Sterilizing Safely

The focus of this article is ethylene oxide, a frequently used and potentially hazardous sterilizing agent.


Ethylene oxide sterilization concerns

Ethylene oxide has been in use for decades. More than 30 years ago, in 1977, the National Institute for Occupational Safety and Health (NIOSH) estimated 75,000 workers nationwide were potentially exposed, with more than 10,000 sterilizers in use.1 Further, during the period 1994 to 1998, ethylene oxide was associated with ten explosions, causing major property damage, scores of injuries, and at least one fatality.2 In August 2004, Sterigenics, a major contract sterilization company, had an explosion in its Ontario, California, facility, injuring four employees and shutting down the plant.3

Abbreviated as EtO (or, less commonly, EO), ethylene oxide is used for sterilization in many medical, dental, veterinary, and animal surgical facilities because it is efficient and performs well at low temperatures compared with steam sterilization. The sterility assurance level (SAL) for EtO sterilizers is 10-6, meaning there is only a one-in-a-million chance that a live microbe survived the sterilization cycle. Also, EtO is very compatible with polymer-based single-use medical devices, procedure kits, and surgical trays as well as most surgical instruments. The average surgical or research facility most likely uses small-chamber, countertop, selfcontained sterilizing cabinets. For larger-volume demands, contract companies have sterilizing units as large as semitractor trailers. In the small-batch sterilizers used in averagesize surgical units, EtO is usually delivered as a gas or from a small cartridge or ampule that is opened and allowed to vaporize for each batch run. However, ethylene oxide is not without its drawbacks. EtO is very toxic and highly flammable. Ethylene oxide sterilizer use requires precise installation, rigorous maintenance, and employee protection.

Hazard recognition—the dangerous properties

Ethylene oxide is a gas at room temperature with a boiling point of 10.7°C (51oF). EtO is also highly flammable. The flammable limits in air are from 3% to 100%, a very wide range. The vapor density is 1.5 (air is 1.0), and if left uncontrolled the vapors will tend to sink to the floor (or lowest available levels) and accumulate, spreading to the nearest ignition source.

In addition, EtO is a serious health hazard. It is colorless, with a characteristic sweet, etherlike odor, but if you can smell it, you are breathing a toxic concentration hundreds of times greater than the exposure limit. The OSHA occupational permissible exposure limit is only 1 ppm for a regular eight-hour work shift. However, the reported odor threshold is between 500 and 700 ppm. Therefore, odor is a very poor warning property for EtO.

Exposure can occur by inhalation or skin absorption and can have severe results. Inhalation exposure symptoms include eye pain, sore throat, blurred vision, difficult breathing, dizziness, and convulsions. Acute effects may lead to central nervous system depression, pulmonary edema, respiratory distress, and coma.

Liquid EtO can produce irritation or blistering of the skin or frostbite from rapid evaporation and consequent cooling. In some persons, ethylene oxide exposure may result in allergic sensitization, and future exposure may cause hives or a lifethreatening allergic reaction.

Ethylene oxide is designated as a suspected human carcinogen that may cause leukemia and other cancers. EtO is also linked to spontaneous abortion, genetic damage, nerve damage, muscle weakness, and peripheral paralysis.

Hazard evaluation—are exposures present?

Occupational use of ethylene oxide is regulated by the OSHA standard 29CFR1910.1047. However, if the employer can demonstrate that employee exposures are below the established action level (AL) of 0.5 ppm during processing, use, and handling based on an eight-hour time-weighted average (TWA), the standard would not apply. If the AL threshold is exceeded, then the additional requirements for routine personal monitoring, medical examinations, training, and recordkeeping would be triggered. There is also a short-term exposure limit (STEL) of 5.0 ppm, a 15-minute average that employees must never exceed. If the AL or STEL is triggered, periodic monitoring is conducted every six months for employees exposed at or above the AL and every three months for those exposed at or above the PEL.

If ethylene oxide sterilizers are used in your facility, potential exposures should be evaluated. The most appropriate method is to conduct initial personal exposure monitoring on representative employees directly involved in handling and using ethylene oxide. Area sampling may be done with direct-reading portable instruments as a screening method and the results used to set up the personal monitoring. If area monitoring is performed, it is a good idea to also perform leak checking while the direct-reading instrument is on-site.

Before conducting initial monitoring it is good practice to check the room or area for proper ventilation and exhaust. For general ventilation, make sure the room is on dedicated, single-pass air without recirculation. Ensure that the sterilizer area air is at a slightly lower pressure than that in adjacent spaces. Check that the supply flow is adequate and volumes are as designed. Also examine the sterilizer equipment thoroughly. Are the door gaskets in place and in good condition? Is the sterilizer exhausted properly according to the manufacturer’s recommendations? Is the exhaust working? How are ethylene oxide gas cylinders handled? Are associated piping, hoses, valves, regulators, and filters inspected and checked for leaks regularly?

After the equipment is checked and the facility ventilation is verified, ensure that all employees using the system have proper operator training. Observe the technician’s operation of a complete sterilization cycle. Pay special attention when the sterilizer is unloaded. Is a purge cycle used? How are the sterilized materials vented and handled? It is best to get all these things in order before spending money on expensive monitoring.

Hazard control—preventing overexposure

First we must use good engineering controls and comprehensive training and work practices to prevent potential exposures. Measure supply and exhaust flows, and conduct a ventilation test and balance. Ensure that equipment is properly installed and exhausted and that there are no leaks. Check for leaks from the sterilizer, gas cylinder, and supply lines (if so equipped).

The task that presents the best chance for uncontrolled release and exposures that we have observed is the venting and off-gassing of sterilized materials. With sampling we have found that significant EtO levels can remain in the packages for up to 30 minutes after they are removed from the sterilizer. We would recommend using an additional purge cycle or transferring materials to a hood or other exhausted enclosure for a short period to allow final off-gassing prior to further handling.

Respirators may be used during installation of engineering controls, during maintenance or repair, and when engineering controls and/or work practices are not feasible. Respiratory protection is also prudent when dealing with emergencies. However, use of respirators triggers another OSHA standard, 29CFR1910.134, Respiratory Protection, and employees must be cleared by a physician, trained, and fit tested prior to use. So this should be a last resort to limit exposures.

Summing up

Additional assistance is available from the EtO Sterilization Association ( as well as the NIOSH and OSHA websites. The keys to working with hazardous materials are first to recognize the hazards, then follow up with evaluation of the potential exposures, and finally design and implement proper controls. Where EtO is used we must ensure proper installation and sufficient facility supply and exhaust, safe cylinder/material handling, and routine leak-check procedures. Finally, conduct periodic monitoring of the engineering systems and air concentrations.


1. Special Occupational Hazard Review with Control Recommendations: Use of Ethylene Oxide as a Sterilant in Medical Facilities. National Institute of Occupational Safety and Health. Atlanta, GA. 1977.

2. Preventing Worker Injuries and Deaths from Explosions in Industrial Ethylene Oxide Sterilization Facilities. National Institute of Occupational Safety and Health. Atlanta, GA. 2007. 

3. Final Report on Ethylene Oxide Explosion at Sterigenics, Ontario, California. United States Chemical Safety Board. March 2006. 

Categories: Lab Health and Safety

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Published: July 10, 2014

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