1. Introduction to Environmental Engineering and Science (3rd ed.)
Chapter 4. Risk Assessment

2. Risk Assessment: National Academy of Science (1983)

3. Risk Assessment
Hazardous identification: the process of determining whether or not a particular chemical is causally linked to particular health effects, such as cancer or birth defects.
Dose-response assessment: the process of characterizing the relationship between the dose of an agent administered or received and the incidence if an adverse health effect.

4. Risk Assessment
Exposure assessment: determining the size and nature of the population that has been exposed to the toxicant under consideration and the length of time and toxicant concentration to which they have been exposed.
Risk characterization: the integration of the foregoing three steps, which results in an estimate of the magnitude of the public-health problem.

5. Nomenclature for toxic effects
Figure: 04-01

6. Fate of chemical toxicants in the body
Figure: 04-03

7. Hazardous Identification
Acute toxicity: effects that are caused within a short period of time after a single exposure to the chemical ↔ chronic toxicity
LD50: dose that will kill 50% of a population
Mutagenesis: genotoxic, carcinogens teratogens (Figure 4.5)
Carcinogenesis: initiation and promotion
(Table 4.7)

9. Dose-response morality curves for acute toxicity
Figure: 04-04

10. Possible consequence of a mutagenic event
Figure: 04-05

11. Toxicity Testing in Animals
Ames mutagenicity assay (short-term test)
Intermediate testing (several months)
Chronic carcinogenesis bioassay
- Two species of rodents (mica and rates)
- At least 50 males/50 females for each dose
- At least two doses + no-dose control
(exposure 6 weeks of age to 24 months)

12. Carcinogens: By U.S. EPA Group A: human carcinogen
Group B: probable human carcinogen
Group C: possible human carcinogen
Group D: not classified
Group E: evidence of noncarcinogenicity
Figure: 04-06

13. Human Studies Relative risk (>1 ?) Attributable risk (>0 ?)
Figure: 04-07
Relative risk (>1 ?)
Attributable risk (>0 ?)
Odds ratio (>1 ?)
(Example 4.1)

14. Figure: 04-07

15. Dose-Response Assessment
Dose-response curves: the results of chronic toxicity; the organism is subject to a prolonged exposure over a considerable fraction of its life
X-axis: dose (mg/kg-day); the average milligrams of substance per kilogram of body weight per day

16. Hypothetical dose-response curves
Figure: 04-08

17. Two methods of fitting for dose-response curves
Figure: 04-09

18. Potency Factor (PF) for Carcinogens
Incremental lifetime cancer risk
= CDI×PF
CDI: chronic daily intake
= average daily dose/body weight (4.13)
Incremental lifetime cancer risk = an upper-bound estimate of the actual risk
(Table 4.9, PF oral/inhalation)
(Example 4.2)
Figure: 04-10

19. Figure: 04-10

20. Drinking Water Equivalent Level (DWEL)
To estimate the concentration of a contaminant in drinking water that would result in a politically acceptable risk level = 10-6
DWEL = the concentration that will produce that risk
assumption: 70kg, 2-L of water /day
(Example 4.3)

22. RfD (ADI), NOAEL, LOAEL, & threshold
Figure: 04-11

23. RfD, Hazard Index RfD = NOAEL / (uncertainty factor)
Hazard quotient = average daily dose during exposure period (mg/kg-day) / RfD
Hazard Index = Sum of the hazard quotients (>1, potential risk)

24. Exposure Pathways
Figure: 04-12

25. Bioconcentration Factors (BCFs) Contaminant Degradation
Concentration in fish =
concentration in water × BCF
(Table 4.12) (Example 4.7)
Half-life (t1/2)
t1/2 = ln2/k = 0.693/k