Module Three - Lecture Notes
What is Risk Assessment
First, risk is defined as the probability that an event will occur. It can also be defined as the probability that a health effect will occur after an individual has been exposed to a specified amount of a hazard. Risk assessment is the process of gathering all available information on the toxic effects of a chemical and evaluating it to determine the possible risks associated with exposure. The process of gathering and evaluating the information can be divided into the following:
- Hazard Identification
- Hazard Evaluation or Dose-Response Assessment
- Exposure Assessment
- Risk Characterization
- Hazard Identification - this first step in
risk assessment consists in collecting data from different
sources to determine whether a substance is toxic. It
involves gathering and examining data from toxicological
and epidemiological studies.
- Epidemiology' is the study of the causative factors that are associated with the occurrence and number of cases of disease and illness in a specific population. Information from these studies should answer these questions:
- Does exposure to the substance produce any adverse effects?
- If yes, what are the circumstances associated with the exposure?
See Handout 3.1 - Types of information collected and considered when performing the risk assessment PDF - [27 KB]. (Obtained from the U.S. Environmental Protection Agency, Risk Assessment Guidelines and Information Directory, Government Institute, Rockville, MD, 1988.)
Information collected and considered when performing a risk assessment are listed below.
- Substance Identification (name)
- Physical/chemical properties of the toxic substance (Does it dissolve? Is it reactive [explosive, flammable, etc.], What is its size?)
- Source of the toxicity information
- Other factors to consider include the species of test animal (was the study done in rats, mice, man, etc.), and other variables affecting toxicity (including age, sex, and health of the study population).
- Exposure to Toxic Substances
Exposure to toxic substances depends on the
- Route of exposure (skin contact, inhalation, ingestion, injection),
- Duration of exposure (acute or chronic),
- Frequency of exposure, and
- Exposure to other toxic substances.
- Other factors to consider when determining potential exposures to toxic substances include diet, lifestyle choices, and occupation.
Epidemiological studies - The two major types of epidemiological studies are retrospective and prospective. Retrospective studies attempt to gather information from the past. Sometimes the information is incomplete because of the way the data was gathered. Because of that, it is sometimes difficult to determine if there is a relationship between the effect and a specific factor, such as exposure to a particular toxic substance. Prospective studies gather information from current, ongoing investigations. For that reason, the results are more complete and accurate than retrospective studies. Both methods are useful in identifying adverse health effects associated with a given toxic substance (1).
Toxicological studies - Different types of studies fall under the category of toxicological studies, including acute toxicity studies, which look at short-term exposures, and chronic toxicity studies, which look at exposures over a long period of time.
If the hazard identification process produces evidence of a hazard, then a hazard evaluation is performed. The purpose of this step is to calculate, if possible, the dose at which a harmful effect will occur. Since an effect in animals may not be the same in humans, at the same dose, "safety factors" are used. Safety factors account for the differences in response of test animals and differences in toxicity. The dose-response assessment tells the toxicologist what dose causes a response, usually illness or death, in the test animal.
An exposure assessment is performed to identify the affected population and, if possible, calculate the amount, frequency, length of time, and route of exposure. Exposure is "an event that occurs when there is contact at a boundary between a human and the environment at a specific (contaminant) for a specified period of time". Units to express exposure are "concentration times time"(1). Factors to consider when performing an exposure assessment include
- General information for each chemical
- Identification of molecular formula and structure (how the chemical looks and is made) and other identifying characteristics
- Chemical and physical properties
- Characterization of production and distribution
- Summary of environmental releases
- Exposure Pathways and Environmental Fate
- Transport and transformation
- Identification of principal pathways of exposure
- Predicting environmental distribution
- Measured or Estimated Concentrations
- Uses of measurements
- Estimation of environmental concentrations
- Exposed Human Populations
- Size and characteristics
- Integrated Exposure Analysis
- Calculation of exposure includes identification of the exposed population and identification of pathways of exposure.
- General Information for Each Chemical - The physical/chemical
properties of the toxic substance affects how it is
transported, how it is accumulated in the environment
and in tissues, and how it is transformed when it
is released into the environment. Some examples of
- vapor pressure (how easily can a chemical change from a solid or liquid to a gas?)
- its ability to dissolve in water
- its ability to stick to soil or sediments
Knowing these facts will help determine the dose and route of exposure.
- Sources of Exposure - Exposure to chemicals can occur anywhere, including the home (cleaning products, paints, pesticides, etc.). Outside the home, exposure to chemical pollutants in the air occurs through inhalation.
- Exposure Pathways and Environmental Fate - Once the source has been identified, the route and nature of the exposure must be determined. For exposure could occur through drinking water (the route could be ingestion of contaminated water).
- Measured or Estimated Concentrations - If possible, it is best to obtain actual samples from the source of exposure to calculate the amount of toxic substance present. However, samples are not always available and estimations of exposure can be calculated using a mathematical model. These models attempt to estimate the concentration of a substance at the point of exposure. Modeling is mostly used when determining concentrations of substances in air, but can be used to determine the amount in lakes or other bodies of water.
- Exposed Population - It is important to identify and characterize the exposed population in terms of sex; age; number of small children, pregnant women, and chronically ill individuals. Other information such as eating, work, exercise, and play habits is also necessary. Some populations are more at risk for illness than others, such as young children and older adults.
- Measuring Exposures - The effects from exposure
to simple and complex mixtures are very important,
as well as the health impact of these substances on
susceptible populations (e.g., children, elderly,
people of color). Exposure can also vary greatly within
geographic areas. Measurement of exposure is often
determined through questionnaires or surveys, employment
records, and evaluation of environmental contamination
data for areas in which a study population lives (10).
A problem seen in most communities is the absence
of actual data, because no personal monitoring was
conducted. This could lead to exposure calculations
that could be too high or too low (10). How much exposure
occurred and how much of a dose a person received
is significant in documenting exposure.
Two major approaches for assessing total exposure include indirect methods and direct methods. Indirect methods include environmental monitoring; use of fate and transport (migration); computer models; (use of questionnaires, and/or surveys for residents). Direct methods, include the use of personal workplace monitoring equipment and biologic markers (2). The extent of exposure may depend on the size of the population, its proximity to the contamination source, a person's degree of personal contact with the site, and the extent of the release of hazardous substances. Children are one population particularly susceptible to the toxic effects of contaminants at hazardous waste sites. While playing outside, young children come into contact with environmental toxicants via dermal contact and subsequent hand-to-mouth activity. Therefore, children who play in areas where there is little to no vegetative cover, as in many urban areas, and pica children (those who ingest greater than an average amount of non-food items [dirt] per day) are particularly sensitive to contaminants in soil.
- Calculation of Exposure (1) - Once the information is available, exposure can be estimated. Exposure can occur through more than one route, and when that is the case, the total exposure may be measured by adding the contributions of all routes. When data is not available, certain guesses are made, using standard reference values.
The last and final step in the risk assessment process is putting all of the information gathered from the other steps together to determine the actual risk of exposure to a specific toxic substance. This step relies on the expertise of the assessor in analyzing the information.
Based on information obtained from the risk assessment, decisions are made about the best way to address environmental contamination and exposure. The risk manager also includes an evaluation of social, legal, economic, and policy issues to determine the best approach to address an exposure issue.
SDR's public health assessment is an evaluation of environmental data, health outcome data, and community concerns associated with a site where hazardous substances have been released. The health assessment identifies populations living or working on or near hazardous waste sites for which more actions or studies are needed.