Alarm Levels
It is important to note that whereas portable gas detection instruments measure and alarm at the TWA levels, instantaneous alarms set at the same numerical values of the 8hr TWA 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.
The following data has been extracted from EH40 and OSHA (see below) for some common toxic gases:
Workplace Exposure Limits:
Workplace Exposure Limits (taken from EH40/2005) | OSHA- PELs | |||
Substance | Chemical Formula | LTEL (8hr TWA) ppm | ppm | LTEL (8hr TWA) ppm |
Ammonia | NH3 | 25 | 35 | 50 |
Arsine | 0.05 | - | 0.05 | |
Bromine | Br2 | 0.1 | 0.3 | 0.1 |
Carbon Dioxide | CO2 | 5000 | 15000 | 5000 |
Carbon Monoxide | CO | 30 | 200 | 50 |
Chlorine | Cl2 | 0.5 | 0.5 | 1 |
Chlorine Dioxide | ClO2 | 0.1 | 0.3 | 0.1 |
Diborane | B2H6 | 0.1 | - | 0.1 |
Ethylene Oxide ETO | C2H4O | 5 | - | 1 |
Fluorine | F2 | 1 | 1 | 0.1 |
Germane | GeH4 | 0.2 | 0.6 | - |
Hydrogen Cyanide | - | 10 | 10 | |
Hydrogen Chloride | HCl | 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 | 5 - See Note 1 | 15 - See Note 1 | 25 |
Nitrogen Dioxide | NO2 | See Note 2 | See Note 2 | 5 ceiling* |
Ozone | O3 | - | 0.1 | 0.1 |
Phosgene | COCl2 | 0.02 | 0.06 | 0.1 |
Phosphine | PH3 | - | 0.3 | 0.3 |
Silane | SiH4 | 0.5 | 1 | - |
SO2 | See Note 2 | See Note 2 | 5 | |
Note 1: NO alarm levels currently advised in the UK are based on those defined for tunnelling operations.
Note 2: NO2 and SO2 TWA levels are currently subject to temporary Chemical Hazard Alert Notices (CHAN’s) issued by the UK Health and Safety Executive.
USA - Occupational Safety and Health Administration (OSHA):
In general, PELs refer to substances that may be inhaled, although some can be absorbed through the skin or eyes. The employer is duty bound to protect the work place and the requirements are very similar to those in the UK as listed above.
Threshold Limit Values (TLVs) are exposure guidelines developed by the American Conference of Governmental Industrial Hygienists (ACGIH). They are not legally enforceable, but because they are updated regularly, they represent good professional practice. They are expressed as follows:-
TLV-TWA- Time-Weighted Average: The time-weighted average concentration for a normal 8 hour work day and a 40 hour work week, to which nearly all workers may be repeatedly exposed, day after day, without adverse effect.
TLV-STEL - Short Term Exposure Limit: a 15 minute time-weighted average exposure which should not be exceeded at any time during a work day even if the 8 hr TWA is within the TLV. Exposures at the STEL should not be repeated more than 4 times a day and there should be at least 60 minutes between successive exposures at the STEL.
*TLV-C- Ceiling: the concentration that should not be exceeded during any part of the working exposure.