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Textbook Chapter 2
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Risk Concepts


by Fred Arnold & Gail Froiman with John Blouin

Risk: the probability that a substance or situation will produce harm under specific conditions. Risk is a combination of two factors: the probability that an adverse even will occur and the consequences of the adverse event.

The Presidential/Congressional Commission
on Risk Assessment & Risk Management, Vol. 1, 1997


Risk is a concept that used in the chemical industry and by practicing chemical engineers. The term risk is multifaceted and is used in many disciplines such as: finance (rate of return for a new plant or capitol project, process improvement, etc), raw materials supply (single source, back integration), plant design and process change (new design, impact on bottom line), and site selection (foreign, political stability). Though the term risk used in these disciplines can be discussed either qualitatively or quantitatively, it should be obvious that these qualitative or quantitative analyses are not fields (financial risk ¹ process change risk). This chapter will focus on the basic concept of environmental risk and risk assessment as applied to a chemical's manufacturing, processing, or use, and the impact of exposure to these chemicals on human health or the environment.

Risk assessment is a systematic, analytical method used to determine the probability of adverse effects. A common application of risk assessment methods is to evaluate human health and ecological impacts of chemical releases to the environment. Information collected from environmental monitoring or modeling is incorporated into models of human or worker activity and exposure, and conclusions on the likelihood of adverse effects are formulated. As such, risk assessment is an important tool for making decisions with environmental consequences. Almost always, when the results from environmental risk assessment are used, they are incorporated into the decision-making process along with economic, societal, technological, and political consequences of a proposed action

Introduction of the previous chapter Introduction of the next chapter


Chapter 2. Example Problem

Example 2.6-3: Reference-Dose

Reference doses are used to evaluate noncarcinogenic effects resulting from exposure to chemical substances. The reference dose (RfD) is the threshold of exposure below which protective mechanisms are believed to guard an organism from adverse effects resulting from exposure over a substantial period of time. When valid human toxicological data are available, it forms the basis for the reference dose. When human exposure data are not available, the animal species believed to be most sensitive to the chemical of concern is used to determine the lowest level at which an adverse effect is detected, often called the LOAEL. Similarly the NOAEL is the greatest test-dose level at which no adverse effect is noted. When animal data are used the reference dose for human populations is adjusted by extrapolation factors to convert the NOAEL or LOAEL into a human subthreshold or reference dose.



FA is an adjustment factor to extrapolate from animal to human populations;
FH is an adjustment factor for differences in human susceptibility;
FS is an adjustment factor used when data are obtained from subchronic studies;
FL is an adjustment factor applied when the LOAEL is used instead of the NOAEL; and,
FD is an adjustment factor applied when the data set is dubious or incomplete.

Each adjustment factor should account for the systematic difference between the two measures bridged by the extrapolation and incorporate a margin of safety in accordance with the uncertainty associated with the extrapolation. For example, in a 3-month subchronic study in mice, the NOAEL for tris(1,3-dichloro-2-propyl) phosphate was 15.3 mg/kg body weight per day; the LOAEL was 62 mg/kg at which dose abnormal liver effects were noted. See Kamata E. et al., Acute and subacute toxicity studies of tris(1,3-dichloro-2-propyl) phosphate on mice. Bull Natl Inst Hyg Sci, 107:36-43 (1989). If each of the adjustment factors is equal to 10, the reference dose for this chemical is:

Using the NOAEL:

RfD=NOAEL/FAFHFS=15.3/(10)(10)(10)=0.015 mg/kg-day

Using the LOAEL:

RfD=LOAEL/FAFHFLFS=62/(10)(10)(10)(10)=0.0062 mg/kg-day

The lesser of the two values, 0.0062 mg/kg-day, would be selected as the reference dose for humans in this instance.

Example problem from the previous chapter Example problem from the next chapter


Chapter 2. Sample Homework Problem

1. A colleague has requested your advice on selection of a safe solvent for a photoresist. A photoresist consists of an acrylate monomer, polymeric binder, and photoinitiator applied to the surface of a copper-clad laminate or silicon wafer. After the solvent evaporates, the photoresist is exposed to ultraviolet light through a mask containing the pattern to be etched on the circuit board or silicon wafer. When exposed, the resist polymerizes and becomes insoluble to the developer. The circuit board or silicon wafer is subsequently washed with the developer solution to remove unpolymerized photoresist exposing the pattern to be etched with acid into the copper metal or the silicon wafer. Your colleague has identified the following solvents as suitable for formulation of the photoresist.

CAS Number
Vapor Pressure
kPa at 25°C
OSHA Concentration Limit
parts per million
furfuryl alcohol
ethyl acetate
monomethyl ether
methyl ethyl ketone
n-butyl acetate

a. Using the OSHA Permissible Exposure Limit as a surrogate for relative hazard, a higher OSHA PEL connoting a lower hazard, rank these solvents from highest hazard to lowest.

b. Using the vapor pressure as a surrogate for the magnitude of worker exposure to the solvent vapors, rank these solvents from highest exposure potential to lowest.

c. Considering both hazard and exposure potential, which of these solvents would you recommend to your colleague for the photoresist solution?

d. What alternatives can be used to reduce the risk associated with solvents even further?

Sample homework problem from the previous chapter Sample homework problem from the next chapter


Chapter 2. Lecture Notes

Under construction. Please come back soon.
Lecture notes for the previous chapter Lecture notes for the next chapter


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