Appendix A Principles Of Gas Detection; Short Term Exposure Limit - MSA ORION Manual

Four-Head Gas Detector: MSA Orion
APPENDIX A:
A.1

Short Term Exposure Limit

The Short Term Exposure Limit (STEL) is the gas concentration an unprotected
worker can be continuously exposed to for 15 minutes without adverse health
effects.
This limit should not be repeated more than four times per working day of eight
hours. There should be a break of at least one hour between each exposure.
The equation below shows how the Orion calculates your short term exposure. To
calculate STEL:
Orion STEL reading =
Example 1: Assume the Orion has measured a H
minutes:
Orion STEL reading =
Compare this with the STEL for H
Substances Information System (HSIS) database. To work in this environment, this
worker must wear respiratory protection.
Example 2: Assume the Orion has measured a CO concentration of 15 ppm for 10
minutes and 10 ppm for 5 minutes:
Orion STEL reading =
Due to its persistence in the blood stream – with a biological half life of 3 to 4
hours – carbon monoxide doesn't have an established STEL. However the time
weighted average (TWA) published by HSIS is 30 ppm. This worker doesn't need
respiratory protection.
D.I. Recommended Practice
FILE: W:\NB\RP HMRU\Orion\Frame Ver 02\Appendix.fm
PRINCIPLES OF GAS DETECTION
(Exposure time in minutes) x (Exposure concentration in ppm)
(15 minutes) x (45 ppm)
= 45 ppm
(10 mins x 15 ppm) + (5 mins x 10 ppm)
= 13.3 ppm
NOTE
For carbon monoxide, Safe Work Australia recommends excursion
limits of:
• 100 ppm for a sum of 30 minutes exposure in an eight hour day
• 60 ppm for a sum of 60 minutes exposure in an eight hour day.
These excursion limits assume the worker will have no further
exposure to carbon monoxide. This is not a conventional STEL.
15 minutes
S concentration of 45 ppm for 15
2
15 minutes
S of 15 ppm published in the Hazardous
2
15 minutes
Version 02 Issued February 2011
© Copyright State of NSW (Fire and Rescue NSW) 2011
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