Mixed Solvent Exposure in a Spray-Paint Operation
At an automotive parts painting facility, an industrial hygienist sampled three spray-paint operators over a full shift. The paint contains toluene and methyl ethyl ketone (MEK). Personal samples: Operator A - toluene 30 ppm, MEK 120 ppm; Operator B - toluene 22 ppm, MEK 165 ppm; Operator C - toluene 45 ppm, MEK 90 ppm. All are 8-hour TWA results. The facility's OELs are toluene 50 ppm and MEK 200 ppm (8-h TWA). The spray booths have dry filters and a downdraft design, but the operator's breathing zone is outside the booth during gun cleaning with solvent-soaked rags. Two samples were collected during a week when production ran at 60% because of a parts shortage; the third was during a full-rate day. The plant physician reports two workers with persistent headaches and one with elevated liver enzymes. Management wants to know if they are 'in compliance' and whether respirators alone would fix the problem.
1. Evaluate compliance with the occupational exposure limits for the mixture. Show your calculations.
Read the worked answer
Because the two solvents act on the same target (both are central-nervous-system depressants and both are absorbed, with MEK known to modify toluene's effects), the exposure must be assessed as a mixture using the additive formula: sum of (C_i / OEL_i) must be 1 or less.
- Operator A: 30/50 + 120/200 = 0.60 + 0.60 = 1.20
- Operator B: 22/50 + 165/200 = 0.44 + 0.825 = 1.265
- Operator C: 45/50 + 90/200 = 0.90 + 0.45 = 1.35
All three exceed 1.0, so all three operators are overexposed to the mixture even though no single component exceeds its own OEL. Operator C is closest to the toluene limit (90% of it) but the combined index puts the crew at 120-135% of the acceptable mixture exposure.
The additive formula is valid only if the OELs share the same averaging time and are based on comparable health effects; both here are 8-hour TWAs, so the calculation stands. The results also fit the symptoms: headaches consistent with solvent CNS effects, and the elevated liver enzymes warrant follow-up because the exposure is uncontrolled.
One caution: TWA results can hide peaks. Gun cleaning with solvent-soaked rags outside the booth is a short, high-concentration task that a full-shift average dilutes. Short-term or task-based samples should be taken against any ceiling or short-term limit before concluding the picture is complete.
2. Critique the sampling strategy and state what additional data are needed before the plant declares compliance status.
Read the worked answer
Weaknesses:
- Only three workers sampled, with no defined similar exposure groups. The plant needs to group workers by task and exposure profile (sprayers inside booths, gun cleaners, touch-up, booth maintenance) and sample within each group, using the variability of prior results to decide how many samples are needed to be confident about the group's exposure.
- The samples are not representative of the worst case. Two were taken during a 60% production week caused by a parts shortage. Low-throughput weeks understate exposure for a task whose rate depends on parts flow. Sampling must cover normal full-rate production and the highest-exposure tasks.
- No task-based or short-term samples. Gun cleaning outside the booth is the obvious peak exposure and is invisible in a full-shift TWA.
- Dermal exposure is ignored. Both solvents can be absorbed through skin; solvent-soaked rags with bare hands are a real route, so skin assessment and glove selection matter as much as air sampling.
- No documentation of sampling and analytical method, calibration, or field blanks in the summary, which undermines defensibility.
What to do next: repeat sampling over at least three full-rate days, add short-term samples for gun cleaning, sample each exposure group, record task durations and ventilation conditions, review the Booth airflow and filter loading, and check the solvent composition on the safety data sheet against the OELs used. Only then can a compliance determination be made. Meanwhile, the exceedance already justifies interim controls - the mixture index over 1.0 is not a borderline result.
3. Management asks whether supplying respirators would resolve the issue. Give your recommendation with a control strategy.
Read the worked answer
Respirators are the last line of defense, not a solution. They depend on fit testing, medical clearance, correct cartridge selection for both solvents, change-out schedules, and constant supervision - and they do nothing for the solvent absorbed through the skin during rag cleaning. Relying on them alone would also leave the underlying emission source in the building.
Control strategy, in hierarchy order:
- Elimination/substitution. Evaluate waterborne or high-solids coatings, or a toluene-free formulation. This is the single most effective step and should be evaluated first, with a trial batch.
- Engineering. Enclose or ventilate the gun-cleaning station with a dedicated local exhaust hood (a closed gun washer with a lid is a common, low-cost fix); keep the operator's breathing zone inside the booth during spraying; check downdraft velocity and filter loading; and stop using open pans of solvent.
- Administrative. Limit the quantity of solvent in use, keep lids on, segregate solvent rags in self-closing containers, and rotate the gun-cleaning task if engineering controls cannot be installed immediately.
- PPE. Solvent-resistant gloves selected for both chemicals, and respirators as an interim measure with a full written respiratory protection program - fit test, medical evaluation, cartridge change schedule, and training.
Verification: repeat personal sampling after controls, including task-based samples, and confirm the mixture index is below 1.0 with margin. Track the physician's findings - the liver enzyme case and the headache reports should be reviewed with the medical provider, and the exposure records kept for the required retention period.