A PEL of 5 ppm, a Unit Risk of 1.1E-4, and a Co-Worker with Neuropathy
You are the IH at a plant that hand-applies an epoxy coating containing a solvent, Compound A. Full-shift personal exposures average 0.6 ppm, with a 95% upper confidence limit of 0.9 ppm. Compound A has an OSHA PEL of 5 ppm, unchanged since 1971. Current manufacturer literature cites a cancer unit risk of 1.1E-4 per ppm (from a 1998 animal study) and a chronic RfC of 0.3 ppm. A 2019 study in a similar industry reported a fivefold higher unit risk, but it is a single unreplicated study. Company risk acceptance criterion is 1E-4. The plant manager says, 'We're at 0.6 against a PEL of 5 - we're fine.' There are 11 applicators, all Spanish-speaking, several of whom have asked whether a co-worker's recent peripheral neuropathy diagnosis could be work-related.
1. Compute and interpret the risk estimate both ways, and explain what the numbers do and do not support.
Read the worked answer
Cancer, manufacturer value: 0.6 ppm x 1.1E-4 per ppm = 6.6E-5, about 7 per 100,000. That is below the 1E-4 criterion and above 1E-5 - the top of the commonly accepted range.
Cancer, 2019 value: 0.6 x 5.5E-4 = 3.3E-4, about 3 per 10,000, which exceeds the company criterion. Using the 95% UCL of 0.9 ppm, the same calculation gives 9.9E-5 and 5.0E-4 respectively.
Noncancer: HQ = 0.6 / 0.3 = 2.0 (3.0 at the UCL). This exceeds unity and is the more robust finding - the RfC carries explicit uncertainty factors, while a unit risk is a low-dose-linear upper bound.
What the numbers do not support: 'We're fine.' The 5 ppm PEL is a 1971 limit derived from older data and is not a health-based risk threshold; comparing an exposure to it says nothing about risk. The estimate is also incomplete - it ignores dermal absorption, mixture effects, and short-term peaks during hand application. A defensible position is that cancer risk is at or above the company criterion depending on which potency estimate is used, and noncancer hazard is exceeded. Both point in the same direction: reduce exposure.
2. The plant cannot enclose the operation for 12 months. Recommend a risk management approach and justify it.
Read the worked answer
Manage to a target, not to the PEL, and be explicit about the interim period.
Hierarchy:
- Substitution - qualify a lower-solvent or waterborne coating. Even a partial switch on high-volume parts reduces exposure for all 11 applicators.
- Engineering - local exhaust at the application bench, a glove-bag or mini-enclosure for small parts, and ventilation of the storage/mixing area.
- Administrative - rotate applicators, reduce task duration, and segregate mixing from application.
- PPE - a PAPR or supplied-air hood as the interim measure; this is defensible only as time-limited protection while the higher-order controls are installed.
Targets. Interim target of 0.3 ppm or less (HQ of 1.0), with a stretch goal toward roughly 0.09 ppm, the level corresponding to 1E-5 at the manufacturer's unit risk. Set these in writing with dates.
Supporting elements. Dermal protection and glove selection (this agent is likely skin-absorbed), biological monitoring if a validated marker exists, medical surveillance aimed at neurological endpoints, and employee communication in Spanish. Re-survey after each control is in place - a control that is not verified is an assumption, not a control. Document the interim period as ALARA-based, not as compliance.
3. Several applicators ask whether their co-worker's neuropathy could be work-related. How do you respond?
Read the worked answer
This is a medical question I am not qualified to answer, and I should say so plainly rather than reassure or alarm.
- Refer, do not diagnose. The correct response is referral to an occupational physician with a copy of the exposure assessment, the agent identity, and the exposure duration - the physician makes the causation judgment.
- Take the question seriously on the IH side. A noncancer HQ of 2.0 means effects at this exposure level are plausible, and solvent neuropathy is a recognized outcome for several classes of compounds. Premature reassurance would be both wrong and damaging to trust.
- Offer surveillance. Make medical surveillance available to all 11 applicators on a consistent, non-punitive basis, including neurological symptom screening and an occupational history, with the results handled confidentially.
- Close the exposure loop. Confirm dermal exposure is controlled (gloves selected for this solvent, not just nitrile by default), check short-term peaks, and record the case appropriately in the OSHA 300 log if it meets the recording criteria.
- Communicate. Deliver the message in Spanish, in a group setting, with the same information to supervisors and workers, so no one is singled out.