Chapter 5 Basic Concepts Regarding Risk Assessment and Toxicology

105

CHAPTER 5

Basic Concepts Regarding Risk Assessment and Toxicology 5.1.

RISK ASSESSMENT PRINCIPLES

General aspects Risk assessment is the common first step in the risk management process. It determines the quantitative and/or qualitative value of risk [1]. Risk management is the process of defining and implementing measures to control a risk [2]. Risk assessment takes into account factors relevant to the situation assessed, such as the current or proposed human activities, specifics related to exposure to the agent assessed, and, in case of chemical hazard, the agent’s physicochemical characteristics. The essential steps involved in risk assessment, beyond the identification of an issue, are (1) hazard identification, (2) dose–response assessment, (3) exposure assessment, and (4) risk characterization. After an issue has been identified, the specific problems are identified, involving information related to the potential causes and context of the problem, the associated types of adverse effects, the frequency and duration of the problem, public perceptions, etc. The hazard identification step involves evaluating the available data to define the adverse health effects that are related to the problem including frequency and duration of exposure. Dose–response assessment evaluates the qualitative and quantitative toxicity data to estimate the incidence and severity of adverse effects occurring at different exposure levels. Exposure assessment involves devising estimates that take into account potential exposure scenarios, magnitude, duration, length of exposure, and frequency of exposure to a hazard. The risk characterization step details the effects (incidence, nature) for the specific exposure scenarios. Besides capturing all the findings from steps 1, 2, and 3, this step also defines the uncertainties (magnitude, nature) and assumptions used in the risk assessment. Risk management takes into account all the risk assessment findings and considers pertinent external factors before reaching a decision [2].

Uncertainty and variability in risk assessment Uncertainty and variability are usually part of risk assessment. Uncertainty is usually the result of insufficient knowledge (lower data quality, insufficient studies, or lack of studies), the result of parameter uncertainty (e.g., random measurement, systematic errors, multiple uncertainty errors from incorrect or unrealistic model application), and decision-related uncertainty (prediction interpretation inability or limitation). Variability may result from differences in people or populations and can occur when a single value is used to describe something characterized by multiple or variable values (e.g., body weight, other interindividual differences). Uncertainties in data sets and data gaps are the weak points of risk assessment. One way of dealing with uncertainty relies on incorporation of safety and uncertainty factors when deriving risk levels. This is the route probably used most often, although, within some limits, the specific factors used to select a given uncertainty or safety factor often have an arbitrary basis for being chosen.

Risk assessment guidance documents Different countries have different authoritative bodies that have developed their own set of criteria for risk assessments. For example, there are various approaches for conducting carcinogen assessment [3]. In the United States, the most elaborate current risk assessment guidelines have been produced by the Environmental Protection Agency (EPA). EPA has guidelines for carcinogen risk assessment,

106

Pyrolysis of Organic Molecules

mutagenicity, neurotoxicity, developmental toxicity, reproductive toxicity, chemical mixtures, and ecological risk assessment [4].

5.2.

TOXICOLOGY PRINCIPLES

General aspects Toxicology is the study of the adverse effects of substances on organisms, their mechanisms of toxic action, and the application of the collected information to protect the health of human and other species [5]. In order for a toxicant to generate toxic effects on an organism, either the compound by itself or its metabolites or biotransformation products must come in contact with the affected organism via one or more exposure routes [6]. The exposure route refers to the way individuals come in contact with a potentially toxic substance. The main routes of exposure are eating/drinking (ingestion via gastrointestinal tract), breathing (inhalation via respiratory system), or skin contact (dermal). In terms of duration and frequency of exposure, toxic effects can be divided into acute, subacute, subchronic, and chronic. Acute exposure takes place for a short period of time, such as less than 1 day. Subacute exposure generally occurs for less than 1 month, subchronic exposure generally occurs for less than 3 months (1–3 months), and chronic exposure is generally more than 3 months [2]. The exposure can take place in a single administration or under repeated administration. Acute toxic effects are often different from chronic toxic effects. Chronic toxic effects occur when the system is repeatedly or continuously exposed to toxic doses that induce irreversible changes at the site of action and/or if there is not enough time for the biological system to repair the damage in the time intervals between repeated exposures. Chronic effects can also occur if accumulation occurs (e.g., if absorption exceeds biotransformation and/or excretion).

Dose and response The dose can be defined as the amount of chemical to which an organism or a population is exposed. The usual unit of measure for dose is mg/kg body weight/day. The routes of exposure for which dose is measured in these units include oral, intravenous, intramuscular, subcutaneous, and dermal. Sometimes, dose can be given on weight per body surface area basis or mg/cm2. Concentration is used when inhalation exposure is involved and is usually expressed in units of mg/m3 [2]. The response, or the adverse or toxic effect, can be any departure from normality at any level of a biological system. An adverse or toxic effect, for example, can occur at the smaller biological levels (e.g., molecules in DNA), or it can occur at higher biological levels (e.g., cells, organs, systems). Responses or effects can be localized or generalized (systemic), immediate or delayed, reversible or irreversible, and graded or continuous. Toxic effects or responses can affect one or more particular organ systems. Usually, the type of toxicity is categorized on the basis of the major organ systems affected. Most chemicals generally induce systemic toxic effects in one or two main organs (target organs). However, the target organ is often not the same as the organ with the highest concentration of the chemical.

The relation between dose and response Almost any agent given in a high enough dose and/or for long enough time can elicit toxicity (dose makes the poison). Typically, as the dose increases, the toxicological response increases. Each chemical has its own specific dose–response curve for a given effect. The dose–response curve is generally sigmoidal. However, for some compounds, the total absence is detrimental, a favorable biological response is shown for a low exposure, and the compounds are toxic at large doses. This effect is known as hormesis, and, in this case, the typical dose vs. response behavior is not followed.

Basic concepts regarding risk assessment and toxicology

107

Also, the dose–response relationship does not necessarily hold true if the response is an allergic reaction. The description of the dose vs. response regarding the toxicity of a specific compound can be done by many procedures. A simple parameter used for the characterization of dose and response relation is the median lethal dose LD50 (lethal dose, 50%), which is the dose of a toxic substance (or radiation) required to kill half of the members of a tested population (usually of rats). The LD50 value is in general used for the description of acute toxicity. It should be noticed that LD50 is not the lethal dose for all the subjects, some subjects being killed by less toxicant and others surviving even higher doses. Also, the method of administration of a toxic compound may influence the LD50 value. To the LD50 value, sometimes is added a specification regarding the timing of lethality (e.g., LD50/30 indicates 50% lethality after 30 days). A similar descriptor for dose–response relation is LCt50 (lethal concentration and time, 50%). Another parameter is lCt50, which is the dose that will cause incapacitation rather than death, and it can be applied more easily to humans. Some toxicity studies generate information known as NOAEL (no observed adverse effect levels), which gives the lowest tested dose of a toxicant below which no response is observed. This dose is typically expressed as a level or concentration, for example, in units parts per million (ppm). In addition to the level, description of the type of animals in which the study has been performed, time of exposure, as well as the system affected (respiratory, hematological, dermal, etc.) are indicated together with the NOAEL value. A similar descriptor to NOAEL is LOAEL (lowest observed adverse effect levels). A more quantitative procedure for dose–response assessment is benchmark dose (BMD) approach (see, e.g., [7]). In this procedure, dose–response curves are typically used for the evaluation of a predetermined dose that results in a certain level of adverse response or critical effect size [8]. All the previously described parameters are based on the assumption that there is a dose below which no toxic effect occurs (i.e., there is a threshold for noticeable toxicity). An exception to this assumption is carcinogenicity, in which case it is generally assumed that there is no threshold. Most current cancer risk assessment approaches assume that there is no threshold below which the risk of an adverse effect occurring is negligible. In cancer risk assessment, extrapolation models either statistical (based on probability distributions) or mechanistic (based on mechanism of response) can be used to generate risk at very low levels. This low risk is called de minimis risk, meaning a risk so small that it does not pose concern. This type of small risk usually is assumed to have a probability of occurrence below 104 to 106 and can be interpreted as ‘‘virtually safe.’’

Carcinogenic potency A numerical description indicated as TD50 typically has been used to estimate carcinogenic potency. Similar to LD50 and with NAOEL or LAOEL, this parameter is evaluated for specific animals. The parameter is defined as the dose rate in mg/kg body weight/day, which, if administered chronically for the standard life span of the species, will reduce to half the probability for the animals to remain tumorless. TD50 is analogous to LD50, and a low value of TD50 indicates a potent carcinogen, whereas a high value indicates a weak one. TD50 can be computed for any particular type of neoplasm, for any particular tissue, or for any combination of these. The determination of TD50 values can be further complicated if the measurement experiments are terminated before the standard life span of the tested animals. Various correction procedures were developed for the evaluation of acceptable TD50 values [9,10]. A relative carcinogenic potency can be calculated to compare the carcinogenic potency of specific compounds. This relative potency can be calculated by comparing the values of 1/TD50 for the compounds of interest. For the relative potency, the higher numbers indicate a more potent carcinogen.

Classifications of toxicants by various agencies There are various agencies in the United States and abroad that have different classification schemes (qualitative descriptors) for the toxic effects of chemicals, and for chemicals that are carcinogens and reproductive toxicants. In the United States, these agencies include National Toxicology Program (NTP), US EPA, American Conference of Governmental Industrial Hygienists (ACGIH), Occupational

108

Pyrolysis of Organic Molecules

TABLE 5.2.1. IARC and EPA (2005) categories of carcinogens [12,13] IARC

EPA 2005

Category

Descriptor

Evidence

Category

Descriptor

Evidence

1

Carcinogenic to humans

Sufficient in humans

A

Carcinogenic to humans

Sufficient in humans

2A

Probably carcinogenic to humans

Limited (or occasionally inadequate) in humans Sufficient in animals

B1

Likely to be carcinogenic to humans

Limited in humans Sufficient in animals

2B

Possibly carcinogenic to humans

Limited or inadequate in humans Sufficient in animals (level of evidence 2AW2B)

B2

Likely to be carcinogenic to humans

Inadequate or no data in humans Sufficient in animals

C

Suggestive evidence of carcinogenic potential

No data in humans Limited in animals (more data needed)

3

Not classifiable as to its carcinogenicity to humans

Inadequate in humans Inadequate or limited in animals (more data needed)

D

Inadequate information to assess carcinogenic potential

Inadequate human and animal data or no data

4

Probably not carcinogenic to humans

Demonstrated lack of carcinogencity in animals and humans (or, occasionally, insufficient in humans, sufficient evidence for lack of effects in animals and supporting mechanism)

E

Not likely to be carcinogenic to humans

Evidence of noncarcinogenicity in at least two animal studies in different species or adequate human and animal data

Safety and Health Administration (OSHA), and Office of Environmental Health Hazard Assessment (OEHHA) of the California EPA, which issued what is known as the ‘‘Proposition 65 list of chemicals known to the state to cause cancer for purposes of the Safe Drinking Water and Toxic Enforcement Act of 1986’’ or ‘‘Proposition 65’’ [11]. A number of international agencies also have classifications for known toxicants, carcinogens, and other classes of hazards, one of these being the International Agency for Research on Cancer (IARC). Although most classifications are similar, each agency classification has specific differences, and the classifications can be based on different criteria. Also, these classifications are evolving, and they may have differences in the guidelines from one year to another. As an example, the differences in the classification of carcinogens by IARC [12] and EPA in 2005 [13] are shown in Table 5.2.1.

Factors influencing toxicity Toxicological assessment is a complex process due to the many factors affecting toxicity. Among these factors can be listed: physicochemical properties of the agent (or vehicle), dose, length of exposure, frequency of exposure, route of exposure, absorption, distribution, metabolism, excretion, species,

Basic concepts regarding risk assessment and toxicology

109

strain, interindividual susceptibility, preexisting condition of the exposed individual or population, and other coexposures. Other factors include coexposure-related conditions (e.g., temperature, light, food) and biological system-related conditions (e.g., coexistent stress, other environmental conditions, age, health status, hormones, sex). For these reasons, a number of different tests on several animal species are necessary to arrive to an acceptable understanding of the toxicological profile of a given compound. One other aspect that must be considered in toxicity assessment includes possible interactions of chemicals. This may occur when more than one chemical is present in a given system and the response can be modified by potentiation, additivity, synergy, or antagonism. Chemicals can interfere by altering each other’s absorption, metabolism, distribution, excretion, or protein binding, directly or indirectly. Tolerance is another effect that must be considered. Tolerance can be seen in situations in which a chemical (or structurally related compound) is administered repeatedly, and it involves decreased responsiveness of a system (e.g., organ, tissue, or cell) to the toxic effects of that chemical. Tolerance depends on the chemical characteristics, individual characteristics, and administration conditions.

Information used in toxicological evaluations Toxicological evaluations usually include two main types of information: (1) chemical related and (2) effect/mechanism related. The main sources for toxicological information are in vivo studies (in living organisms), in vitro studies (outside of living organisms, in isolated systems), and in silico data (computer simulation). Studies in vivo can be performed in humans, in animals, or in other living organisms. Studies in humans typically include epidemiological studies and controlled clinical studies. Epidemiology studies are studies on human populations that attempt to link human health effects to a cause (e.g., cancer linked to exposure to a specific chemical, such as asbestos). Clinical studies usually involve studies conducted as part of the safety assessment of medications or devices. Studies in animals include studies in various species and conditions. In vitro studies usually involve studies in bacteria or mammalian cells and are generally used as supporting studies to the interpretation of the in vivo studies. In silico data involve understanding structure–activity relationships (quantitative or qualitative). These are also used as supporting information that is taken into account in an assessment.

The use of toxicological information for risk assessment The descriptors used to assess the dose–response relation are further used in risk assessment with the goal to generate parameters that can be applied to estimate admissible levels of exposure to toxicants or drugs, levels that are considered protective of the public health. Such parameters include reference dose (RfD), reference concentration (RfC), and acceptable daily intake (ADI). These parameters usually are generated from NOAEL, LOAEL, or BMD values by applying correction factors such as safety factors (SF), uncertainty factors (UF), and/or modifying factors (MF) [14]. As an example, the ADI is defined as the amount of chemical to which a person can be exposed daily over extended time (usually lifetime) with minimal risk of adverse effects. There are different formulas for the calculation of ADI. One calculation uses the NOAEL and SF values: ADI ¼

NOAEL SF

(5.2.1)

Other calculations generate an ADI1 value by the formula: ADI1 ¼

NOAEL UF  MF

(5.2.2)

110

Pyrolysis of Organic Molecules

The use of LOAEL instead of NOAEL is used for the calculation of an ADI2 value: ADI2 ¼

LOAEL UF  MF

(5.2.3)

The safety factor, SF, is selected based on an estimated extreme worst case or can be generated as UF  MF, where the uncertainty factor, UF, accounts for different types of extrapolations or compensations depending on the source of NOAEL and LOAEL values (e.g., NOAEL generated from animal studies, or using adult–child extrapolation, etc.), and where the MF values are adjusting factors for UF.

5.3.

REFERENCES

1. http://www.epa.gov/risk/ 2. C. D. Klaassen, Casarett and Doull’s Toxicology: The Basic Science of Poisons, 6th edition, McGraw-Hill Companies Inc., New York, 2001. 3. R. J. Moolenaar, Regul. Toxicol. Pharmacol., 20 (3) (1994) 302. 4. http://cfpub2.epa.gov/ncea/cfm/recordisplay.cfm?deid ¼ 55907 5. R. A. Lewis, Lewis’ Dictionary of Toxicology, Lewis Publishers, CRC Press, Boca Raton, FL, 1998. 6. A. W. Hayes, Principles and Methods of Toxicology, 4th edition, Taylor & Francis, Philadelphia, PA, 2001. 7. http://www.epa.gov/NCEA/bmds/bmds_training/methodology/intro.htm 8. K. Z. Travis, I. Pate, Z. K. Welsh, Regul. Toxicol. Pharmacol., 43 (2005) 280. 9. C. Sawyer, R. Peto, L. Bernstein, M. C. Pike, Biometrics, 40 (1984) 27. 10. R. Peto, M. C. Pike, L. Bernstein, L. S. Gold, B. N. Ames, Environ. Health Prospect., 58 (1984) 1. 11. http://www.oehha.ca.gov/prop65/prop65_list/Newlist.html 12. http://monographs.iarc.fr/ENG/Preamble/currentb6evalrationale0706.php 13. EPA. Guideline for carcinogen risk assessment. EPA 630/P-03/001F. Washington, DC, 2005. 14. http://www.epa.gov/iris/gloss8.htm