Environmental Engineering FOURTH EDITION

Risk Analysis 45

For all causes of death, the annual SMR is

106

95.8

SMR(t0tal) = - = 1.1.

Without performing a test of statistical significance, we may assume that the annual

SMR for cancer is significantly greater than 1, and thus Beaverville residents display

excess cancer deaths. Moreover, a BeaverviUe resident is only a little more likely to die

in any given year from any cause as the average resident of the United States. We may

also calculate whether cancer deaths per 1000 deaths are higher in Beaverville than in

the United States as a whole. In the United States, cancer deaths per 1000 deaths are

(G) (1000) = 167,

while in Beaverville, cancer deaths per thousand deaths are

(g) (1000) = 340.

We may thus conclude further that a death in Beaverville in any given year is about
twice as likely to be a cancer death than is the case in the United States as a whole.

Risk assessment is usually used to compare risks, because the absolute value of
a risk is not very meaningful. EPA has adopted the concept of unit risk in discussion
of potential risk. Unit risk for airborne and waterborne pollutants is defined as the risk
to an individual from exposure to a concentration of 1 pg/m3 of an airborne pollutant,
or g/L of a waterborne pollutant. Unit lifetime risk is the risk to an individual
from exposure to these concentrations for 70 years (a lifetime, as EPA defines it).
Unit occupational lifetime risk implies exposure for 8 h per day and 22 days per month
every year, or 2000 h per year, for 47 years (a working lifetime).
EPA’s concern with respect to somatic risk from a number of hazardous substances
is the carcinogenic potential of these substances, so the “consequence” part of the risk
is given as latent cancer fatalities (LCF). We can then write equations for the different
expressions for unit risk, and use these to calculate the estimated risk. In the example
below, these calculations assume that risk increases linearly with time and concentra-
tion. EPA considers this a conservative assumption for low exposure to a carcinogen
over a period of years. Nonlinear dose-response relationships imply more complex
relationships between risk, concentration, and exposure time; examples of such more
complex relationships will not be considered here. For waterborne pollutants:

LCF/year

Unit annual risk =

10-~ g/~

LCF

Unit lifetime risk =

g/L)(70 yrs)

(3.4a)

(3.4b)

(3.4c)

LCF

Unit lifetime occupational risk =

( g/L) (47 yrs) (2000/8760).