In January 2022, the United Kingdom adopted ISO 10993-7:2008 / Amd 1:2019 as BS EN ISO 10993-7:2008 / A1:2022 without modification.

Prior to the implementation of the revised standard, no obvious consideration was given to patient age when setting the appropriate limits for ethylene oxide residues in medical devices.  Arbitrary limits given as milligrams of exposure to residues per device were simply prescribed based on the physical contact time allowed for each category of device. It was also not apparent without the scrutiny of the many appendices how these figures had been derived. The new standard differentiates limits to be applied to babies and infants based on body mass, which we consider to be an improvement to the previous “one size fits all” recomenndations.

As might be expected, the limits for babies and infants calculated, taking account of the new standard, are significantly lower and may be below the quantitation limit of some current analytical methods developed and validated by manufacturers. For example, for a device with a maximum contact time of 24 hours, the “old” limits for ethylene oxide (EO) and ethylene chlorohydrin (ECH) for exposure, given a body mass of 70kg and a single device was involved were 21 and 60mg respectively. The new limits for a baby of mass 4kg (single device) would be 1.2 and 2.56mg respectively.  

While the present article outlines how the limits for ethylene oxide sterilization residues based on body mass are to be calculated for babies and infants, it will also discuss how the analytical results determined for residues generated by even the most precise GC analytical methodology, are only ever as good as the sampling, sample preservation and sample preparation procedures employed.  This is especially true when analytes of high volatility and reactivity such as ethylene oxide (EO) are concerned. When dealing with such a highly toxic agent, Butterworth Laboratories has always recommended a worst-case approach regarding this standard.

The present article is applicable to solid medical devices but not fabrics, drapes, contact lenses or some other particular devices such as dialysis blood flow components.


Sterilization by the application of EO gas is commonly used in the healthcare industry.  Developed in the 1940s by the US military, it is used for sterilization of articles such as plastic instruments and medical devices that are unable to tolerate excessive heating from simpler and cheaper sterilization procedures such as autoclaving.  EO is also commonly used for the sterilization of other items such as fabric dressings and drapes.

EO is known to have a number of biological impacts including irritation, organ damage, mutagenicity and carcinogenicity as well as negative reproductive effects. The main drawback of using EO as a sterilization agent is that it can leave toxic residues on the articles being processed. It is therefore essential that EO sterilized articles are subjected to sufficient aeration to remove such residues prior to final packaging by manufacturers.   

Further to EO residue itself, toxic reaction and degradation products such as ethylene chlorohydrin (ECH) and ethylene glycol (EG) can result from the use of EO as a sterilization agent.  ECH, is a residue that can form when EO comes into contact with free chloride ions and EG when EO reacts with water. 

The use of EO is addressed by ISO 11135-1 which specifies requirements for the development, validation and routine control of EO sterilization processes for medical devices. ISO 10993-7 is the complimentary standard detailing allowable limits for residual EO, ECH and EG. It also outlines procedures for the measurement of EO and ECH in order to determine compliance so that devices may be safely released for use.

Categories of Device and Analytical Extraction Procedures

ISO 10993-7 categorizes devices into three exposure categories:

(A) Limited Exposure:  devices whose cumulative sum of single, multiple or repeated contact is up to 24 hours.

(B) Prolonged Exposure:  devices whose cumulative sum of single, multiple or repeated contact is likely to exceed 24 hours but not 30 days.

(C) Permanent Contact: devices whose cumulative sum of single, multiple or repeated contact exceeds 30 days.

The ISO standard specifies the maximum levels of release of EO and ECH residues over specific time periods for each category of device.

The standard also defines two types of extraction of residues from samples prior to analysis: Simulated-Use and Exhaustive. Care needs to be taken in assessing which option should be chosen for a specific medical device.  Simulated-use is based on water extraction to evaluate residue levels actually available to a patient or user of a device during its routine use and is mainly applicable to categories (A) and (B).  Exhaustive extraction is defined slightly differently in that the definition states: extraction until the amount of EO or ECH in a subsequent extraction is less than 10% of that detected in the first extraction, or until there is no analytically significant increase in the cumulative residue levels detected.  It typically can be applied to category (C) devices. The ISO reference procedure is Simulated-Use extraction.

Permissible Limits

Calculation of Limits in General:

The following general discussion of limits for residuals (EO and ECH) is only found from the scrutiny of the appendices of the standard. A number of definitions are required at this point:-

Final limits are obtained in terms of total mg of exposure per day to residues for each category of device. The limit is defined as the Tolerable Exposure (TE).

Tolerable Intake (TI) is defined in terms of exposure mg/kg/day (derived from animal toxicological studies).

Concomitant Exposure Factor (CEF); where multiple devices are used, the limits should be proportioned downwards by the CEF. For example, if 5 devices are used, then limits shall be multiplied by a CEF of 0.2.

Proportional Exposure Factor (PEF); where a device is used for only a part of the time assumed for a device type, the limits can be proportioned upwards by the PEF. For example, if a Limited Exposure Device is used for only 12 rather than 24 hours, then the limits can be multiplied by a PEF of 2

The final calculation of specific limits is then obtained as follows;

TE = TI x Body mass (kg) x CEF x PEF

Using this calculation the actual limits can be calculated if all variables are known. Figures given in the main body text are given assuming a body mass of 70kg for the adult population, a CEF of 0.2 and a PEF of 1.  While the appendices of the standard gives the detail for calculation of limits for EG, the standard states that it is not usual to find significant residues of EG given that any residual EO is within limits. In any case, the calculated limits would be very high and it is unusual to investigate EG residues present (if any).

The following Generic limits are given as examples in the standard main body text.

The following has been lifted “as is” from the standard.

Limits for Permanent Contact Devices:

The average daily dose of EO should not exceed 0.1 mg/day and in addition not exceeding 4 mg in the first 24 hours, 60 mg in the first 30 days, and 2.5 g in a lifetime.

If the device is intended for use in babies and infants, the limits shall take account of body mass as follows: 0.02 mg/kg/day for EO, 0.029 mg/kg/day for ECH.

Limits for Prolonged Exposure Devices:

The average daily dose of EO should not exceed 2.0 mg/day and in addition not exceeding 4 mg in the first 24 hours, 60 mg in the first 30 days.

If the device is intended for use in babies and infants, the limits shall take account of body mass as follows: 0.3 mg/kg/day for EO, 0.27 mg/kg/day for ECH.

Limits for Limited Exposure Devices:

The average daily dose of EO and ECH should not exceed 4 and 9 mg/day respectively.

If the device is intended for use in babies and infants, the limits shall take account of body mass as follows: 0.3 mg/kg/day for EO, 0.64 mg/kg/day for ECH.


Even though the actual limits calculated in accordance with the appendices would give higher figures, the generic limits regarding EO for adults, in terms of mg/day for Permanent Exposure and Prolonged Exposure devices, have been maintained from the “old” standard. Another failing of the standard is that it only gives generic limits for ECH for adults for Limited exposure devices, with limits for other device categories now being neglected. The only actual change in generic limits for average mg/day, is the generic limit for EO of 4mg/day replacing the “old” standard 2mg/day for Limited Exposure devices.

Frank D. Judge, Consultant Chemist Chromatography at Butterworth Laboratories comments: “I have to confess that I was somewhat disappointed when I realised that the new standard, supposedly to introduce limits for children, simply gives the same TI (mg/kg/day) values as have been used for calculation of adult limits. The only difference being that the child’s body weight rather than an assumed 70kg should be used in calculation. It could simply have stated that where children are concerned, actual body mass should be used. Unless I have missed a figure somewhere in the depths of the standard appendices, I was surprised that the new standard, including limits for children, does not give the assumed body mass to be used for the calculation of generic limits for children. I would like to think that I have simply missed this and it is in fact stated somewhere in the new standard.”

The majority of samples received for analysis at Butterworth Laboratories are category (A) Limited Exposure types such as dressings, clamps, speculums and other plastic articles. This article will focus further on the sampling and analysis of these items and the use of the reference Simulated-Use extraction procedure. 

Sampling Selection, Presentation of Samples for Analysis and Pre-Analysis Sample Storage  

While the analytical procedure is of course of major importance, the most accurate and carefully designed methodology is superfluous if representative articles are not selected and supplied for testing intact. Once received by the testing facility, maintenance of sample integrity is also paramount.

Butterworth Laboratories make the following recommendations to their clients:

  • When samples are selected for analysis at the sterilization facility a worst-case approach should be adopted.
  • Sampling should be carried out given the minimum period of time usually allowed for aeration of the products and dissipation of any sterilization residues prior to release into the supply chain.
  • It is important if packaged treated products are stacked on pallets for example, that samples are not only taken of the outermost articles or packs of the bulk but also from the innermost ones that are liable to have had the least chance to outgas.
  • The selected samples should be immediately packed in dry-ice along with a temperature recording device and delivered to the analytical laboratory as quickly as is possible.  
  • The package containing the samples should be clearly marked to alert sample reception staff that the contents are under dry ice so that they are alerted to the requirement for prompt processing.
  • Immediately after the ‘booking in’ procedure, samples should be stored deep frozen at a maximum temperature of – 20°C prior to analysis.  If no temperature monitoring device is included in the sample packaging it is considered acceptable that the presence of dry ice on receipt is noted.

Simulated-Use Extraction Procedure

The process of simulated-use extraction of limited exposure devices is carried out by adding the sample articles to glass bottles which are then filled with a recorded volume of high purity water and capped.  Extraction is then carried out by incubation of the bottles at 37°C for articles that directly contact the body when in use such as dressings.  25°C is used for articles which do not directly contact the body such as syringes. While it is only required to extract samples for the amount of time they will be in contact with the body in use, with a minimum prescribed by the ISO standard of one hour, again Butterworth Laboratories  recommend a worst-case approach to extract samples for the full 24 hours maximum usage time allowed for limited exposure type devices.

For plastic items such as long speculums which may not be easily accommodated in normal laboratory bottle types, it may be required to break the article into smaller pieces for extraction.  It is recommended that example samples additional to those for actual testing are supplied, so that this operation can be practiced.  This is recommended so that when the process is carried out on actual samples, that a minimum amount of time is taken between removing the samples from frozen pre-analysis storage conditions until the samples are sealed in bottles ready for extraction. It is also vital when filling and capping the extraction bottles that minimum air space is left that EO or ECH vapours could partition into during the incubation period and which would lead to lower recovery of analytes.

In order to determine whether EO is consumed by reaction with the sample material itself or with water to form ethylene glycol during the incubation period, additional unsterilized samples of each type should be provided so that the extra sample extraction bottles used for these can be artificially spiked with EO and then analysed along with the routine samples in order to qualify recovery.

Analytical Procedure

The standard states that the simulated-use extraction procedure is the reference method for extracting EO residues from samples. In practice, for limited exposure devices, the efficiency of this process is not evaluated at the time of use as long as the process and conditions described in the standard are followed. This means that the actual analysis is simply EO and ECH in water extracts and therefore a validation based on each device type is not required. One exception to this general rule actually encountered by the author, was where a permanent contact bone graft sponge actually absorbed and did not liberate EO during simulate-use extraction. The analytical procedure used at Butterworth Laboratories determines both EO and ECH using a single Headspace Gas Chromatography method.

Author: Frank D. Judge BSc (Hons), CSci, CChem, MRSC, MRQA.