In recent months, we have seen certain people touting systems using gas chromatography (provided with a photo-ionization detector) for area monitoring of ethylene oxide in Supply, Processing, and Distribution (SPD) departments. [Sometimes SPD stands for Sterile Processing Department or Sterile Processing and Distribution.] 

Such systems, despite their expense, were reasonably popular 15–20 years ago. However, for a variety of good reasons, which we will detail in this article, they quite justly fell out of favor, and essentially disappeared more than ten years ago. 

To better understand this matter, it will be helpful to examine the regulatory history of ethylene oxide (EtO)…

In 1968, the American Conference of Governmental Industrial Hygienists (ACGIH) recommended a long-term exposure limit of 50 parts-per-million (ppm) as a time-weighted average (TWA), calculated over eight hours. In 1971, pursuant to 29 U.S.C. Sec. 655(a) (1982), which permits the promulgation of “national consensus standards,” the Secretary of Labor adopted the 50 ppm PEL (Permissible Exposure Limit).

By the mid to late 1970s, area monitoring systems for EtO were in place, based mostly on inexpensive, interference-prone detectors, which were hard-pressed to achieve even the modest sensitivity required.

In 1981, ACGIH lowered its recommended TWA to 10 ppm. Moving quickly, the organization designated EtO as a suspected carcinogen and proposed an even lower value of 5 ppm. In June of 1982, ACGIH recommended a TWA of 1 ppm, to take effect in 1984.

In January of 1982, OSHA issued an advance notice of proposed rule-making, inviting interested individuals to submit data or comments on revising the OSHA EtO standard. A year later, this led to OSHA publishing a proposed rule, suggesting a PEL of 1 ppm. After public hearings and some delay, in June of 1984, this PEL became a final rule.

Here are the current OSHA regulations, per 29 CFR:


8-hour time-weighted average (TWA).” The employer shall ensure that no employee is exposed to an airborne concentration of EtO in excess of one (1) part EtO per million parts of air (1 ppm) as an (8)-hour time-weighted average (8-hour TWA).



Excursion limit.” The employer shall ensure that no employee is exposed to an airborne concentration of EtO in excess of 5 parts of EtO per million parts of air (5 ppm) as averaged over a sampling period of fifteen (15) minutes.


Now, consider the implications. Although not explicitly stated in official OSHA guidance documents, the only way to determine an eight-hour time-weighted average of exposure is by first collecting full-time continuous monitoring data. The subsequent calculation can be performed manually, or as is more common, via specialized software.

Here is the problem with a gas chromatography/photo-ionization (GC/PID) system: Assuming that more than one sampling point is involved—and that is an excellent assumption in the SPD application—GC/PID provides but a single sensor, which must be time-shared among the various points. Thus, by definition, this is not continuous monitoring.

For example, on an eight-point system, a valve/timer apparatus conveys sample to the single detector, perhaps spending two minutes or so on each given point. In other words, in a 16-minute cycle around all of the points, only two minutes out of sixteen are being devoted to a given point. Therefore, no active monitoring data is being obtained for each point 87.5% of the time!

Yet, the purveyors of the GC/PID approach seem to embrace this deficiency, and attempt to transform it into a benefit. Superficially, you the customer are now able to “monitor” many points with one system. Ironically, the high cost (“sticker shock”) of a GC/PID monitoring system is lessened by adding more points—even though with more and more points, one is obtaining less and less data per point.

But, that’s not all.

Beyond the large safety issue raised by the monitoring system being offline for a given sampling point a substantial period of time, what about the accuracy and relevance of exposure calculations, based on such part-time monitoring data? In the eight-point system described above, how can the eight-hour average exposure be calculated?

Consider a given sampling point. As we have already established, data is recorded for only 12.5% of the time. Therefore, we must employ some sort of “placeholder” for the calculation. What should this placeholder be? How do we extrapolate the data?

One way would be to invoke the average value during those two minutes for the rest of 16-minute cycle. Another approach might be to invoke the highest value obtained. Yet another might involve some sort of trending algorithm, which seeks to anticipate future data.

Which method is correct? We at Interscan have no idea, and neither does anyone else.

While we’re at it, given these limitations, how in the world can an Excursion Limit (averaged over 15 minutes) be calculated? Simply put, under these circumstances, it cannot be done.

So much for GC/PID problems. You may well ask how these systems became popular in the first place. As noted above, early vintage EtO monitors were prone to cross-interference from many compounds that could be present in SPD departments. Despite the drastic change in regulatory standards, a goodly number of these original monitors were pressed into service for the 1 ppm compliance level.

Span adjustments were turned all the way up, and false alarms became the rule, rather than the exception. At about this time (late 1980s), PIDs were introduced that offered appropriate sensitivity, and when combined with a GC, offered specificity, as well. These systems were expensive, but the high cost could be amortized over the number of sampling points, even if this number had to be grossly inflated—based on what was actually required.

Thus, the GC/PID systems were deployed. Most of the time, they appeared to be doing a fine job, and were standing up to all the interferences, including isopropyl alcohol—the most notorious.

On the other hand, GC column replacement was costly, and if there were enough isopropyl alcohol, it could “swamp” the column, so the interference and false alarm would occur anyway. Recall that PIDs are not at all specific in themselves, and must rely on a GC column to achieve specificity.

In addition, end-users were noticing that alarm events were sometimes being missed during the off-time of a particular sampling point. A few end-users even realized that the data acquisition reporting left much to be desired.

By 2004, or thereabouts, it would be difficult to find a GC/PID system in the SPD application.

We can explain the mini-resurgence of GC/PID by noting that biases seem to exist in all fields of endeavor, and the world of analytical instrumentation is no exception. In the laboratory, GC is considered an “elegant” method. In this case, though, such elegance disintegrates under the demands of workplace occupational health monitoring.

It would be most unfortunate if this notion of analytical elegance blinds specifiers to the quite substantial limitations such elegance might incur. These limitations must not be imposed on vulnerable workers in SPD departments.

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