Risk can be defined as the likelihood (or probability) of an undesirable occurrence under specific circumstances. In TACO, the circumstances (often called a scenario) are environmental contamination and its possible adverse effects on human health and/or the environment.
The concept of risk is incorporated into TACO to provide adequate protection to human health and the environment while incorporating site-specific information to allow for more cost-effective remediation.
In determining environmental risk there are three basic components; all must be present for a risk to exist:
Contaminant(s) i.e., chemicals,
Exposure route(s) e.g., air or drinking water, and
Receptor(s) i.e., people, plants or animals.
When using TACO, all three factors must be considered. The contaminants of concern are identified based on products released on site and program-specific site investigation requirements. At a minimum, the exposure routes of inhalation, ingestion of soil and ingestion of groundwater as drinking water must be evaluated. The potential of contaminants to migrate to groundwater is evaluated as part of the migration to groundwater route. It may also be necessary to evaluate exposure from chemicals crossing the skin in some circumstances. The receptors (those populations possibly exposed) are identified through the determination of present and post remediation land use.
Risk under TACO is quantified in two separate ways:
For non-carcinogenic effects, risk is expressed as a hazard quotient or the ratio of the expected exposure to the regulatory limit, and
For carcinogenic effects, risk is associated with the probability of an individual developing cancer over a lifetime.
Remediation objectives for carcinogens and non-carcinogens are derived using different assumptions about their effect on the human body. It is important to note that carcinogenic contaminants may have non-carcinogenic toxic effects which are also considered in Tiers 2 and 3 of TACO. The frequency and duration of the exposure along with the concentration of the contaminant determine the health effects.
For non-carcinogenic contaminants, the measure used to describe the potential for non-carcinogenic toxicity in an individual is not expressed as a probability. Instead, the potential for effects is evaluated by comparing an exposure level (E) over a specified time period (e.g., lifetime) with a reference dose (RfD) derived for a similar exposure period. This ratio of predicted exposure to acceptable exposure is called the hazard quotient (HQ).
In the development of a RfD, the upper bound of the human tolerance range must first be identified or estimated. Called the threshold, this upper bound represents a level that, if exceeded, could cause the protective mechanisms that exist in the body to be overcome and an adverse effect could occur.
Next, based on a "subthreshold" level, the "no observed adverse effect level" (NOAEL) is identified. The NOAEL is then divided by one or more uncertainty factors (usually multiples of ten) to account for such things as sensitive sub-populations and extrapolation from animal studies to humans. The RfD values are provided by the U.S. EPA.
An example of the determination of HQs follows:
HQ (acetone) = 0.01 mg/kg-day / 0.1 mg/kg-day = 0.1
HQ (dalapon) = 0.02 mg/kg-day / 0.03 mg/kg-day = 0.7
The HQ assumes that there is a level of exposure (i.e., a threshold) below which it is unlikely that adverse health effects would occur. If the exposure level exceeds this threshold (i.e., if E/RfD > 1) there is a concern for potential noncancer effects. It is important not to interpret HQ ratios as statistical probabilities; a ratio of 0.01 does not mean that there is a 1 in 100 chance of an effect occurring.
TACO also requires that if contaminants of concern are known to have the same target organ (e.g., liver, kidney) the HQ's for those contaminants must be added to form a Hazard Index (HI). This HI must be below one (1.0) to assume no adverse health effects on the target organ. For example:
HQ (acetone) = 0.1
HQ (dalapon) = 0.7
HI = 0.8 (target organ kidney)
For carcinogens, risks are estimated as the probability of an individual developing cancer over a lifetime as a result of exposure to a contaminant.
A risk evaluation based on the presumption of a threshold is inappropriate for carcinogens. The underlying presumption for carcinogens is that the introduction of even one molecule of the contaminant can cause cancer in an individual even if the probability is very low. This conservative, "nonthreshold" concept is used because it is presumed that there is no level of exposure to a carcinogen that does not pose a certain level of risk.
Because the threshold concept is not acceptable for carcinogens, a value different from a RfD that quantifies the relationship between dose and response must be developed. This value is known as a slope factor (SF), and it converts daily intakes of a carcinogen averaged over a lifetime directly to the upper bound risk of an individual developing cancer. That is, risk is equal to chronic daily intake (CDI) averaged over 70 years (lifetime) multiplied by the SF.
The SF is derived through the plotting of a curve that compares dose to response. Statistical procedures usually calculate the SF as the upper 95th percent confidence limit of the slope of the dose-response curve (i.e., there is only a 5% chance that the cancer risk could be greater). Because this is the upper bound risk, the actual risk is between that value and zero. The SF is roughly equivalent to the risk per unit dose, expressed as (mg/kg/d)-1. As with the RfD, the SF is provided by the U.S. EPA.
An example of the determination of cancer risk follows:
Risk = 0.0005 mg/kg-day x 0.03 (mg/kg-day)-1
Risk = 1.5 x 10-5
The risk of cancer due to exposure to a contaminant is commonly expressed in exponential terms, e.g., 10-6 and 10-4. These terms equate to a risk of 1 in 1,000,000 and 1 in 10,000 respectively. In the benzene example, the risk estimate of 1.5 x 10-5 means that 1.5 additional cases of cancer above background might occur among 100,000 exposed persons (or 15 cases in 1,000,000 persons) as a result of benzene exposure. The background cancer rate is 1 in 3, meaning that over a lifetime, an American's probability of getting cancer is 0.333333. Adding a 10-6 risk would increase the probability of an individual getting cancer to 0.333334. With the addition of a 10-4 risk, the probability of an individual getting cancer would be 0.333433.
For both carcinogenic and non-carcinogenic contaminants, it is important to identify the location of the risk on a site. Essentially the risk is at the point of human exposure, because without exposure there is no risk. In the TACO procedure, it is assumed that the point of human exposure, i.e., the risk, is at the contaminant source. If, however, an institutional control or an engineered barrier is in place (see Fact Sheet 4 & Fact Sheet 5), the point of human exposure is moved to the edge of such controls.
The goal of risk management is to reduce risk to an acceptable level while balancing the benefits of corrective action with its associated costs.
TACO provides several options for risk management if there is a need for risk reduction. The three most common options are:
Engineered barriers, and
Active remediation would include any cleanup activities which would reduce contaminant levels to either an acceptable risk level or to a level that would allow the use of one of the other options. Engineered barriers and institutional controls serve to prevent exposure to the remaining contaminants.