Sterilant Gas Monitoring

Sterilant gases are compounds used by health care and other facilities to sterilize medical supplies that cannot be sterilized by heat or steam methods. The current FDA approved sterilant gases are ethylene oxide, hydrogen peroxide [http://www.fda.gov/cdrh/pdf7/K071385.pdf] and ozone [http://www.fda.gov/cdrh/pdf2/k020875.pdf] . Sterilization means the complete destruction of all biological life (including viruses and sporoidal forms of bacteria) and sterilization efficacy is typically considered adequate if less than one in a million microbes remain viable.

Since sterilant gases are selected to destroy a wide range of biological life forms, any gas which is suitable for sterilization will present a hazard to personnel exposed to it. The NIOSH immediately dangerous to life and health values (IDLH) for the three sterilant gases above are 800 ppm, 75 ppm and 5 ppm for ethylene oxide, hydrogen peroxide and ozone respectively. [ [http://www.cdc.gov/niosh/idlh/intridl4.html NIOSH: Documentation for Immediately Dangerous to Life or Health Concentrations (IDLH) / NIOSH Chemical Listing and Documentation of Revised IDLH Values (as of 3/1/95) - intri... ] ] . For comparison, the IDLH of cyanide gas (hydrogen cyanide) is 50 ppm. Thus exposure to even low levels of sterilant gas should not be treated causually and most facilites go to great lengths to adequately protect their employees.

In addition, the sterilizer manufacturers go to great lengths to make their products as safe as possible, but sterilizers (as with any mechanical device) can and sometimes do fail and leaks have been reported (see for example FDA's Manufacturer and User Facility Device Experience Database [available at http://www.fda.gov/cdrh/maude.html] . Continuous gas monitors are used as part of an overall safety program to provide a prompt alert to nearby workers in the event that there is a leak of the sterilant gas.

The monitor alarms are typically set to warn if the concentrations exceed the OSHA permissible exposure limits (PELs), 1.0 ppm for ethylene oxide [29 CFR 1910.1047] and 1.0 and 0.1 ppm for hydrogen peroxide and ozone respectively [29 CFR 1910.1000, App. Z-1] . The PELs are calculated as 8 hour time weighted average values (i.e. the average exposure over a typical shift).

In addition to providing continuous monitoring, the better gas monitors include a computer based data acquisition system provides automatic data logging to assist with compliance with OSHA's regulations [29 CFR 1910.1020] and impending alarms that allow users to rectify problems before they become a hazard.

There are several technologies that are commonly used for the detection of sterilant gases, the main three being electrochemical, gas chromatography and metal oxide semiconductor. All three technologies are suitable for ethylene oxide, but only electrochemical sensors are used for hydrogen peroxide and ozone detection.

For ethylene oxide, metal oxide semiconductor sensors provide a low cost, long life detector, but they lack the sensitivity necessary to detect ethylene oxide at the OSHA PEL. Instead these sensors are used only as gross leak detectors. Gas chromatography offers excellent sensitivity and selectivity towards ethylene oxide, but the monitors tend to maintenance intensive and usually sample and analyze from several measuring points in sequence as opposed to providing continuous monitoring. Electrochemical sensors offer very good sensitivity, fast alarm response times and small size. ChemDAQ and most other gas monitor manufacturers mainly use electrochemical sensors for toxic gases because of their many advantages.

Electrochemical sensors have traditionally suffered from a cross sensitivity problems with other vapors (alcohols are especially troublesome in health care). Cross sensitivity results in false alarms, unnecessary evacuations and eventually a loss of confidence in the gas monitoring system.

One manufacturer, ChemDAQ [ChemDAQ's Inc. 135 Industry Dr, Pittsburgh, PA 15275, Tel. 412-787-0202, Fax. 412-788-2526, www.chemdaq.com] has developed a proprietary chemical filter that allows the ethylene oxide to pass through but removes most common interferent gases and vapors (including carbon monoxide, ethanol and IPA). The use of this filter allow the ethylene oxide to be detected with all the advantages that electrochemical sensors have to offer, but without the problems of cross sensitivity.

There are several other manufacturers of gas monitoring equipment and users should review the information available from several companies before purchasing a gas monitoring system to ensure that it will be suitable for their application.

References


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