Assessment of Exposure-Response Functions for Rocket-Emission Toxicants (1998) / Chapter Skim
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Evaluation of the LATRA Exposure-Response Functions
Evaluation of the LATRA Exposure-Response Functions
Pages 45-66
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From page 45... ...
· What is the appropriate process for distinguishing among mild, moderate, and severe health effects in an exposure-response framework? · What is the optimal process for quantifying the relation between exposure concentration and health effect using existing toxicological information on the likely incidence, sensitivity of individuals, and severity of effect?
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From page 46... ...
at least for severe effects and mortality for HC} and NO2 (see Chapter 6~. Use of Me product of exposure concentration and tune as the ~ndependent variable allows estimation of the risks associated with exposures of different durations using a single ERF.
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From page 47... ...
That construct includes the implicit assumption that a safe exposure dose established by applying safety factors to animal or human toxicity data can be interpreted as a stochastic measurement. Because safety factors are used only to establish safe levels for healthy or sensitive populations, using safety factors to construct a stochastic exposure-response distribution goes beyond the intended use of those
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From page 48... ...
represents each inctividual as having a different dose at which he or she will exhibit the response. Thus, the subcommittee considered whether the sensitive populations and healthy populations should be represented by separate ERFs or should be considered members of a single population that can be represented by a single distribution ranging from the more sensitive to the more resistant individuals (as depicted in Figure tIA)
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From page 49... ...
The Air Force proposes to Include separate ERFs for healthy and sensitive populations so that the estimated number of people affected will be lower as the proportion of sensitive individuals in the area over which an exhaust plume is expected to pass decreases. However, with no ciata on how the slopes of the ERFs should differ between sensitive and normal subgroups, it is difficult to justify any particular relation between the ERF for sensitive individuals and the ERF for a healthy population beyond the relation between the concentrations associated with a threshold for response.
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From page 50... ...
By developing ERFs in value-based severity categories before adequate data are available, the Air Force is implicitly incorporating value judgments about severity into the ERFs in the absence of a documentable rationale. Moreover, if the Air Force were to change its valuation of the severity of different types of health effects because of a change in policy, the ERFs would have be recalculated.
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From page 51... ...
That is done in Figure 5-1, where three ERFs for moderate effects for NO2 are derived for healthy and sensitive populations under the assumption that the concentration at which moderate effects are seen (assumed to be 4 ppm for healthy adults and 2 ppm for sensitive populations for purposes of illustration only) correspond to the 0.~%, I%, or 5% lower bounds of a lognormal distribution and that We upper bound of the ERF (assumed to be 40 ppm for healthy adults and 10 ppm for sensitive populations for purposes of illustration only)
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From page 52... ...
52 ~ ~ E :0 / \~^ _ , EO ~ to ~~ 2 ~ AD ~ _ _ .C - to _ ~ E 2 E ° ~W = \ -lo CO 1- of ~ ~ ..... oooo~ CO ~o ...
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From page 53... ...
OTHER FEATURES OF LATRA The subcommittee notes that the method of estimating the potential for additive effects of simultaneous exposure to different compounds with similar modes of action In the LATRA mode} is likely to underestimate risks. The LATRA model simply estimates the joint probability of an effect from the independent probabilities; it does not allow for the possibility that Me compounds are dose additive (i.e., exposure to sub
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From page 54... ...
For the three rocket-emission toxicants reviewed In this report, it is the opinion of the subcommittee that the dose-additivity mode} would be appropriate. The subcommittee is concerned with the procedure of combining risk estunates from normal launch scenarios, multiplied by the probability of a normal launch, with risk estimates from accident scenarios, multiplied by the probability of an accident, to determine the expected risk from a launch event.
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Given that atmospheric models can at best only predict concentrations within a factor of 2 or 3 at a single location (Turner 1994) , the subcommittee is concerned about the reliability of the overall process with such models for estimating incidence of health effects in exposed populations.
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From page 56... ...
for each y.2 Setting g = ~ would result in a strict Haber's rule model. Haber's rule states that the biological effects of some types of toxicants tend to be related to He total cumulative exposure (area under the concentrationtime curve)
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The aggregate group of subjects at any particular exposure concentration and duration is cIassifled as giving evidence of a specific severity of response. A model, such as the logistic regression model, is applied using the severity code as the dependent variable and the exposure concentration and exposure duration as the independent variables.
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From page 58... ...
58 o ~ E To cat E In ~ o CL a)
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is simply a comparison of the estimated exposure concentration (EEC) to a NOEL or other reference toxicity
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. Me Air Force's comparison of its tier limits to the exposure concentrations estimated by REEDM is an example of the hazard-quotient model.
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A few recommendations on over features of LATRA also are provided. SELECTION OF THE INDEPENDENT VARIABLE The ERFs in LATRA are currently based on I-hr TWA concentrations and ceiling values.
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From page 62... ...
A key weakness of the current derivation of LATRA ERFs, however, is that a dose-response model for predicting incidence is derived from concentrations that are used to define safe levels (at the lower bound) and dangerous levels (at the upper bound)
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In such a process, a group of selected experts would be interviewed using carefully developed procedures to elic* their individual professional opinions concerning what health effects would be expected at each of a series of exposure durations and concentrations.
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, a small but still significant probability of incidence occurs below the ~ % level. With a large nearby population, dozens of people could be affected at or below the exposure concentrations associated with a I% incidence level.
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more appropriately based on sound data. OTHER FEATURES OF LATRA The subcommittee recommends that the Air Force evaluate potential health effects resulting from simultaneous exposure to more than one toxic rocket emission assuming the potential for additive effects.
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From page 66... ...
For example, the maximum 30-m~n concentration passing over an exposure location should be the value compared with a 30-min ERF or effect threshold. The subcommittee also recommends mat the Air Force evaluate the relative accuracies of the exposure estimates from REEDM and the ERFs (or effect thresholds suggested here)
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