2. Materials and Methods
2.1. E/IH Model Focus
- Implanting efficacy test samples into selected boxes of materials on pallets to be sterilized
- Loading pallets of materials to be sterilized into a chamber
- Adding EO as a mixed or pure gas to the chamber to the desired level (kill concentration)
- Running the chamber under the desired temperature and relative humidity conditions for a duration to achieve the desired level of microbial mortality
- Exhausting some EO gas from chambers to in-room water traps (inefficient traps released EO into work room air)
- In some cases, implementing air/nitrogen washes to reduce remaining EO concentrations in chamber air and residue levels in sterilized materials before unloading the chamber (no or few washes were used during the time frame of interest)
- Unloading the processed materials from the chamber (released EO into work room air)
- Removing test samples from boxes for efficacy analysis (operators in close contact with off-gassing packaging and materials emitting EO into work room air)
- Transporting the sterilized materials to a storage area in or separate from the work room, and working in the storage area where materials remained until returned to customers (sterilized packaging and materials continue to off-gas during storage contributing additional EO to work room or storage room air)
2.2. E/IH Model Structure and Parameters
- accumulation of EO in a room in which sterilization operations occurred, resulting from emissions from chambers and water traps for exhausted gas;
2.2.1. Compartment Volumes
2.2.2. Air Exchange Rate
2.2.3. EO Concentration in Sterilization Chambers
2.2.4. Wash Cycles
2.2.5. Storage of Sterilized Materials
2.2.6. Rate of EO De-Gassing
2.2.7. Fraction (Fch) of Mair Attributable to EO in Chamber Void
2.2.8. EO Concentration in Worker Breathing Zone Due to EO Density
2.3. E/IH Model Calibration
2.4. Model Prediction Uncertainty
Conflicts of Interest
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|Date Range||Source a||Acceptable EO Concentration (ppm)|
|1978 (proposed)||FDA Maximum Residue Level||5–250|
|1984–present||ACGIH TLV-TWA, A2 suspected human carcinogen||1|
|2012–present||EC SCOEL implied 10−6 risk||0.0015|
|2016–present||EPA IRIS estimated 10−6 risk||0.0000001|
|Model Parameter a||Sterilized Product Type|
|Chamber, pallet volume per pallet||3.0, 2.0 m3/pallet a||18, 12 m3/chamber a|
|Void fraction||1/3 a||1/3 a|
|Air exchange rate (AER) b||0.25–4.0/h||0.25–4.0/h|
|Container/packaging mass||1.5 kg/box a,c||0.5 kg/bag a|
|Total container/packaging mass||24 kg/pallet d||48 kg/chamber|
|EO ad/absorbed in packaging, and in TM||10%, 2% w/w e||10%, 2% w/w e|
|TM mass||300 kg/pallet f||4,800 kg/chamber f|
|E/IH Model Assumption a||Early Period |
|Late Period |
|Commercial operation||Starting (small rooms for chambers and material storage)||Expanding (larger rooms for chambers and material storage)||Further expanding (large rooms for chambers)||Further expanding (large rooms for chambers)|
|Sterilizer operation||Manual||Manual||Manual and automated||Manual and automated|
|Chamber size||Small (self-manufactured)||Small or mixed||Mixed size or large||Mixed size or large|
|Chamber operation||Under pressure||Under pressure||Under pressure or vacuum||Under pressure or vacuum|
|Loading/unloading||Hand||Pallet jack||Pallet jack||Pallet jack|
|Separate rooms for sterilization versus storage||No||No||Yes (no aeration)||No|
|Vacuum/wash cycles per treatment||0 a||1 a||1–2 a||2 b|
|Final EO concentration in sterilizer chamber air c||17,800 ppm||17,200 ppm||11,500 ppm c||5560 ppm|
|Sterilizer size and number: a #sterilizers/#palettes||4/2||9/2||2/2 + 3/3 + 1/6||2/6|
|Sterilization (warehouse/storage) room volume||4800 (NA) m3 d||11,600 (NA) m3 d||11,000 (8500) m3 a||3100 (NA) m3 b|
|Hours per shift||8||8||8||8|
|Min/day in sterilization (warehouse/store) room||480 (NA)||480 (NA)||280 (180) a||480 (NA) b|
|Residual EO in sterilizer chamber air (Mair)||32.0 g||31.0 g||20.4 g||59.4 g|
|Total packaging mass||24 kg/pallet||24 kg/pallet||24 kg/pallet c||48 kg/chamber|
|% EO retained post-cycle by packaging and TM e||21% w/w||14% w/w||10.5% w/w||7% w/w|
|EO retained by packaging (Mpack)||504 g/pallet||336 g/pallet||252 g/pallet||67.2 g/chamber|
|EO retained by TM (Mprod)||1260 g/pallet||840 g/pallet||630 g/pallet||14.4 kg/chamber|
|90th Percentile 8-h TWA EO Concentration, C90 a (ppm)||C90/(C90 Scenario 3) (ppm)||90th Percentile 8-h TWA EO Concentration, C90 a|
|C90/(C90 Scenario 3) (ppm)||Fraction Due to Storage|
|1||Medical/health||Early||11.9 (7.90, 14.3)||0.33||187 (124, 224)||3.9||0.97|
|2||Medical/health||Middle||30.0 (19.9, 36.0)||0.58||118 (78.2, 141)||2.5||0.96|
|3||Medical/health||Late||47.4 (31.4, 56.8)||1.0||47.8 (31.7, 57.3)||1.0||0.95|
|4||Spices||Late||–||–||46.7 (31.0, 56.0)||0.98||0.92|
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