Toxic gas exposure limits and alarm levels

Alarm Levels

It is important to note that whereas portable gas detection instruments measure and alarm at the TWA levels, instantaneous alarms are included to provide early warning of an exposure to dangerous gas concentrations. Workers are often under risk of gas exposure in situations where atmospheres cannot be controlled, such as in confined space entry applications where alarming at TWA values would be inappropriate.

You must perform your own risk assessment to ensure alarms are set to appropriate levels for your application and in accordance with local legislation and practices.

The following data has been extracted from EH40 and OSHA  for some common toxic gases:

Workplace Exposure Limits:

WORKPLACE EXPOSURE LIMITS
(EH40/2005)
OSHA-PELs
SUBSTANCECHEMICAL FORMULALTEL (8hr TWA) PPMSTEL (15 minute TWA) PPMLTEL (8hr TWA) PPM
Ammonia NH3 25 35 50
Arsine AsH3 0.05 – 0.05
Bromine Br2 0.1 0.2 0.1
Carbon Dioxide CO2 5000 15000 5000
Carbon Monoxide CO 20 100 50
Chlorine Cl2 – 0.5 1 ceiling *
Chlorine Dioxide ClO2 0.1 0.3 0.1
Diborane B2H6 0.1 – 0.1
Ethylene Oxide ETO C2H40 5 – 1
Fluorine F2 1 1 0.1
Germane GeH4 0.2 0.6 –
Hydrogen Cyanide HCN 0.9 4.5 10
Hydrogen Chloride HCI 1 5 5 ceiling*
Hydrogen Fluoride HF 1.8 3 3
Hydrogen Sulphide H2S 5 10 20 ceiling*
Nitrous Oxide N2O 100 – –
Nitric Oxide NO 2 – 25
Nitrogen Dioxide NO2 0.5 1 5 ceiling*
Ozone O3 – 0.2 0.1
Phosgene COCl2 0.02 0.06 0.1
Phosphine PH3 0.1 0.2 0.3
Silane SiH4 0.5 1 –
Sulphur Dioxide SO2 0.5 1 5

 


 

*C – Ceiling Limit

References:

OSHA (Occupational Safety and Health Administration) Permissible Exposure Limits

https://www.osha.gov/dsg/annotated-pels/index.html

EH40/2005 Workplace exposure limits (Third Edition, published 2018)

http://www.hse.gov.uk/pubns/priced/eh40.pdf