Ask Dr. ALOHA: Working with Toxic and Odor Thresholds, Step 3
Estimating the "Phone Call Zone" (Fictional Example)
Jim has not finished his work with ALOHA for this scenario. He knows that ethyl mercaptan has a very strong, unpleasant odor and that people may associate it with a gas leak. He also knows that during previous incidents, people have become very concerned when they smelled a spilled chemical. Jim would like to estimate the area where people might smell ethyl mercaptan vapors—even when chemical concentrations are well below toxic levels. Within this "phone call zone," people may call emergency services for information, report to local hospitals, or—in this case—call to report a gas leak. Jim wants to account for this possibility in his decision-making.
An odor threshold can be useful in emergency planning and response as a rough estimate of the phone call zone. A chemical's odor threshold is the lowest concentration of that chemical in air that people can smell. Ethyl mercaptan has a very low odor threshold, so most people will smell it even at very low concentrations. Jim realizes that odor thresholds are imprecise measurements (because people naturally vary widely in their sensitivity to odors), but he just wants to get an approximate estimate of the size of the phone call zone.
To do this, Jim will use ethyl mercaptan's estimated odor threshold as his toxic Level of Concern (LOC) in ALOHA. Odor thresholds are not listed in ALOHA, so he reviews the CAMEO Chemicals datasheet for ethyl mercaptan again. Jim notices that there are two odor thresholds listed:
- Near the top of the CAMEO Chemicals datasheet, the three-letter USCG CHRIS Code links off to a PDF of the ethyl mercaptan datasheet from the U.S. Coast Guard CHRIS manual. Jim reviews the PDF and sees that 0.001 parts per million (ppm) is listed as the odor threshold of ethyl mercaptan.
- In the Physical Properties section of the CAMEO Chemicals datasheet, Jim sees that there is also a value (0.00014 ppm) listed as a Level of Distinct Odor Awareness (just below the AEGLs table).
Actually, since odor thresholds are so imprecise, researchers normally report them as ranges rather than exact numbers (check Table 1, below, to see that these ranges can be very wide for some chemicals). Jim decides to use each value as an LOC to model threat zones that would approximate the phone call zones for each odor threshold.
Because the information about the release has already been entered into ALOHA (step 1), Jim only needs to modify the LOCs and redisplay the threat zone estimate. To do this, he begins by selecting "Threat Zone" again from the Display menu. He chooses the "Toxic Area of Vapor Cloud" option, and then the Toxic Level of Concern dialog box appears again with the AEGL defaults selected. In the Yellow Threat Zone LOC pulldown list, Jim selects "User specified", types in 0.00014 in the box that appears, and then selects ppm units. In the Orange Threat Zone LOC pulldown list, he also selects "User specified", and then specifies 0.001 ppm. For the Red Threat Zone LOC, Jim chooses "none" in the pulldown list. After he clicks "OK", he sees the following threat zone estimate.
He sees that ALOHA predicts that the phone call zone for the CHRIS Manual value (orange zone) could extend for more than 2 miles downwind, and that the zone for the AEGL Level of Distinct Odor Awareness value (yellow zone) extends more than 6 miles downwind. (The sharp edge on the end of the yellow threat zone is because ALOHA will not estimate threat zones beyond 6 miles—for more information on this restriction, refer to the ALOHA FAQs.)
Note: In step 2, the red, orange, and yellow threat zones each represented a different level of hazard. For instance, for the toxic LOCs provided by ALOHA, the red threat zone is typically a life-threatening effects threshold, the orange threat zone is an escape-impairment threshold, and the yellow threat zone is typically a mild effects threshold. However, the threat zones generated here as "phone call zones" are different: In this estimate, both the orange and yellow zones represent the same mild effects threshold (in this case, an odor threshold). They are both representing the same hazard, but just showing how the results for that hazard vary based on the LOC values provided by the two different sources. Instead of overlaying the two values on a single picture, Jim could have run them separately and produced two pictures, each with a yellow threat zone representing the area where each source's odor threshold was exceeded at some time after the release began.
In the case of either odor threshold value, the threat zone prediction suggests to Jim that widespread public concern could develop in Oceanside City, if the vessel were to dock at the port. Again, he bears in mind the fact that these threat zones are just a ballpark estimate of the real area where people may detect ethyl mercaptan. People with especially sensitive noses may detect ethyl mercaptan's odor at distances farther from the vessel than the threat zones indicate. Others (with poorer senses of smell) may not detect the odor even if they are well within the areas covered by the threat zones. Still, Jim is glad to have at least a general idea of the extent of the area where public anxiety could be high.
Having obtained ballpark estimates of the potential area where a toxic threat could develop if the vessel were to enter port, as well as the potential area where people might smell the chemical and contact emergency services, Jim now feels better prepared to decide whether to allow the vessel to enter port.
|Chemical||U.S. Coast Guard CHRIS Manual||American Association of Railroads||American Industrial Hygiene Association|
References for Table 1:
- American Association of Railroads (AAR): The official AAR site. The AAR odor threshold values in the table above are from the AAR Bureau of Explosives' 1996 Emergency Action Guides. (Washington, DC: AAR.)
- American Industrial Hygiene Association (AIHA): The official AIHA site. The AIHA odor threshold values in the table above are from a list AIHA published in 1989 called Odor thresholds for chemicals with established occupational health standards. (Akron, OH: AIHA.)
- Chemical Hazards Response Information System (CHRIS) Manual: The CHRIS odor threshold values in the table above are from the June 1999 PDF excerpts from the U.S. Coast Guard CHRIS Manual that are available from the CAMEO Chemicals website.
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