Ethylene oxide monitoringEthylene Oxide (EtO) is a colorless gas which is known to be an eye, skin, and respiratory irritant in low concentrations, as well as presenting carcinogenic, mutagenic, reproductive, and neurologic hazards to workers. The odor of EtO cannot be detected below 700 ppm.  As such, ethylene oxide monitoring—via instrumentation—has long been practiced.

Although the primary application of EtO is as a feedstock for many organic syntheses, the major occupational health exposure concerns those who work in hospital sterilization facilities (Central Service areas or sterile processing departments).

EtO is used as a sterilant for steam and heat sensitive materials. In this category would be the various flexible scopes, now used extensively in less-invasive surgical procedures.

While EtO alternatives are available, such as peracetic acid and hydrogen peroxide gas plasma, there are still questions as to their effectiveness and suitability for use with all materials. Thus, at this point, EtO sterilization remains the method of choice.


In an effort to decrease confusion, a review of a few terms would be helpful.

  • ppm = part per million. 1 ppm can be compared to one second in 11 days. 1% = 10,000 ppm.
  • TLV® = Threshold Limit Value. (TLV is a trademark of the American Conference of Governmental Industrial Hygienists)


There are three categories of TLV’s:

  • TLV-TWA (Time Weighted Average): This refers to the concentration for an 8-hour work day and 40-hour work week, to which most workers may presumably be exposed on a daily basis without adverse effect.
  • TLV-STEL (Short Term Exposure Limit): This refers to the concentration to which workers can be exposed to CONTINUOUSLY for a “SHORT” period of time, without suffering from irritation, chronic or irreversible tissue damage, narcosis to the degree to cause impaired self rescue or accidental injury. All this is provided that the daily TLV-TWA has not been exceeded. The STEL is further defined as a 15 minute TWA exposure which should not be exceeded any time during the work day, even if the 8-hour TWA is within the TLV-TWA. Exposures above the TLV-TWA up to the STEL should not occur more than four times a day. There should be at least one hour between such successive exposures.
  • TLV-C (Ceiling): This is the concentration which should not be exceeded during ANY part of the working day.


Although the ACGIH TLV’s are essentially the same as OSHA’s PEL’s (Permissible Exposure Limits), the OSHA standards form the legal basis for determining occupational exposure, and are summarized here…

OSHA Regulations, per  29 CFR 1910.1047

  • The TWA (8-hour time-weighted average) for EtO is 1 ppm.
  • The Excursion Limit for EtO is 5 ppm, as averaged over a sampling period of 15 minutes.
  • The Action Level for EtO is 0.5 ppm, calculated as an 8-hour time-weighted average.

“Action Level” means a concentration designated in 29 CFR part 1910 for a specific substance, calculated as an eight (8)-hour time-weighted average, which initiates certain required activities such as exposure monitoring and medical surveillance.   [29 CFR 1910.1450(b)]

OSHA’s Substance Safety Data Sheet for EtO

OSHA’s Medical Surveillance Guidelines for EtO

OSHA publication — Ethylene Oxide (EtO): Understanding OSHA’s Exposure Monitoring Requirements

Our commentary on OSHA’s Small Business Guide for Ethylene Oxide

Full record on ethylene oxide from Hazardous Substances Data Bank (HSDB)

Prudence would dictate that you only consider an EtO monitoring system that can detect very low concentration levels, and be as specific to EtO as possible.

  • First, choose a manufacturer who is extremely knowledgeable about toxic gas detection.

Not only should this manufacturer be able to answer all your questions in a satisfactory manner, but they should be able to ask you thought-provoking questions about your requirement as well. Their ability to do this will demonstrate their true knowledge of your requirement, and their product.

  • Second, are the answers they are giving you “feel good type answers,” or are they honest answers?
  • Third, although lists of users may make you feel more confident about your decision, they may not be very reliable.

Naturally, if most manufacturers are keeping such lists up to date, they are certainly not going to give you a list of UNhappy users. More importantly, the responsibility of the equipment may have changed hands so often that you may not be getting an accurate appraisal of the situation, nor will you be able to contact a single source who actually knows something about the equipment.

What Is Out There And What Should You Buy?

The most inexpensive equipment uses the metal oxide semiconductor (MOS) device. These sensors may have been appropriate over 20 years ago when the TWA was 50 ppm. Now, however, they can only be considered as a catastrophic leak detector. Since the MOS technology was originally developed to measure combustibles and hydrocarbons in the percent level, it is not capable of measuring within or below the TWA compliance level of 1 ppm.

Some users refer to the MOS sensor as being “sensitive” because it responds to EVERYTHING. However, since the MOS device was designed to respond to ALL hydrocarbons, specificity and accuracy were never a prime requirement or necessity, and therefore not characteristic of this type of device. Interscan prefers to define sensitivity as “being capable of indicating minute differences” (as defined in Webster’s dictionary).

The general characteristics of monitors using MOS technology:

  1. Typically they will not have a direct ppm readout.
  2. The alarm levels are factory set at 20 ppm and 50 ppm.
  3. Usually, the monitors cannot be directly calibrated against a known EtO calibration standard.

Another method used by hospitals for detecting EtO is a gas chromatograph (GC). Some very important background information would be helpful before going any further.

First, it must be emphasized that there are NO monitoring techniques available which are truly and totally specific to EtO. There really is no magic way of getting around interferences. This is primarily due to the hospital environment itself which involves the use of many cleaning agents, solvents, detergents, and other compounds. Generally these substances are used in percent levels. Remember, the EtO monitor must measure in low to sub ppm levels. Whereas some monitors may be more specific than others, no technique is actually 100% specific.

A GC combines a chromatographic column with a detector such as photoionization (PID) or flame ionization (FID). When a substance passes through the column, it is sensed by the detector, and causes a “peak” on the readout or recording device. Although PID’s and FID’s can measure low concentrations, unless they are used with a column, they are completely NON-specific.

Degradation of the columns is caused by repeated exposure to certain chemicals and solvents such as isopropyl alcohol and most particularly, glutaraldehyde. Other solvents typically found in Central Service attack the columns, as well. Replacement costs of the appropriate columns can run anywhere from $100.00 to $925.00 each.

A single point (1 area) gas chromatograph (PID included) sells for approximately $40,000.00. This extremely high cost means that if you want to monitor EtO at more than one point, which is usually the case, the manufacturer will add a system of valves and timers to “stream-switch” the sample line inputs. Thus, despite the high cost, you will not be getting continuous monitoring at all points.

For example, in a 4-point system, you are NOT monitoring a given point three-quarters of the time! This very serious flaw renders any occupational exposure reports based on such a system to be suspect, at best.

Another monitoring technique which is available but is not used by hospitals as much as the two previously mentioned, is infrared. This technique is expensive (approximately $20,000 for a single point unit) and is known to have problems with steam, moisture, and ammonia. Sensitivity can also be an issue.

EtO badges are often employed in Central Service. Badges are not instruments, and do not give real time data. Most badges must be mailed to a laboratory to determine personal exposure results. Not only is this after-the-fact information, but it can also be misleading. The badges only yield a single-number integrated average. Short term high exposures are simply averaged into the single number. Thus, a true time history cannot be provided. As such, in some cases, the results could actually be worse than the badge is able to indicate.

How, Then, Can One Decide On Which Method To Use?

Let’s review the options:

MOS sensor — completely non-specific; cannot detect below 20 ppm (the TWA is 1 ppm); are not quantitative in that they can’t actually be calibrated against a known standard of EtO. They are useful as catastrophic leak detection in the tank storage area. The PID device can measure within and below compliance levels but without the use of chromatography, they are non-specific.

The gas chromatograph is specific only in that the “unwanted” samples or interferences are removed from the sample before being detected by the PID. They are extremely expensive, and are difficult to operate and maintain. Continuous monitoring at more than one point is not possible.

There is one alternative left which should be strongly considered — the electrochemical principle of operation (which is the method used by Interscan). Although we cannot claim the system is 100% specific to EtO, it is far more specific than the MOS type sensor. Furthermore, Interscan’s electrochemical sensor can measure within AND below the compliance levels. Alarms are user-adjustable, and the monitor can be calibrated against a known standard of low concentration EtO.

If the use of isopropyl alcohol (IPA) is curtailed or eliminated, then it is just as specific as a fully functioning gas chromatograph. A single point system is approximately $3,000.00. Multipoint systems are also available.

Interscan has been a manufacturer of toxic gas monitors for over 25 years. We developed and manufacture our own sensors. This affords us the unique capability of providing monitors appropriate for many applications. Interscan’s analyzers are easy to operate. The maintenance is low and simple, and relatively inexpensive.

So, How Does All This Sum Up?

  1. There is no such thing as a maintenance free monitor.
  2. There is no such thing as an EtO monitor which is 100% interference free. Manufacturers who make such a claim focus—at best—on certain key interferents, and are typically silent on the attendant maintenance and high cost of column replacement.
  3. Many hospitals which have run the entire gamut from the $1,100 MOS system to the $40,000 GC systems, have ultimately found that IPA is a problem for all EtO monitoring applications. For this, and other reasons, many hospitals have switched from using IPA, to an alternative germicidal, manufactured by Steris called LpH®se. Interscan’s analyzers do not respond to this compound. (MOS systems won’t either, but you are still left with a monitor which is unable to detect BELOW 20 ppm).
  4. If you have concluded that you are going to switch to an alternative to IPA (such as LpHse), then you certainly don’t need a $40,000 gas chromatograph.

Once you have made the decision to purchase a reliable and manageable EtO monitoring system, the next step toward making your monitoring package complete is to include a data acquisition / archiving system.

Considering that we are in a heavily litigious society, especially involving frivolous Worker’s Compensation lawsuits, one would be well-advised to reliably document and archive events, or more importantly, LACK of events.

Interscan has developed a versatile data acquisition/archiving/reporting system called Arc-Max®, that is currently in use in many medical facilities. A Cloud version is now in beta testing.

Interscan’s analyzers, when all other options are considered, meet compliance level requirements and, along with Arc-Max®, can provide an affordable, viable, and realistic approach to your EtO monitoring needs.