.

The ChemDAQ peracetic acid sensor module includes a proprietary chemical filter that selectively allows peracetic acid to pass, but removes essentially all of the hydrogen peroxide. This selective filter is exchanged every SXP calibration cycle to ensure peak performance. Again, if you are looking at getting a peracetic acid monitor, make sure that it does not suffer from cross sensitivity to hydrogen peroxide otherwise your gas readings will always questionable.