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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
SUBSTANCE CHEMICAL FORMULA LTEL (8hr TWA) PPM STEL (15 minute TWA) PPM LTEL (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