Engineering Controls, Ventilation, and Regulatory Exposure Limits

Why Solvent Vapor Control Matters

Dry cleaning and laundry operations use powerful solvents—most commonly perchloroethylene (PERC) and petroleum-based hydrocarbon solvents—to remove soils that water alone cannot. These chemicals do their job well, but airborne vapor poses serious health risks: short-term dizziness and nausea, and with repeated long-term exposure, potential organ damage and, for PERC, recognized carcinogenicity. Controlling vapor concentration in the breathing zone of workers is therefore a legal requirement and a professional responsibility.

The Hierarchy of Controls

OSHA and the National Institute for Occupational Safety and Health (NIOSH) recognize a ranked system for managing workplace hazards called the hierarchy of controls. You always try higher-level controls first because they protect more workers more reliably than lower-level controls.

1. Elimination – physically remove the hazard entirely (e.g., stop using PERC and convert to a non-solvent process such as water-only cleaning).

2. Substitution – replace the hazardous material with a less hazardous one (e.g., switch from PERC to a hydrocarbon or silicone-based solvent).

3. Engineering controls – use physical barriers, enclosures, ventilation, or equipment design to prevent vapor from reaching workers (e.g., closed-loop dry cleaning machines, local exhaust ventilation, makeup-air systems).

4. Administrative controls – change work practices or schedules to reduce exposure duration or frequency (e.g., rotating job assignments, scheduling high-exposure tasks when fewer workers are present, posting warning signs).

5. Personal protective equipment (PPE) – provide respirators, gloves, and eye protection as a last line of defense when other controls alone are not sufficient.

PPE is the least reliable tier because it depends on correct fit, consistent use, and maintenance. Engineering controls are the professional standard for a permanent fix.

Engineering Controls in a Dry Cleaning Shop

Closed-loop machines are the single most important engineering control for PERC facilities. In a closed-loop (or "closed-loop dry-to-dry") machine, the garment load is dried inside the same sealed drum used for washing. Solvent vapor is recovered by refrigerated condensation and carbon adsorption before the door is opened, drastically reducing fugitive emissions.

Local exhaust ventilation (LEV) captures vapor at the source—right at the machine door, the still, the muck cooker, and the separator—before it can disperse into the room. LEV uses a hood, ductwork, and a fan to pull contaminated air outside or through a filter.

General dilution ventilation (sometimes called makeup-air ventilation) brings fresh outdoor air into the work area. It does not eliminate vapor at the source, but it dilutes whatever vapor is present. EPA guidance for PERC facilities and DLI technical bulletins specify that makeup-air inlets should be positioned to move air across the work area away from the worker's breathing zone and toward the exhaust point, not toward the worker.

Worker positioning matters even when ventilation is well-designed. Workers should stand where cross-ventilation moves air away from them. Avoid standing between a vapor source and an open door or window that could draw contaminated air across your face.

Regulatory Exposure Limits

Two sets of limits govern daily vapor exposure:

• OSHA Permissible Exposure Limit (PEL): A legally enforceable maximum. For PERC, the OSHA PEL is expressed as a time-weighted average (TWA) over an 8-hour shift. Always consult the current OSHA reference table because regulatory values are periodically reviewed.
• ACGIH Threshold Limit Value (TLV): Issued by the American Conference of Governmental Industrial Hygienists. The TLV is a professional recommendation, not a legal limit, and is typically more health-protective than the OSHA PEL. ACGIH also publishes short-term exposure limit (STEL) values for peak exposures lasting 15 minutes or less.

For hydrocarbon solvents, both OSHA and ACGIH publish separate values. Always match the limit to the specific solvent in use—do not mix limits between chemicals.

Direct-Reading Instruments

A photoionization detector (PID) uses an ultraviolet lamp to ionize gas molecules in a sample. The resulting electrical current is proportional to vapor concentration and is displayed in parts per million (ppm). PIDs give real-time readings and are useful for quickly finding high-concentration zones or confirming that ventilation is working. They must be calibrated to the specific solvent being measured, because the instrument response factor varies by chemical.

A colorimetric tube pump draws a fixed volume of air through a sealed glass tube containing a chemical reagent. The reagent changes color, and the length of the color stain indicates concentration. Tubes are solvent-specific and single-use. They are lower in cost than PIDs but provide a reading only for the sampling period, not in real time.

How to read a PID: Turn on the unit, allow the warm-up period, zero the instrument in clean air, select the correct response factor for the solvent, hold the probe near the source or breathing zone, and read the stabilized ppm value on the display. Record the reading, note the time and location, and compare the result to the published PEL and TLV.

Deciding Whether Controls Are Adequate

If your PID reading is below the TLV, conditions are generally acceptable. If the reading is above the TLV but below the PEL, additional engineering or administrative controls are strongly recommended. If the reading is at or above the PEL, work in that area must stop until engineering controls reduce vapor to a safe level; respirators may be used temporarily while repairs are made, but they are not a permanent solution.

Safety Reminders for This Demonstration

• The instructor operates the PID. Students observe and record; they do not handle solvents.
• Safety Data Sheets (SDS) for all solvents present must be accessible in the shop.
• If the PID alarm sounds during the demonstration, follow the facility's emergency action plan: stop work, move to fresh air, and alert the instructor immediately.
• Wear nitrile gloves and safety glasses whenever you are near open solvent containers, even in a training simulation.