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
