Plastics and composites: styrene has three limits, not one

Styrene is one of the few substances where a clean 8-hour time-weighted average (TWA) tells you almost nothing about whether you are in compliance, because OSHA's limit is not only an average.

Styrene has three limits, and that changes the survey

Most people quote OSHA's permissible exposure limit (PEL) for styrene as 100 parts per million (ppm) and stop there. The full entry in OSHA's Table Z-2 is:

• 100 ppm as an 8-hour TWA
• 200 ppm as an acceptable ceiling concentration
• 600 ppm as the acceptable maximum peak, for 5 minutes in any 3 hours

A full-shift TWA cannot demonstrate compliance with a ceiling or with a 5-minute peak allowance. An open-mold shop with a comfortable 45 ppm TWA has answered one of three questions.

The health-based limits are lower still. ACGIH, the professional body that publishes the threshold limit values (TLVs), sets a 10 ppm TWA with a 20 ppm short-term exposure limit (STEL). That is a tenth of the OSHA TWA. In 2019 the World Health Organization's International Agency for Research on Cancer (IARC) placed styrene in Group 2A, its category for agents that are probably carcinogenic to humans.

Styrene is also ototoxic, meaning it can damage hearing. Solvent-related hearing loss is a recognized health effect, and styrene combined with noise does more harm than either one alone. A composites shop that keeps its hearing conservation program and its styrene program as unrelated files is missing the connection between the two. That is a reason to run the noise survey and the air monitoring survey in the same visit.

→ Solvent vapor air monitoring · Noise surveys

Where the styrene exposure actually is

The common mistake is to sample the laminator at an open bench and treat that result as representative of the whole shop.

The geometry of the work determines exposure, not the process. Rolling out resin inside a boat hull, a tank or a deep mold traps vapor around the worker's head, and the same task on a flat panel does not. Gel coat spray is typically the highest short-term exposure in the building, and mold cleanup, trimming green laminate and acetone wipe-down all add to the same shift.

The catalyst, methyl ethyl ketone peroxide (MEKP), has no OSHA PEL. ACGIH sets a 0.2 ppm ceiling limit, a level that must not be exceeded at any time. The more urgent hazard is MEKP's reactivity. MEKP and cobalt accelerator must never be mixed directly. The reaction is violent, and it has started fires in resin rooms that were otherwise well run. If your shop pre-promotes its resin (adds accelerator ahead of time), that mixing order needs to be in a written procedure, not in one person's memory.

Polyurethane, foam and spray foam

Two-part polyurethane brings MDI (methylene bisphenyl isocyanate), one of the isocyanates. OSHA sets a 0.02 ppm ceiling limit, and ACGIH sets a TLV of 0.005 ppm as a TWA.

High-pressure spray polyurethane foam (SPF) is the difficult case, because it generates both vapor and a fine mist. A sampling method that captures only vapor misses the isocyanate in the mist entirely. Spray foam also raises the re-entry question: how long after spraying can unprotected people be in the space, and what evidence supports that answer?

Isocyanate sampling requires derivatizing media, which is a specially treated filter. The wrong filter does not give you a wrong number that looks wrong. It gives you a clean number from a dirty room.

→ Isocyanate air sampling

Thermal decomposition: hot plastic makes chemicals of its own

Extrusion, thermoforming, hot-wire cutting, trim sealing and any hot knife or torch work on plastic put new chemicals in the air. They are not on the raw material's safety data sheet, because they did not exist until the plastic was heated. This is thermal decomposition, and it needs a hazard inventory of its own.

• Polyvinyl chloride (PVC) releases hydrogen chloride, which has a 5 ppm OSHA ceiling limit. This is the classic case of an acid exposure in a plant with no acids on site, and the usual sources are hot-wire cutting and shrink-sealing of PVC-coated material.
• Acrylonitrile butadiene styrene (ABS) releases styrene, acrylonitrile and butadiene. Acrylonitrile has an OSHA standard of its own, with a 2 ppm TWA and a 10 ppm ceiling limit over 15 minutes.
• Polyethylene and polypropylene release aldehydes, including acrolein and formaldehyde.
• Nylon releases caprolactam. Heated fluoropolymers cause an illness known as polymer fume fever.
• Hot-wire cutting of expanded polystyrene foam (EPS) releases styrene, in a shop that thought it had no chemical hazard at all.

Purging, start-up, screw pulls and die changes generate far more of these decomposition products than steady running does. If you sample only during a stable run, the problem stays hidden.

→ Acid mist air monitoring · Formaldehyde air monitoring

Finishing, dust and the rest

Grinding, routing and trimming cured laminate generate dust, and that dust is not harmless. For continuous filament glass fiber, ACGIH's TLV has two parts: 1 fiber per cubic centimeter of air (fiber/cc), plus 5 milligrams per cubic meter of air (mg/m³) of inhalable dust. Machined carbon fiber is electrically conductive, which makes it a problem for equipment as well as for lungs.

Pneumatic trim tools and routers easily put most finishing areas above 85 decibels (dBA), the action level in OSHA's noise standard.

On the reporting side, styrene, methanol and glycol ethers are all listed for Form R, the annual Toxics Release Inventory (TRI) report to EPA. Resin used in laminating normally counts as "otherwise used" for that report, which puts the reporting threshold at 10,000 pounds, not 25,000.

We help with ISO 14001, the international standard for environmental management systems, and we also handle the Form R calculations and filing.

→ Form R and TRI reporting · ISO 14001 consulting