Peracetic acid appears on the WEL list as a new airborne contaminant (`new_no_previous_WES`) with an 8-hour TWA of 1.24 mg/m³ (0.4 ppm). Until 30 November 2026, WES remains the compliance reference. From 1 December 2026, WEL applies once implemented — verify with the WHS regulator for your jurisdiction.
What you are measuring
Peracetic acid is a reactive oxidising sanitiser. Airborne exposure is typically vapour, with mist / aerosol possible during spray, fog and vigorous transfer. Monitoring design should state:
- Target analyte and reporting basis (mg/m³ and/or ppm)
- That the WEL comparison is an 8-hour TWA of 1.24 mg/m³ (0.4 ppm)
- Whether short-duration make-up / spray / fog samples will be time-weighted into a shift TWA or reported separately as peaks for control diagnosis
- That no STEL is taken from the CSV row for this listing — do not invent one
Peak control still matters for irritation and acute effects — address via task observation and engineering, without fabricating a numeric STEL.
Site patterns that drive strategy
- CIP make-up / dosing — IBC connection, day-tank charge, titration sampling
- Filler / equipment spray — brief but high proximity; easy to miss if you only sample sealed automated CIP
- Fogging / room sanitation — often night shift or contractors
- Pharma / biotech wipe-spray programmes — smaller volumes, sometimes weaker LEV
- Maintenance — line breaks after imperfect flush on sanitiser circuits
- Spill response — episodic SEGs that still need a plan
Reactive chemistry means media selection and sample integrity need lab agreement early — do not treat PAA like a stable VOC analogue without checking recovery and interferences (peroxide / acetic backgrounds).
SEG walkthrough
Example survey plan for a beverage site:
- SEG A — CIP / dosing operators (full-shift personal + task sample during make-up and open sample).
- SEG B — filler sanitation spray (task-based; may nest inside A if same people and profiles match).
- SEG C — night fog / contract cleaners (usually separate).
- SEG D — maintenance line-break on sanitiser circuits.
- Covariates: stock vs use-dilution, spray rate, room volume, LEV on/off, temperature, RPE worn (context only).
Pre-define “representative” for sanitation chemicals: sampling only between CIP cycles answers the wrong question.
Sampling strategy
- SEG definition — make-up, spray, fog, maintenance, lab as separate or nested SEGs where profiles differ.
- Full-shift samples on representative sanitation days plus task-based open pour / spray / fog samples.
- Sorbent / media / impinger selection agreed with the analytical laboratory for PAA stability, recovery and interference management.
- Flow, breakthrough and blanks documented; note humidity, temperature and co-oxidants.
- Simultaneous process context — open time, dilution, fog dwell, indoor vs outdoor make-up.
- Enough replicates across sanitation cycles to see whether one “good” make-up was luck or design.
Where ppm and mg/m³ both appear on the WEL listing, report both when the method supports it, and show conversion assumptions if you calculate one from the other.
Design detail worth writing into the plan
- Breakthrough controls when concentrations may spike during open pour or fog start-up
- Storage and transport conditions that respect analyte reactivity
- Clear instruction on whether RPE is worn during sampled tasks
- Separate diagnostic static samples at make-up points for LEV performance — labelled diagnostic, not personal TWA
Interpretation
- Planning phase (now): compare measured TWAs to 1.24 mg/m³ (0.4 ppm) as the incoming limit.
- Compliance phase (from 1 Dec 2026, once implemented): the WEL is a limit not to be exceeded for persons at the workplace.
- Non-detects: state LOD/LOQ relative to a meaningful fraction of the WEL. A non-detect at a high LOD is not evidence of comfort under 0.4 ppm.
- Variable shifts / night sanitation: document how partial shifts and split crews were handled.
- Peaks: report task concentrations for control diagnosis even when the shift TWA is the WEL comparator — without inventing a STEL.
Surface contamination and dermal risk may dominate even when air is low. Air monitoring alone does not close the risk assessment.
Method hygiene (peer expectations)
- Cite laboratory method ID and accreditation scope
- Avoid claiming “AS X requires …” unless verified for this analyte
- Keep field notes sufficient to reconstruct SEG logic
- Separate compliance comparison from control-performance diagnostics
- Do not back-calculate a STEL or invent figures beyond 1.24 mg/m³ (0.4 ppm)
Control hierarchy cues for recommendations
When results approach 1.24 mg/m³ (0.4 ppm):
- Eliminate / substitute — closed dosing; reduce fog where efficacy allows
- Engineering — sealed make-up, LEV at spray booths, purge-before-break on filters
- Administrative — written open-time limits; contractor induction naming PAA
- PPE — vapour/mist RPE with change schedules; compatible gloves — listed last
State what evidence would show the control worked (repeat personal TWAs on a representative make-up + spray day; capture check at the day tank).
Peak tasks vs TWA
Clients often ask whether a three-minute open pour “failed the WEL.” Answer in two layers:
- The WEL comparator is the 8-hour TWA of 1.24 mg/m³ (0.4 ppm).
- Task peaks still drive irritation risk and control design; report them as diagnostics, time-weight with explicit assumptions, and do not invent a STEL.
If unsampled time is assumed zero, say so.
Common failure modes
- Mobilising with generic “acid vapour” media before lab confirmation for PAA
- Full-shift sampling on a day with no make-up, spray or fog
- Treating odour logs as quantitative exposure data
- Filing ppm results without stating measured vs converted
- Ignoring night sanitation SEGs because “production CIP was automated”
- Inventing a STEL for client convenience
- Early-adoption claims without regulator confirmation
FAQ — technical
Should every report quote both 1.24 mg/m³ and 0.4 ppm?
Prefer both when the method supports it; keep them paired as the published WEL expressions.
How do we handle short fog tasks on a long shift?
Collect task samples and time-weight explicitly into the 8-h TWA, stating assumptions for unsampled time.
Is there a published STEL on this CSV row?
No. Do not invent one.
What is the minimum defensible n?
Enough to characterise sanitation-cycle variability for the decision you are making. One sealed automated CIP day rarely ends a control argument near 0.4 ppm.
Sources
- Safe Work Australia, Changes to Workplace Exposure Limits – May 2025 (PDF): https://www.safeworkaustralia.gov.au/sites/default/files/2025-05/changes_to_workplace_exposure_limits_-_may_2025.pdf
- SWA WEL / WES changes page: https://www.safeworkaustralia.gov.au/safety-topic/managing-health-and-safety/workplace-exposure-limits-airborne-contaminants/changes-between-wes-and-wel
- `/workspace/wel/wes-to-wel-changes-2026.csv` — Peracetic acid WEL 1.24 mg/m³ (0.4 ppm); no STEL on row
Technical support
Need a monitoring plan or report review against the incoming WEL? Start at [occhygiene.com.au](https://occhygiene.com.au).
