An internal dose model for interspecies extrapolation of immediate incapacitation risk from inhalation of fire gases

Assessing the application of acute toxic gas standards

OBJECTIVE

In this paper, we compare acute toxic gas standards developed for occupational, military, and civilian use that predict or establish guidelines for limiting exposure to inhaled toxic gases.

CONTEXT

Large disparities between guidelines exist for similar exposure scenarios, raising questions about why differences exist, as well as the applicability of each standard. The motivation and rationale behind the development of the standards is explored with emphasis on the experimental data used to set the standards.

METHODS

The Toxic Gas Assessment Software (TGAS) is used to quantitatively compare current acute exposure standards, such as: Acute Exposure Guidelines (AEGL), Immediate Danger to Life or Health (IDLH), Purser, International Organization for Standardization (ISO 13571), and Federal Aviation Administration (FAA). The TGAS software does this by calculating the body-mass-normalized internal doses of each gas exposure in each standard, which is then plotted on a cumulative distribution function for a normal or susceptible population to visualize the relationship of the standards to each other. To focus the comparison, acute toxic gas standards for five common fire gases, carbon monoxide (CO), hydrogen cyanide (HCN), hydrogen chloride (HCl), nitrogen dioxide (NO₂), and acrolein (C₃H₄O), are explored.

RESULTS

It was found that differences between standards can be reconciled when the target population, effect endpoint, and incidence level are taken into account.

CONCLUSION

By analyzing the standards with respect to these factors, we can acquire a better understanding of the applicability of each.

Incorporation of Acute Dynamic Ventilation Changes into a Standardized Physiologically Based Pharmacokinetic Model

A seven-compartment physiologically based pharmacokinetic (PBPK) model incorporating a dynamic ventilation response has been developed to predict normalized internal dose from inhalation exposure to a large range of volatile gases. The model uses a common set of physiologic parameters, including standardized ventilation rates and cardiac outputs for rat and human. This standardized model is validated against experimentally measured blood and tissue concentrations for 21 gases. For each of these gases, body-mass-normalized critical internal dose (blood concentration) is established, as calculated using exposure concentration and time duration specified by the lowest observed adverse effect level (LOAEL) or the acute exposure guideline level (AEGL). The dynamic ventilation changes are obtained by combining the standardized PBPK model with the Toxic Gas Assessment Software 2.0 (TGAS-2), a validated acute ventilation response model. The combined TGAS-2P model provides a coupled, transient ventilation and pharmacokinetic response that predicts body mass normalized internal dose that is correlated with deleterious outcomes. The importance of ventilation in pharmacokinetics is illustrated in a simulation of the introduction of Halon 1301 into an environment of fire gases.

An internal dose model of incapacitation and lethality risk from inhalation of fire gases

A mathematical model for estimating the likelihood of incapacitation and lethality from the inhalation of toxic gases is presented. The model computes an internal dose, equal to retained toxic gas per body mass, which is used to extrapolate outcomes across species. Account is taken for ventilation changes due to species, activity, and chemical response. The internal dose is correlated with each outcome using a cumulative, log-normal, probability distribution, which allows the estimation of tolerances for any population incidence. No internal interactions of gases are modeled and probabilities are combined independently. The model compares favorably with combined gas and large animal data.

A mathematical model of ventilation response to inhaled carbon monoxide

A comprehensive mathematical model, describing the respiration, circulation, oxygen metabolism, and ventilatory control, is assembled for the purpose of predicting acute ventilation changes from exposure to carbon monoxide in both humans and animals. This Dynamic Physiological Model is based on previously published work, reformulated, extended, and combined into a single model. Model parameters are determined from literature values, fitted to experimental data, or allometrically scaled between species. The model predictions are compared with ventilation-time history data collected in goats exposed to carbon monoxide, with quantitatively good agreement. The model reaffirms the role of brain hypoxia on hyperventilation during carbon monoxide exposures. Improvement in the estimation of total ventilation, through a more complete knowledge of ventilation control mechanisms and validated by animal data, will increase the accuracy of inhalation toxicology estimates.

Pharmacokinetic–pharmacodynamic models for categorical toxicity data

We propose a pharmacokinetic-pharmacodynamic (PK/PD) model (with possibly different choices for the PD link) for categorical toxicity data analysis. This is extension of the one-comportment model that applies to toxic endpoints categorised by grades (e.g., benign, mild, severe, and very severe). The model assumes that the area under the curve (AUC) of the internal quantity of the chemical substance is the critical dose-metric that drives the acute toxic phenomenon. That model handles time-varying concentrations and takes into account follow-up time, i.e., time at which effects are observed. Moreover the model bridges mechanistically based dose-response models and standard dose-response models, retaining the advantages of both. We use Markov chain-Monte Carlo (MCMC) simulations to fit the model to mortality data for mice exposed to chlorine, rats exposed to ammonia, and categorical data (different severity levels) from acute exposures of rats and humans to hydrogen sulfide.