Within sterile processing departments, the predominant modality for ethylene oxide sterilization is the sealed EtO cartridge approach, as sold by 3M. In its Product Profile on the Steri-Gas™ EO cartridges, 3M puts forth these recommendations:
- Keep all sources of ignition such as matches, lighted cigarettes, sparks and static discharge away from the sterilizer and cartridges.
- Store cartridges at a temperature of 15–30°C (59–86°F) in an upright position.
- Keep only one day’s requirement or a maximum of twelve (12) cartridges (one box) in the immediate sterilizer area. This area needs to have at least ten air changes per hour.
- Additional Steri-Gas cartridges should be stored in an approved flammable liquid storage cabinet vented to the outside atmosphere, through a non-recirculating, continuously operating, dedicated exhaust system, or in an area suitable for storage of flammable liquids appropriately vented to the outside atmosphere. 3M recommends that you do not use unventilated flammable storage cabinets.
ChemDAQ advocates the practice of monitoring inside the storage cabinet, which, owing to their products being limited to diffusion rather than sample-draw sensor design, requires the use of an intrinsically safe sensor head, mounted directly inside the cabinet. The supposed danger is that concentrations of EtO in excess of the lower explosive limit (LEL) could build up in the cabinet, and thus be ignited by a general purpose (not intrinsically safe) sensor.
However, this notion flies in the face of logic, right reason, and the recommendations of 3M itself (as above).
3M—and ChemDAQ’s own documentation—emphasize that the storage cabinet must be vented, or as ChemDAQ puts it:
It is important that the cabinets be actively vented, i.e. the gas inside the cabinet is sucked out. Simply opening the vents on the side of the cabinet and relying on air diffusion does not suffice since air diffusion is relatively slow especially if there are flame arrestor grids in the vent holes. In summary, EtO cartridges should be stored in a flammable cabinet, but since EtO has such a low PEL, the cabinet should be actively vented to prevent exposure of workers using the EtO cartridges to EtO.
As such, with the cabinet properly vented, there is no possibility that the EtO concentration could approach anywhere near the 3% (30,000 ppm) LEL. Moreover, the 3M cartridges have been proven out in thousands of facilities, and carry a host of official governmental approvals.
Intrinsic safety would be a consideration if a sensor had to be deployed in a sealed area in which greater than LEL concentrations could build up. But such a situation does not exist here. Besides, alarms for a toxic gas detection system, as opposed to a combustible gas monitoring system, would have set point levels orders of magnitude below the LEL, providing an additional early warning. Notably, leaks from within a cabinet would affect other monitoring points within the department, long before any LEL danger could occur.
So why, then, does ChemDAQ continue to recommend monitoring the inside of these cabinets with an intrinsically safe sensor head? For the answer, consider this excerpt from one of their blog entries:
Most hospitals that we visit do store their single use cartridges in flammable cabinets, but a surprising number do not vent the cabinets. These single use cartridges often seep small amounts of EtO. If the cartridges were in the open, the EtO seepage though not good would be dispersed into the air and with the high air turn over in most sterile processing departments, not pose a great threat. However, if the cartridges are in an unvented flammable cabinet, the EtO that seeps from the cartridges can accumulate within the cabinet to be released only when the door is opened. We have seen a surprisingly large number of unvented flammable cabinets being used to store EtO single use cartridges.
In other words, for those hospitals that wish to continue the dangerous practice of not venting the storage cabinet, please deploy one of our intrinsically safe sensor heads inside the cabinet. And please do this even though in the same paragraph we also tell you that: “Most flammable cabinets have vents built into the side of them by the manufacturer, and so exhaust ducts can easily be connected to the flammable cabinets using standard plumbing fixtures.”
For those familiar with confined spaces in gas detection, an unvented cabinet left as such, but provided with an EtO sensor represents nothing less than an intentionally created and completely unnecessary confined space! A not too farfetched analogy would be purposely running your car inside a sealed garage, but having a carbon monoxide detector monitoring the air for “safety.”
Thus, the air in the room containing the cabinet can simply be monitored at any convenient point within the room. Alternatively, taking advantage of Interscan’s sample draw design, sample can be extracted from either the cabinet or its exhaust. In either case, a general purpose sensor, rather than an intrinsically safe set-up would more than suffice.
Certainly, an elevated reading within this room could indicate excessive EtO cartridge leakage, or a massive ventilation failure, and should be investigated—using the proper protective gear—as soon as possible. In the event of a complete ventilation failure, power to any potentially arcing devices should be shut down in the room, and any operational sterilizer cycles should also be terminated.