Yesterday 4 employees at the LaPorte, TX DuPont plant died, apparently as a result of exposure to methyl mercaptan (CH3SH, AKA ethanethiol, or MeSH); another employee was taken to the hospital for exposure. News reports (here and here) indicate that they were responding to a leak due to a malfunctioning valve. The Chemical Safety Board is deploying a team to initiate an investigation.
Methyl Mercaptan Standards
MeSH is a toxic chemical that is widely used to ‘odorize’ natural gas and propane. It has a strong, easily detectable odor that can be added to odorless flammable gasses to make leaks more easily detectable. It is this very strong odor that makes it a ‘friendly’ toxic gas. With a human detection limit somewhere in the neighborhood of 1 ppb, very small amounts of this material are very easily detectable.
There are some conflicts between the OSHA and NIOSH short term exposure limits for this chemical (10 ppm and 0.5 ppm respectively), but all government sources agree with the current IDLH (Immediately Dangerous to Life and Health) limit of 150 ppm. What is not readily available is the level at which the human gag reflex makes it uncomfortable to be around the odor; almost certainly below 10 ppm. Thus anyone approaching a leak site without personal protective equipment (PPE) is almost certain to turn around and leave the area well before the 150 ppm IDLH is reached.
Avoiding Deadly MeSH Concentrations
Which leaves the question as to how someone could enter an area with lethal concentrations of MeSH? One way (speculation on my part, the CSB will determine the actual root cause) would be for the responders to be wearing cartridge respirators (the most common kind of respirator used in the chemical industry). Those cartridges are typically activated charcoal. According to a 2002 article by Svetlana Bashkova, et. al., the three types of adsorbents that they investigated all had break through times greater than 100 minutes. This should be more than long enough for an initial look at the situation to determine if an easy fix (close the valve, for instance) was possible.
Of course, if anyone knew or suspected that the concentration of MeSH was 150 ppm or greater, the use of a cartridge respirator should not have been authorized; any respirator failure would have placed the wearer in a potentially deadly situation. A supplied air respirator should be required in those circumstances.
I have long advocated in this blog that any CFATS covered facility include toxic gas detectors at any site with a toxic release hazard DHS chemical of interest (COI) on site. These detectors would be deployed as an array of gas detectors on (and off) site as part of their immediate response plan for their site security plan. This would allow responders to know where the toxic gas cloud was heading and determine what level of protection would be needed for security and safety responders.
EPA vs DHS Regulations
Looking at this incident I ran across an interesting discrepancy between how EPA and DHS look at methyl mercaptan. The EPA lists methyl mercaptan as a toxic chemical under their Chemical Accident Protection regulations (and that include the Risk Management Plan regulations), but DHS lists it as a flammable release COI under the CFATS program. It is certainly a flammable liquid (flammable gas at temperatures above 45°F) and apparently DHS figures that the risk from an explosion of a methyl mercaptan storage tank attack is greater than the risk from a toxic release from the same successful attack. EPA, on the other hand, is more concerned with the toxic release.
As DHS looks at updating their COI list (Appendix A to 6 CFR Part 27) they might want to reconsider the either/or dichotomy of this and other flammable toxic chemicals and list them under both hazards. This would ensure that chemical security efforts deal with both concerns. Similarly, EPA should consider also listing the flammability characteristics in 40 CFR Part 68 so that safety efforts are formally required to be targeted at both characteristics.
We should start to hear some informal information from CSB and the other investigating bodies in the weeks to come. The formal determination of the root cause of the deaths will, unfortunately be months or years in coming.