Chapter 1
Risk Assessment Basics

Understanding how risk assessments are done is key tounderstanding their results, interpreting them for your audience, and detecting bogus, overblown, minimized, or unsubstantiated claims.

What is Risk Assessment?

Risk assessment is a scientific process of evaluating the adverse effects caused by a substance, activity, lifestyle, or natural phenomenon.

In this handbook, risk assessment refers to the process of estimating the type and magnitude of risk to human health posed by exposure to chemical substances.

Assessments of environmental human health risks combine information on the amount of a substance humans are ex-posed to and the toxicity of that substance in order to state what is likely to happen.

Risk assessment is characterized by uncertainty. Although scientists have learned much about environmental contaminants, limited data and knowledge still require researchers to make assumptions throughout the risk assessment process.

The results of a risk assessment should report what assumptions were madeas well as the type of harm and itsmagnitude.

The uncertainty inherent in risk assessment means that risk assessors cannot precisely describe the risk. Rather, they should state the range of probabilities which they found.

Example of risk stated as a range of probabilities: "Our best estimate of the risk of cancer from chemical X is one additional case in 10,000 people, but the risk could be as high as one additional case in 1,000 people, or as low as no additional cases."

The Steps in Risk Assessment

A risk assessment consists of four steps.

• Problem Identification. What substances should be tested? Every natural or manmade substance cannot be tested at once, due to limited resources. Scientists and regulatory agencies must identify for study those substances that apparently cause health problems in humans.
• Exposure Assessment. How much of a substance are people actually exposed to? Once a suspect substance is identified, scientists conduct studies to estimate the amount of the substance a particular population is exposed to.
• Toxicity Assessment. How much of the substance causes what kind of harm to humans? Scientists also study the type and degree of harm different amounts of the substance cause in humans.
• Risk Characterization. What is the risk posed by this substance in its present (or predicted) use patterns?

  In risk characterization, scientists combine information from exposure assessments and toxicity assessments to estimate the type and magnitude of risk faced by the exposed population. The risk characterization should also state areas of uncertainty in the exposure and toxicity assessments that may affect the outcome.

Problem Identification

The problem identification step evaluates whether a substance may cause harm.Scientific or public concerns about harm from a particular substance often initiate the problem identification process. Direct evidence of harm to humans is seldom available. Instead, evidence is gathered by:

• animal studies. If the substance causes health problems in animals, it is assumed that it may be harmful to humans. Animal studies are the most common type of problem identification study.
• in vitro (test tube) studies. Experiments on isolated cells or microorganisms are used to explore how chemicals may exert toxic effects.
• comparison studies. The properties of the substance in question are compared with substances known to be harmful. (In vitro and comparison studies are less conclusive than animal studies, but they can augment them.)
• epidemiological studies . If available, these provide the most reliable evidence. However, their relevance to the present population of concern (perhaps a particularly sensitive one such as children) must be judged.

Ideally, if a substance is found to be possibly harmful to humans, it will be further studied in exposure and toxicity assessments.

Exposure Assessment

The exposure assessment step estimates how much of a substance a population inhales, ingests, or absorbs through the skin.

Long-term exposures to chemicals in the environment are usually assessed by environmental exposure studies. These estimate the amount of the substance in the environment; then estimate how much of it people take in.

Some of the factors an exposure assessment must consider are:

• how long people have been exposed.
• whether the exposure was continuous or intermittent
• how they were exposed-inhalation, ingestion, or absorption through the skin.

Again, many uncertainties may enter into these measurements, given incomplete knowledge of:

• how chemicals behave in the environment.
• people's lifestyles and how these affect exposure. An exposure assessment should describe the uncertainties present.

Toxicity Assessment

The toxicity assessment step looks at how much of a substance causes what kind of harm to humans.Toxicity to humans is not usually measured directly by intentionally exposing people, for obvious ethical reasons. Rather it is determined indirectly, usually by extrapolation * of animal studies to humans.

The extrapolation process is one of the areas in risk assessment where many assumptions must be made. These may include assumptions about:

• the effects of size and biological differences between animals and humans.
• the effects of high doses fed the test animals versus the low doses humans usually encounter in their environment.

A toxicity assessment should describe the assumptions used and indicate the uncertainties inherent in these assumptions.In some cases, epidemiological studies may also be used for toxicity assessment (see Chapter 4).

Risk Characterization

Ideally, the risk assessor has access to several toxicity and exposure studies. The risk assessor analyzes the sum total of information from these studies to develop the best possible judgment of risk.

Part of the risk assessor's job is to review the assumptions and uncertainties in the different studies and how they may have affected the study findings.

In the end, the risk assessor provides an estimate of risk, along with a description of the uncertainties that cause his/her report to be a "best guess," not an irrefutable statement of fact.

The estimate of risk is usually referred to as a "risk assessment." So the term "risk assessment" refers to both the entire process of estimating risk-problem identification, exposure assessment, toxicity assessment, and risk characterization-and to the estimate itself.

Uncertainty Unavoidable

Uncertainties are involved in all steps of a risk assessment, particularly in the toxicity assessment and the exposure assessment.

• In the exposure assessment, uncertainties arise because it is difficult to measure the amount of a pollutant in the environment over time, and how much is taken in by individuals.
• In the toxicity assessment, uncertainties arise when the findings are extrapolated from animals to humans.

Combining these uncertainties in the risk characterization step leads to a statement of risk that is not necessarily a statement of fact. However, it does indicate whether a substance may be of concern. The combined evidence of several studies is required before a statement of risk can be made with reasonable reliability.

Results of Risk Assessments

The uncertainty inherent in risk assessment means that the risk assessors cannot precisely describe the risk. But they can and should describe the limits of the risk-how low to how high it could possibly be.Scientists, regulators, and interest groups may describe risk as a single estimate or probability (e.g., 1 case in 10,000), rather than as a range of likely probabilities. Anyone stating a single probability may have a variety of motivations:

• Scientists or other experts may state the probability that appears most likely.
• Officials charged to protect public health may state the higher limit of risk, or give the range in a statement that emphasizes the higher limit: "The risk can be as high as [x], but may be much lower."
• Individuals, organizations, or others with special interests may report only the highest or only the lowest limit, depending on the point they wish to make.

Example of reporting risk as a range: In a report on asbestos, the National Academy of Sciences stated that the risk of lung cancer in non-smoking males from asbestos levels commonly found in air is 6 additional cases for every million men exposed. However, they also indicated that the risk could be as low as zero or as high as 22 additional cases per million men exposed.

What Risk Assessment is Not

Sometimes a study is represented as a risk assessment when it is not.

• A report on the incidence of cancer to rats from a certain substance is not a risk assessment. It only defines the toxicity to rats.
• An extrapolation of this information to the incidence of cancer in humans at postulated levels of exposure is a toxicity assessment, not a risk assessment.
• Risk assessment occurs only when information from toxicity assessments is combined with information from exposure assessments (how much of the substance people are actually exposed to).

Ideally, the risk assessment describes what is likely to happen to a particular population under real world conditions.

Risk Assessment versus Risk Management

Risk assessment is distinct from risk management.

Risk assessment is a scientific process of investigating phenomena to estimate the level of risk.

Risk management is an effort to reduce the risk through education, regulation, and clean-up.

Risk managers use the results of risk assessments, plus economic, social, and legal considerations to make regulatory and policy decisions.

While economic, social, and legal considerations have a legitimate place in risk management, they have no place in the scientific process of risk assessment.

Example of risk management: Scientists study the effects of chemical X on rats and find that adverse effects do occur. They also find that when the rats are given up to 10 grams of the chemical, there are no adverse effects.Risk managers then use this highest no-effect value-10 grams-to determine an acceptable level of chemical X in humans. This would likely be done by dividing one or more safety factors into 10 grams. If the safety factor were 100, this would yield 0.1 gram as the acceptable level in humans.Risk managers then use the acceptable level value to set standards. For example, they may use the value to set the maximum amount of chemical X that will be allowed in drinking water, so that no more than 0.1 gram accumulates in human tissue. By establishing these limits, risks from chemical X in drinking water will be reduced, if not eliminated.

Important Characteristics of Risk Assessments

Although economic, social, and legal factors should not figure in risk assessment, risk assessment is not completely devoid of what might be termed policy decisions.

The choices of models, data, assumptions, and methodology may be scientifically based; yet different scientists may make different decisions.

These decisions are sometimes "logical" in that they explore assumptions, data, methodology, etc. previously unexplored.

Sometimes they are intuitive-based on the scientist's best judgment.

Sometimes they are influenced by exter-nal considerations, (e.g., the public's great fear of cancer may influence researchers to select worst-case models of how cancers are formed).

These decisions affect the outcome of the risk assessment, although they are often not described.

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