Sometimes, we are asked about the 0-50 ppm measuring range offered on our ethylene oxide (EtO) instruments. After all, a founding principle for direct-reading instruments, used in occupational health work, was that the range should be set up to cover one-tenth to 10 times the TLV-TWA, or as OSHA would later call it, the PEL or permissible exposure limit.

At present (September, 2009) the level is 1 ppm, with OSHA also mandating an excursion limit of 5 ppm on a 15 minute average, so the question is certainly warranted. Let us examine some reasons for measuring at higher ranges.

Respirator guidelines for Ethylene Oxide

Up until recently, the following table appeared in 29 CFR 1910.1047, the section of occupational health standards applicable to EtO…

Airborne Concentration or Condition of Use

Required Respirator

< or =  50 ppm (parts per million)

Full-facepiece respirator with ethylene oxide approved canister, front- or back-mounted.

< or =  2,000 ppm

(1) Positive-pressure supplied-air respirator equipped with full facepiece, hood, or helmet; or (2) Continuous-flow supplied-air respirator (positive-pressure) equipped with hood, helmet, or suit.

> 2,000 ppm or unknown concentrations

(1) Positive-pressure self-contained breathing apparatus equipped with full facepiece; or (2) Positive-pressure full-facepiece supplied-air respirator equipped with an auxiliary positive-pressure self-contained breathing apparatus.


Positive-pressure self-contained breathing apparatus equipped with full facepiece.


Any respirator described above.

It is unclear why this table was deleted from 29 CFR 1910.1047, as the information presented in it is still valid, and it appears on the NIOSH website.

The point is that there might be times when re-entry must be made into an area that has had an EtO leak. 50 ppm is the dividing line between a relatively simple full-facepiece respirator and more elaborate equipment. As such, some users—especially those whose EtO monitoring systems are provided with remote concentration displays—would like to be able to discern a 50 ppm level.

By the same token, emergency personnel who are already wearing full self-contained breathing apparatus could bring in a portable analyzer capable of reading up to 50 ppm, to monitor the status of the clean-up and aeration of the contaminated area.

The 20 ppm reference

In a document entitled Ethylene Oxide (EtO): “Understanding OSHA’s Exposure Monitoring Requirements [OSHA 3325-01N 2007],” the following text appears under the section “Is there a specific EtO level that I should use to trigger an alert?”

OSHA has not established an “alert” level; you should choose an alert trigger level that is appropriate for your workplace. When evaluating alarms, it is important to remember that the alarm’s purpose is to alert employees to unintended and hazardous EtO releases, rather than to average concentrations measured over an 8-hour work shift. It is not necessary to base the EtO alarm trigger specifically on the OSHA action level (0.5 ppm) or permissible exposure limits for 8 hours (1 ppm) or 15 minutes (5 ppm).

You should also be aware that there is a large range in the cost and sensitivity of commercially available monitors. Some systems alert employees to EtO levels greater than 20 ppm, while other highly sensitive monitoring devices can trigger an alarm at much lower levels, such as 1 ppm or even lower (NIOSH, 1989).

Researchers suggest that alarms that alert employees to accidental EtO spills or leaks can be reasonably set to levels of 20 ppm without compromising employee health and safety (Lamontagne and Kelsey, 1998).

While it is technically correct in the most narrow legalistic sense that the EtO alarm trigger need not be set at any particular value, this advice is extremely foolish and misleading inasmuch as 29 CFR 1910.1047(c)(2) establishes a clear definition of the excursion limit…

“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.

One wonders how the employer can possibly comply with this provision if the “alert” level on his EtO monitor is not set at 5 ppm or lower.

As to the citation (Lamontagne and Kelsey, 1998), this refers to an article published in the American Journal of Industrial Medicine 34:95-104(1998), entitled “OSHA’s Renewed Mandate for Regulatory Flexibility Review: In Support of the 1984 Ethylene Oxide Standard.”

While this article praises the standard (as you might deduce from the title), the authors present some curious points of view…

The lack of clarity over the “means to alert” requirement [1910.1047(h)(2)] may lead to overspending by employers. Some advertisers of EtO alarm systems (the most common means of fulfilling the alert requirement) have interpreted this requirement in favor of their own interests, recommending sophisticated and expensive alarm systems that are sensitive to 1 ppm of EtO or lower in order to comply with the OSHA standard.

Accordingly, we recommend that OSHA (1) clarify its interpretation of “means to alert” (e.g., if it does not necessarily mean “get an alarm,” what are the alternative means of complying?), and (2) clarify that employers using EtO alarms are not required to set their alarms at or below the personal exposure monitoring limits (the PEL, AL, and EL) in order to be in compliance with the standard. Alarms are intended to monitor instantaneous area concentrations, and not time-weighted average personal breathing zone exposures. Alarms that would be triggered in the event of an accidental EtO leak or spill could be reasonably set within the range of 20-100 ppm, as previously recommended by NIOSH [Mortimer and Kercher, 1989].

OK. Let’s start from the end of the above citation. Mortimer and Kercher do not mention a “range of 20-100 ppm.” Rather, they state that…

Relatively inexpensive EtO sensors and alarm systems are available which could alert workers to an emergency situation involving the presence of a high concentration (greater than 20 ppm) of EtO. Other more sophisticated (and more expensive) systems have been developed which could detect elevated concentrations on the order of 1 ppm or less.

The Mortimer and Kircher work, entitled “Control Technology for Ethylene Oxide Sterilization in Hospitals,” is a 179-page report with 82 references in which nine sterilizer control systems were evaluated in real-life hospital situations. The results were excellent in that employee exposures in most cases were less than 2 ppm on excursion, and less than 0.1 ppm on an 8-hour time weighted average. While some material is outdated, there is still much useful information in this report.

Since OSHA set its 5 ppm excursion limit (EL) for ethylene oxide (EtO) in April 1988, we have no idea why Mortimer and Kercher used 20 ppm as their threshold of a “high concentration,” or why Lamontagne and Kelsey cited them nine years later.

While Lamontagne and Kelsey are again technically correct in the most narrow legalistic sense that the EtO alarm trigger need not be set at any particular value, they provide no means for how an employer would comply with the excursion limit. Moreover, the fact that OSHA quotes them on this matter—in light of its own excursion limit standard—is puzzling, to say the least.

Finally, Lamontagne and Kelsey are concerned about “overspending by employers.” Have they figured how much the employer might “overspend” defending a workers’ compensation lawsuit? How about “failure to warn” given a 20 ppm alarm set point and an OSHA mandated 5 ppm excursion limit?

The fact that OSHA has still not established an “alert” level is mentioned in its latest relevant publication: OSHA’s Small Business Guide for Ethylene Oxide [OSHA 3359 – 04 2009]. Interestingly, the reference to “20 ppm” has been modified as follows:

You should also be aware that there is a wide range in the cost and sensitivity of commercially available monitors. Some systems alert employees to EtO levels greater than 20 ppm, while other highly sensitive monitoring devices can trigger an alarm at much lower levels, such as 1 ppm or even lower.


Although permeation tubes were always available for low range ethylene oxide calibration, many customers prefer to use calibration gas blends in cylinders. Some years ago, these blends were not readily obtainable for concentrations below 20 ppm. Thus, a high measuring range was necessary to accommodate such blends.

Nowadays, though, low range gas is available. Traditional practice is to pick a gas blend concentration at one-half of the full scale measuring range of the analyzer. However, for those customers with 0-50 ppm range instruments, considering that most of your readings will (hopefully) be less than 5 ppm, a lower concentration standard can be used.

Overriding the traditional practice in this case, the best practice is to calibrate close to the level at which you expect most of your readings to occur. As always, we recommend that the instrument outlet be vented to the outside or to an exhaust register, ensuring that the discharge tubing is not obstructed.


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