Exposure Limits: PEL, REL & TLV
Exposure limits tell you how much of a chemical, dust or noise is considered acceptable in the air you breathe or the sound you hear. Several organizations set limits, and they don’t always agree. Knowing the alphabet soup helps you read SDSs and air monitoring results.
- All workers
- Supervisors
- Safety staff
- California
Who sets the limits
| Limit | Set by | Legally enforceable? |
|---|---|---|
| PEL (Permissible Exposure Limit) | OSHA | Yes |
| REL (Recommended Exposure Limit) | NIOSH | No, but based on current science |
| TLV (Threshold Limit Value) | ACGIH (a professional organization) | No, but widely used |
Most OSHA PELs were adopted around 1970 and haven’t been updated. Many are higher than current RELs and TLVs. OSHA itself recommends that employers consider the lower limits. Some state plans, like California, have stricter limits.
Kinds of limits
- TWA (time-weighted average): the average exposure over a work shift, usually 8 hours.
- STEL (short-term exposure limit): the average over 15 minutes, which shouldn’t be exceeded even if the 8-hour average is fine.
- Ceiling (C): a level that should never be exceeded at any time.
- Action level: in several OSHA standards (like lead, silica and noise), a lower level that triggers monitoring, medical checks or training.
- IDLH (immediately dangerous to life or health): a level where escape could be impossible or serious harm could happen quickly; only supplied air or SCBA is acceptable.
- “Skin” notation: the chemical can be absorbed through the skin, adding to what you breathe in.
Examples from OSHA standards
| Hazard | Action level | PEL (8-hour TWA) |
|---|---|---|
| Respirable crystalline silica | 25 µg/m³ | 50 µg/m³ |
| Lead (construction) | 30 µg/m³ | 50 µg/m³ |
| Noise (general industry) | 85 dBA | 90 dBA |
Units vary: ppm (parts per million) for gases and vapors, mg/m³ or µg/m³ for dusts and fumes, and fibers per cc for asbestos.
Where to find them
- Section 8 of the Safety Data Sheet. See Safety Data Sheets
- The NIOSH Pocket Guide to Chemical Hazards (free online and as an app)
- OSHA’s air contaminant tables and specific standards
Below the limit doesn’t mean zero risk. Limits are set to protect most workers, not everyone, and some don’t reflect the latest science. Keep exposure as low as you reasonably can. See Hierarchy of Controls.
Standards & references
- OSHA: Annotated PEL tables
- NIOSH Pocket Guide to Chemical Hazards
- 29 CFR 1910.1000: Air contaminants
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