# Osmium tetroxide — workplace exposure limit

For Osmium tetroxide, niosh rel (ppm) is 0.0002; niosh rel (mg/m³) is TWA 0.002; cas no. is 20816-12-0, verified against its source on 2026-09-08.

- **Substance:** Osmium tetroxide _(verified: appears in its own passage below)_
- **CAS No.:** 20816-12-0 _(verified: appears in its own passage below)_
- **NIOSH REL (ppm):** 0.0002 _(verified: appears in the quote below)_
- **NIOSH REL (mg/m³):** TWA 0.002 _(verified: appears in the quote below)_
- **PEL (mg/m³):** 0.002 _(per osha.gov, not stated by the source above)_
- **PEL (ppm):** 0.0002 _(per dir.ca.gov, not stated by the source above)_
- **STEL (ppm):** 0.0006 _(per dir.ca.gov, not stated by the source above)_
- **STEL (mg/m³):** 0.006 _(per dir.ca.gov, not stated by the source above)_

## What the source says

> NIOSH REL TWA 0.002 mg/m3 (0.0002 ppm) ST 0.006 mg/m3 (0.0006 ppm)

## Where each value comes from

This source states these in separate places, so each value is shown with the passage that states it.

### Substance

> CDC - NIOSH Pocket Guide to Chemical Hazards - Osmium tetroxide

### CAS No.

> CAS No. 20816-12-0

## What osha.gov says

> Osmium tetroxide (as Os) | 20816-12-0 | - | 0.002 | - |

Source: https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1000TableZ1

## What dir.ca.gov says

> 20816120 | | Osmium tetroxide, as Os | 0.0002 | 0.002 | | 0.0006 | 0.006

Source: https://www.dir.ca.gov/title8/5155.html

## Source

- https://www.cdc.gov/niosh/npg/npgd0473.html
- https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1000TableZ1
- https://www.dir.ca.gov/title8/5155.html

Last verified: 2026-09-08. Review by: 2027-08-05.
Part of [US workplace chemical exposure limits: federal OSHA, Cal/OSHA and NIOSH, side by side](https://referencesource.org/workplace-exposure-limits/).