Solvent Mixture Overexposure in a Manual Degreasing Operation
At a 60-person metal fabrication shop, parts are degreased manually with toluene and acetone. Personal 8-hour TWA sampling returns toluene at 120 ppm and acetone at 450 ppm. A 15-minute task-based sample during manual wiping shows toluene at 210 ppm. Employees report headaches, dizziness, and dry skin. The only controls are general dilution ventilation and nitrile gloves; there is no local exhaust at the degreasing bench. OSHA PELs are 200 ppm for toluene and 1,000 ppm for acetone; neither substance has an OSHA STEL that applies here, but the company uses ACGIH TLVs of 20 ppm for toluene and 500 ppm for acetone as internal targets. The shop has 14 open-top parts washers and no written respiratory protection program. Management wants to issue N95 respirators and continue the current practice.
1. Calculate the combined 8-hour exposure using the OSHA mixture formula and determine whether the exposure is over the PEL. Explain what the result means.
Read the worked answer
Use the mixture formula for PELs: C1/L1 + C2/L2, where C is the measured concentration and L is the applicable limit. For toluene, 120/200 = 0.60. For acetone, 450/1,000 = 0.45. The sum is 0.60 + 0.45 = 1.05. Because 1.05 is greater than 1.0, the combined exposure exceeds the PEL even though each substance is individually below its own limit. The employer must reduce exposure.
The 15-minute toluene result of 210 ppm is also relevant. It exceeds the 200 ppm 8-hour PEL as a short-term level and is far above the company's ACGIH-based target of 20 ppm TWA. The mixture calculation does not average away the peak; it shows that the 8-hour exposure is already over the limit before considering the peak. The correct response is engineering control, not a respirator-only fix.
2. Evaluate the glove selection and skin exposure. What should the employer do about dermal exposure?
Read the worked answer
Nitrile is a poor choice for sustained toluene contact; aromatic solvents permeate thin nitrile quickly. The employer must select gloves from manufacturer permeation data for the specific chemical, thickness, and task duration. For toluene, PVA or Viton may be appropriate; for acetone, confirm breakthrough times rather than assuming a generic "chemical glove" is adequate. Gloves must be changed at the breakthrough time, not when they feel wet.
Skin is a significant route for toluene and can cause dermatitis, which is consistent with the dry skin reported. Add a face shield if splashing is possible, provide washing facilities, and remove contaminated gloves without touching the inside. But PPE is the last line of defense. The real fix is to eliminate open manual wiping through substitution or enclosure, because glove performance is difficult to verify in a busy shop.
3. Recommend a control strategy and a follow-up sampling plan. Address the proposed N95 respirators.
Read the worked answer
The control strategy should follow the hierarchy. First, substitute an aqueous or less hazardous cleaner. Second, enclose the parts washer with a lid and add local exhaust ventilation at the source. Third, keep containers closed and improve general ventilation. Administrative controls include limiting the number of employees exposed and rotating tasks. Respiratory protection is interim only.
N95 respirators are not appropriate for organic vapors; they do not remove toluene or acetone. If respirators are used before engineering controls are working, the employer needs a written respiratory protection program with medical evaluation, fit testing, training, and organic vapor cartridges. The follow-up plan should repeat personal 8-hour TWA sampling after controls, add task-based 15-minute samples for manual wiping, sample the worst-case worker and shift, and use skin wipe samples if contact continues. Set the target below the ACGIH TLV for toluene (20 ppm), not merely below the PEL.