Use of the reciprocal calculation procedure for setting workplace emergency action levels for hydrocarbon mixtures and their relationship to lower explosive limits.

This paper proposes a novel use of the reciprocal calculation procedure (RCP) to calculate workplace emergency action levels (WEALs) for accidental releases of hydrocarbon mixtures. WEALs are defined here as the concentration in air that area monitors should alarm at to provide adequate warning and be sufficiently protective of health to allow at least enough time to don respiratory protective equipment (RPE) and escape. The rationale for the approach is analysed, and ways of defining suitable substance group guidance values (GVs) for input into the RCP are considered and compared. WEAL GVs could be based on: 3× RCP GVs (i.e. using the 3× rule), the 5× RCP GVs (i.e. using the 5× rule for calculating ceiling values), emergency exposure limits, or immediately dangerous to life or health values (IDLHs). Of these, the method of choice is to base WEAL GVs on health-based IDLH values, which were developed for emergency situations in the workplace. However, IDLHs have only been set for 11 hydrocarbons, so the choice of GVs is also informed by comparison with possible GVs based on the other approaches. Using the proposed GVs, WEALs were calculated for various hydrocarbon mixtures, and the way they vary with the composition of the mixture was examined. Also, the level of health protection given by the current practice of setting emergency area alarms in the oil and gas industry at 10% of the lower explosive limit (LEL) was tested by comparing this with the WEAL. In the event of an accidental release, this comparison suggests that, provided that aromatics constitute <50% of the mixture, an alarm set at 10% LEL should provide adequate warning and be sufficiently protective of health to at allow at least enough time to don RPE and escape. In the absence of better information or specific acute toxicity concerns (such as the presence of hydrogen sulphide), it is proposed that the WEALs be used as a guide for assessing the adequacy of area alarm levels in respect of warning of an acute health risk. This work is exploratory (e.g. other rationales for setting GVs are possible) and the approach needs testing on further real-life samples. Although not explored here, the RCP approach may also lend itself to the calculation of in-house short-term exposure limits for hydrocarbon mixtures and other mixtures where the acute toxic end points of the components are similar.