Chronic Daily Intake

The fraction of ingested arsenic that is bioavailable, i.e., that is taken up into the bloodstream from the intestinal tract 

the exposure (dose) due to drinking water from the contaminated aquifer is the product of (arsenic concentration in drinking water aquifer) x (volume of water ingested) x (fraction of arsenic that is bioavailable). 

To illustrate the kind of variability that is typical of the concentrations of toxic chemicals measured in water—and in environmental media in other risk 

Land Use Exposure Pathway Daily Intake (days year- ) (years) 

Source Reprinted with permission from Ian Pepper, Charles Gerba and Mark Brusseau, eds., Pollution Science (New York Academic Press, 1996), 350. 

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Chronic daily intake (CDI), Hazard Index (HI), and Cancer Risk (CR) for carcinogenic effects were calculated and exposures associated with HI<1 and CRdE-6 were considered negligible. 

The carcinogenic risk may be defined as the chronic daily intake dose (as developed in the exposure assessment) multiplied by the carcinogenic slope factor (as selected by the toxicity assessment). The product is the probability of developing cancer during lifetime from exposure to this chemical. 

GDI = chronic daily intake averaged over 70 years (mg/kg-day)... 

Noncarcinogenic risk is represented by the hazard index (HI), which is the ratio of the chronic daily intake to the RfD ... 

Carcinogenic risk is a function of the chronic daily intake (calculated using Equation (1) and the slope factor (SF)) ... 

Environmental toxicity The risk of accidental exposure to meprobamate, carbamazepine, and phenytoin from consumption of stream water and fish has been studied in children and adults the average hazard quotients (i.e. the ratio of chronic daily intake to acceptable daily intake) showed no potential risks of adverse reactions due to exposure to these substances [140. ... 

Let us calculate the chronic daily intake of arsenic by a future resident on the Superfund site if the groundwater aquifer were not cleaned up. The GDI is calculated per kilogram of body weight and then averaged over a person s lifetime ... 

The chronic daily intake is used to predict both the risk of noncancer health effects and the risk of cancer. Analogous estimates of chronic daily intake can be made for other pathways of exposure in other risk scenarios. For example, exposure to an air pollutant could be estimated as follows ... 

To summarize the noncancer health risk-assessment process, chemicals of concern, pathways of exposure, and exposed populations are identified in the first step of the risk assessment, hazard identification. In the second step, analysis of exposure, the doses are estimated for each population, each exposure pathway, and each chemical of concern in the form of chronic daily intakes, or CDIs. In the third step of the risk assessment, analysis of effect, noncancer health effects are estimated by comparing CDIs to reference doses, or RfDs, derived from animal toxicity studies (with input from human epidemiological studies, when available). If the CDI is greater... 

The chronic daily intake (CDI) estimated in the analysis of exposure, the second step of the risk assessment, is used to calculate the risks of both noncancer health effects and cancer. Risk calculations are also referred to as quantitative risk assessment, a term that is somewhat misleading because the word quantitative implies a high degree of accuracy, which is clearly not the case. In the first risk scenario described in Section 8.3, future residents drink arsenic-contaminated water from the aquifer beneath a former Superfund site. Their CDI by this pathway is estimated to be 0.0I6I mg/kg/day of arsenic. The oral reference dose (RfD) for arsenic is 3 x lO"" mg/kg/day, according to the EPA s Integrated Risk Information System (IRIS) (U.S. EPA 2009). The hazard index (HI) for noncancer health effects caused by this chemical of concern by this exposure pathway is calculated using Equation (8.3) ... 

Discussing the uncertainties involved in risk calculations is an important aspect of risk characterization. Quantitative risk assessments are fraught with uncertainty. Estimates of exposure (chronic daily intake) are probably accurate to roughly a factor of 10. Toxicity values—the reference dose for noncancer health effects and the slope factor for cancer risk—are also essentially order-of-magnitude estimates. As a result of these uncertainties, quantitative risk assessment gives a number that could be... 

The four steps of the risk-assessment process are hazard identification, analysis of exposure, analysis of effect, and risk characterization. In the hazard identification step, the risk assessor identifies chemicals of concern, environmental pathways of exposure, and populations and subpopulations at risk. The exposure analysis develops exposure scenarios and estimates the chronic daily intake of each chemical of concern. In the analysis of effect, the risk assessor combines the chronic daily intake calculated in the exposure analysis with toxicity data from animal studies (and/or human epidemiological studies, if available) to estimate the risk of toxic effects in exposed populations, whereby risks to public health are divided into two broad categories noncancer health effects and cancer. The final step of the risk-assessment process, risk characterization, is a narrative that marshals all the evidence of risk to public health, including quantitative risk assessments and qualitative evidence of risk. The risk assessor weighs all the evidence and uses professional judgment to draw conclusions about risks. 

The hazard index (HI) is used to assess the risk of a noncancer health effect in an exposed population. The hazard index is calculated by dividing the chronic daily intake (CDl) by the reference dose (RfD) HI = CDI/RfD. A hazard index equal to or greater than 1 indicates significant risk. The higher the hazard index, the greater is the risk. 

Calculate the chronic daily intake and divide it by the reference dose to obtain the hazard index ... 

If exposure to the maximum concentration of methylmercury found in the FDA study, 1.300 ppm, is assumed, then the chronic daily intake is increased by the ratio of the maximum concentration to the average concentration (1.300 mg/kg)/(0.383 mg/kg) = 3.39. The hazard index is increased by the same factor, to (3.39) x (0.4) = 1.4. A hazard index valne greater than 1.0 is cause for concern and is consistent with the joint FDA/EPA advisory to women of childbearing age not to consume more than one meal of alba-core ( white ) tuna per week, since albacore tuna tends to have high levels of mercury. 

To estimate noncancer health risk, calculation of the chronic daily intake cannot be avoided ... 

Let us take a closer look at the analysis of exposnre to a toxic chemical. If the ecological receptor is a population of fish, such as coho salmon, a useful measure of exposure is the concentration of the toxic chemical in water. If the ecological receptor is a population of birds, exposure analysis is performed in much the same way as the estimate of chronic daily intake in a human health risk assessment (Chapter 8). The measure of interest is the average daily dose (ADD) in units of milligrams of... 

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