Ryzyko karcenogenozy

(2-27)

P(*> = 1 - er^hw

where h(x) is by definition a hazard ratę function. Researchers typically use a simple polynomial to describe h(*). The one-hit model with linear extra-polation assumes a linear hazard ratę. function of the form

h(*) = ax + b    (2-28)

This expression is evaluated by conducting a linear regression on a log-log plot of P(x) versus x.X

If we analyze Eqs. (2-27) and (2-28) at zero, we see that 1 -£-* is the background level of cancer in the control rats. The actual probability of cancer contraction is then    In the low-dose region where we are examining

the risk, P(x) is approximately linear. Therefore, the risk can be approximated by

P(x) s ax    (2-29)

This linearity is assumed for assessing the risk. The term a in Eqs. (2-28), (2-29) when normalized by division by the body weight, is the carcinogenic potency, in (mg/kg-day)'h

Table 2-12. Health Effeots Associated With a Contaminated Groundwater

Constituent Present	Concentration (M/L)	Health Effect of Interest	Potency (mg/kg-day) ~^1	Dose (mg/kg-day)	Cancer Risk
Chloroform	10	Hepatocellular	0.18	1.4 x 10-^4	2.6 x 10-5
2.3,7,8-	0.001	Hepatocellular	4.25 x 10^	1.4 x 10-8	6 x 10-3
tetra- chloro- dibenzo- p-dioxin Toluene*	1000	carcinoma Central nervous	29.057-	0.14
1,1,2-tri-	100	system depression cardiac arythmias Hepatocellular	0.057	1.4 x 10-3	1 O X oo

chloro-    carclnoma

ethane

^aToluene has an ADI; therefore, the potency is in mg/kg-day.