citation:
Michaels, Robert A. Emergency planning:
critical evaluation of AEGLs for ammonia.
New York City, New York; American Institute of Chemical Engineers; AIChE Technical Manual: Ammonia Plant Safety, 38:179-84, 1998.
Emergency Planning: Critical Evaluation of AEGLs for Ammonia
National Advisory Committee’s Acute Exposure Guideline Levels (AEGLs) for ammonia are critically evaluated. The technical bases for concern about AEGL-2 and AEGL-3 values derived by the committee are summarized and some recommendations are made.
Robert A. Michaels RAM TRAC Corp., Schenectady, NY 12309
“During the 20th century, humanity has almost quadrupled its numbers. Although many factors have fostered this unprecedented expansion, its continuation| during the past generation would not have been at alll \possible without a widespread, yet generally unappreciated, activity: the synthesis of ammonia” (Smil, 1997).
Table 1. AEGL Values for Ammonia
t its June 9, 1997 meeting, after five meetings A« a year of deliberations about ammonia,
the National Advisory Committee on Acute Exposure Guideline Levels (NAC AEGL) decided to propose AEGL values for ammonia, which were lower than RAM TRAC recommendations (Michaels, 1997a,b; RAM TRAC, 1996a,b,c,d, 1997), as shown in Table 1. The values are generally too low by a factor of approximately 4.5, producing Emergency Planning Zone areas generally 20 times greater than necessary around facilities exceeding threshold quantities of ammonia on-site.
AMMONIA TECHNICAL MANUAL
Concentration (ppm-v) by Exposure Duration
AEGL parameter Smin 30min 1h 4h 8h
NAC AEGL Proposals 1 25 25 25 25 — 2. 380 160 110 110 110 3 3,800 | 1,600] 1,100[ 550 [| 390
i 2
RAM TRAC Recommendations 2 1,704 | 696 492 246 174 3 16,869] 6,887 | 4,870} 2,435) 1,722
The proposed values will be published in the Federal Register, and the Committee will then receive and formally respond to comments from members of the public. This article critically evaluates NAC AEGL’s actions, and summarizes the technical bases for concern about AEGL-2 and AEGL-3 values to be proposed by the Committee, as well as RAM TRAC
comments (RAM TRAC, 1996a,b.c, 1997) on the values. Although NAC AEGL may “finalize” the AEGL values despite the arguments summarized herein, further review will be undertaken by an independent entity. namely, the Committee on Toxicology of the U. S. National Research Council (NRC).
AEGL-3
To derive AEGL-3 values, the Committee properly used human data for range-finding, and bioassay data to flesh out the dose-response curve over the full eight-hour range of exposure durations of concern (National Research Council Committee on Toxicology, 1993). However, the Committee applied this proper approach in an improper manner. For range-finding, the Committee gave undue credence to Henderson and Haggard (1927, 1943), which cites no source of its reported lethality concentration of 5,000 to 10,000 ppm-v with five to 10 minutes of exposure. This exposure range is apparently anecdotal, and apparently predates World War I (Flury, 1928; Lehmann, 1886; Ronzani, 1909), when ammonia was marketed to military buyers interested in its high toxic potency against enemy soldiers engaged in trench warfare. Whereas such “data” should have been accorded a low threshold for refutation, the Committee instead adhered to this lethality exposure range, even when presented with reconstructions of several relatively recent industrial accidents involving ammonia release (Environ, 1997; Mudan and Mitchell, 1996; Mulder and Van Der Zalm, 1967; Pederson and Selig, 1989), each €0nflicting with the pre-World War I “data” (Figure 1). A further reason for rejecting the old lethality exposure range is its lack of concordance with bioassays involving five animal species, producing LCsq and LC, values scaled to human equivalent concentrations (HECs). These lethality values are, however, in accordance with the accident reconstruction data (Figure 2). In rejecting accident reconstructions which rely upon air modeling, the Committee improperly overemphasized uncertainties inherent in air modeling (Mazzola, 1997; ORNL, 1997), even though the Risk Management Program (RMP) being implemented by the Committee depends critically and intrinsically upon air modeling. Moreover, the Committee failed to
AMMONIA TECHNICAL MANUAL
understand that meta-analysis of multiple accidents can dramatically reduce uncertainties inherent in modeling single accidents, as should be evident from Figure |.
The flawed rationale for preferring anecdotal lethality data predating World War I over a wealth of accident reconstruction data for range-finding provided the Committee with a basis. spuriously, for deriving AEGL-3 values using mouse data (Kepeghian et al., 1982). Although citing Ten Berge et al. (1986), the Committee ignored the authors’ caveat: “Mice are often more sensitive than other mammals...experiments using mice do not provide an appropriate basis for predicting quantitatively the mortality response in humans.” The Committee’s assumption that humans are as sensitive as mice to ammonia was applied to the Potchefstroom accident, in which 18 people died. Under the Committee’s assumption, at least 180 people should have died (RAM TRAC, 1996). Thus, human experience refuted the Committee’s use of the mouse data. Rat data are more relevant, as acknowledged by NRC’s Committee on Toxicology (National Research Council Committee on Toxicology, 1993), and rats (Appelman et al., 1982) were three times more tolerant of ammonia than mice (Kapeghian et al., 1982), as acknowledged by ORNL. Even using mice, however, the Committee had difficulty deriving AEGL-3 values below concentrations assumed to be lethal for humans. Upwardly scaling ammonia concentrations to which mice were exposed to HEC values produced AEGL-3 values too high relative to the preWorld War.I data selected for lethality range-finding. Consequently, the Committee declined to do HEC scaling, although guidelines of NRC (National Research Council Committee on Toxicology, 1993) and EPA (U.S. EPA, 1994) require it. The resulting values were still so high that their further reduction by a safety factor of three was deemed necessary to protect “sensitive subpopulations” not already protected by mouse LC), (one-per-thousand lethality) values, which are already very protective. Finally, the Committee claimed that it could produce similar AEGL-3 values using rat data (Appelman et al., 1982), but required an interspecies safety factor of three (ORNL, 1997), whereas no such factor is justified given the accident reconstruction data (see Figures |
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and 2). Resolution of AEGL-3 differences
Resolution of the 4.4-fold difference between RAM TRAC and NAC AEGL recommendations can be achieved via two steps, restoring consistency of Committee procedures with those specified by the EPA (U.S. EPA, 1994) and NRC’s Committee on Toxicology (National Research Council Committee on Toxicology, 1993). First, HEC adjustment of mouse (Kapeghian et al., 1982) or rat (Appelman et al., 1982) LC), data (not to be confused with means of multiple LCsq and LC, bioassay data depicted in Figure 2) to HEC values would multiply AEGL-3 values to be proposed by the Committee by a factor of 2.7. If only this adjustment were implemented, this would bring AEGL-3 values to within two-thirds of RAM TRAC recommendations, which were derived based upon halving the five-minute zero-mortality level of 33,737 ppm-yv in the Potchefstroom accident reconstruction. This leaves a difference of approximately 1.6-fold.
Virtually all of this remaining 1.6-fold difference can be eliminated by a second step, that is, adopting the Appelman et al. rat LC), data (Appelman et al., 1982; approximately mouse LC), data times three) with an interspecies uncertainty factor of two rather than three proposed by NAC AEGL. This produces an approximate 1.5-fold increase in NAC AEGL proposed values. This adjustment is technically justified based upon accident reconstruction data, which suggest higher human tolerance to ammonia than assumed by the Committee based upon pre-World War I data. Accident reconstruction data indicate the absence of any need to apply an interspecies uncertainty factor relative to rat LC), data (not relative to LCs) and LC, o data depicted in Figure 2). The resulting harmony of AEGL-3 values with RAM TRAC recommendations would decrease the area of Emergency Planning Zones around ammonia facilities by a factor of 20-fold, with commensurate savings in community emergency planning costs and reductions in the difficulty of permitting or repermitting facilities exceeding threshold quantities of ammonia on-site.
90000 iN \ \ ag00 F-4- = \ » =-O- = Farmland accident \ \ |—-O— mean of four accident \ reconstructions
=-o-- rocttsroom nck 1-0 3)
|—-@ -~Mulder and Van der Zalm (1967)
‘ammonia concentration (ppm-v)
Semin, Bh
3o-min th ‘exposure duration
0,000 ¢§$$ $$$ 0
|. Gecident reconstructions)...
80,000
:
H
= & 3 5 e000 a i
ve on
exposure duration .
Figure 1. Lethality range-finding based upon accident reconstruction vs. anecdotal data predating World War I.
AMMONIA TECHNICAL MANUAL
Figure 2. Lethality range-finding via bioassays of five species, reconstruction of four industrial accidents, and anecdotal data predating World War I.
AEGL-2
With respect to irreversible injury and inability to escape from released ammonia (AEGL-2), the Committee deviated from its defined criteria, deriving values based upon volunteers’ perception that concentrations to which they were exposed were “intolerable” (ORNL, 1997; Verberk, 1977). However, concentrations perceived in the context of voluntary exposure as “intolerable” caused neither injury (not even minor injury) nor incapacitation (Verberk, 1977), and were greatly exceeded in another study of volunteers, also with neither injury nor incapacitation (Silverman et al., 1949).
The proposed AEGL-2 is 160 ppm-v at 30 min and 110 ppm-v thereafter based upon the need to preserve the ability to escape. The 30-min concentration should not be reduced, however, for longer-term exposures of 1h, 4h, and 8 h. These values should remain at 160 ppm-v, consistent with NAC AEGL policy. Thirty minutes is a traditional period assumed for escape from chemical emergencies, and is embodied in the Immediately Dangerous to Life and Health (IDLH) parameter of the National Institute of Occupational and Industrial Health (NIOSH). Moreover, given the extreme volatility and fugacity of ammonia, ammonia releases can be expected to have completely dissipated within 30 min following a release (for example, the preponderance of ammonia dissipated within 10 min at Potchefstroom).
Thus, the generally reasonable 30-minute escapetime assumption is also specifically reasonable, if not more than reasonable, in the case of ammonia. As ORNL (ORNL, 1997) indicated, the “immediate response to individuals exposed to severe irritating concentrations of ammonia is to escape from the area” (August draft, p. 33; emphasis added). Thus, if 160 ppm-v is regarded as a concentration that would permit escape within 30 min, it should also permit escape within | h, 4 h, or 8 h, and under the reasonable assumption that no more than 30 min would be required for escape (of both people and ammonia).
Accordingly, at the suggestion of the ammonia chemical manager (at the September 17, 1996 meeting; Michaels, 1997; RAM TRAC, 1996), the Committee decided to maintain AEGL-2 levels con-
AMMONIA TECHNICAL MANUAL
stant beyond 30 min, except when toxic effects beyond 30 min could be demonstrated. However, in setting AEGL-2 values for ammonia at the June-1997 meeting, the Committee denied having established this policy. Yet, during discussion of a constant value beyond 30 min, the Chair began to indicate that “this would be consistent with...” and then stopped himself, apparently from referring to the Committee's prior policy. recalling its current denial that the prior policy existed. However, all Committee actions eventually can be checked, because under rules of the U. S. Federal Advisory Committee Act (FACA), tapes of Committee deliberations are public documents. Such checking has just now become possible, because previously denied requests under the Freedom of Information Act for transcripts of earlier NAC AEGL proceedings finally have been produced in accordance
with the law.
Resolution of AEGL-2 differences
Resolution of differences between RAM TRAC and NAC AEGL recommendations should begin with Committee adherence to its definition of AEGL-2. AEGL-2 values should be derived to protect against irreversible injury or incapacity to escape (National Research Council Committee on Toxicology, 1993; ORNL, 1997). Further, the Committee should adhere to its policy of maintaining AEGL-2 values constant beyond 30 min, except if necessary to reduce such values to protect against toxic effects associated with prolonged exposure. This step would bring AEGL-2 values to 160 ppm-v at exposure durations of 30 min or longer. A third step would be for the Committee to adopt the 30-min value of 500 ppm-v (Silverman et al., 1949), which produced neither injury nor incapacity to escape among volunteers exposed to it. This would bring the Committee into the range of RAM TRAC recommendations (500 ppm-v vs. 696 ppm-v at 30 min). If the Committee maintained 500 ppm-v at | h, 4 h, and 8 h, then its values would exceed RAM TRAC recommendations. Thus, complete harmonization at the I-h, 4-h, and 8-h exposure durations consistent with Committee definitions and policies should be possible; whereas, harmonization at the 5- and 30-min
durations would be substantial.
182 1998
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AMMONIA TECHNICAL MANUAL
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Publication citation
Michaels, RA. Emergency planning: critical evaluation of AEGLs for ammonia. New York City, American Institute of Chemical Engineers; AIChE Technical Manual: Ammonia Plant Safety, 38:179-84, 1998.