Estimation of Toxic Effects

Estimation of Toxicant Effects in Ecological Risk Assessment.33... 

In this chapter, an overview of the development and use of matrix population models for estimation of toxicant effects is presented. Matrix models are one of the modeling approaches that can be used to determine whether populations of organisms will remain the same, increase, or decline, and thus have great potential for use in ecological risk assessment. 

Consequence Phase 3 Develop Detailed Quantitative Estimate of the impacts of the Accident Scenarios. Sometimes an accident scenario is not understood enough to make risk-based decisions without having a more quantitative estimation of the effects. Quantitative consequence analysis will vary according to the hazards of interest (e.g., toxic, flammable, or reactive materials), specific accident scenarios (e.g., releases, runaway reactions, fires, or explosions), and consequence type of interest (e.g., onsite impacts, offsite impacts, environmental releases). The general technique is to model release rates/quantities, dispersion of released materials, fires, and explosions, and then estimate the effects of these events on employees, the public, the facility, neighboring facilities, and the environment. 

The toxic effects model uses concentration-time profiles from the respiratory and skin protection models as input to estimate casualty probabilities. Two approaches are available a simple linear dose-effect model as incorporated in RAP and a more elaborate non-linear response model, based on the Toxic Load approach. The latter provides a better description of toxic effects for agents that show significant deviations of simple Haber s law behaviour (i.e. toxic responses only depend on the concentration-time product and not on each quantity separately). 

Studies with lambda-cyhalothrin and the freshwater crustacean G. pulex have shown that shorter durations of exposure result in substantially less severe effects than maintained, long-term exposures [15], In this case there was a significant reduction in toxicity with decreasing exposure times, with an 18-fold reduction in toxicity with exposure for 1 h compared to that determined after exposure for 96 h. Given this inverse correlation with exposure duration and toxicity, effects under natural environmental conditions where water column concentrations of pyrethroids are expected to decrease relatively rapidly are again likely to be less than would be predicted from estimates of toxicity under standard laboratory conditions with maintained concentrations (see Sect. 5). 

Consequence—The direct, undesirable result of an accidentsequence usually involving a fire, explosion, or release of toxic material. Consequence descriptions may be qualitative or quantitative estimates of the effects of an accident in terms of factors such as health impacts, economic loss, and environmental damage. 

For the purposes of estimating the potential toxicity of the chemical mixture, it is assumed the toxicity of the individual component compounds is additive. Data from the Registry of Toxic Effects of Chemical Substances (RTECS) and from the Hazardous Substances Data Bank will be accepted, as well as peer-reviewed primary data. 

Pharmaceuticals have provided numerous examples of genetic susceptibilities to toxicants. This is because major pharmaceutical drugs are given to hundreds of thousands, or even millions, of people so that genetic defects that result in toxic effects will show up even if only very small fractions of the population (estimated to be 1 in 10,000 or less for cases of toxic effects to the liver) are genetically predisposed to adverse effects. Unfortunately, at such low levels of occurrence, there is as of yet no good way to predict such rare adverse effects in advance. 

Where there is usable estimation of human exposure, the lowest level should exceed this. Ideally, the intermediate-dose level(s) should produce minimal observable toxic effects. If more than one intermediate dose is used, the dose levels should be spaced to produce gradation of toxic effects. In the low and intermediate groups and in the controls, the incidence of fatalities should be low to permit a meaningful evaluation of the results. If application of the test chemical produces severe irritation, the concentration may be reduced. This reduction may result in a subsequent reduction in, or absence of, other toxic effects observed at the high-dose level. If the skin shows severe damage, it may be necessary to terminate the study and undertake a new study at lower concentrations. 

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