Environmental Engineering FOURTH EDITION

46 ENVIRONMENTAL ENGINEERING

The factor 2000/8760 in Eq. (3.4~) is the fraction of hours per year spent in the
workplace. For airborne pollutants:

Unit annual risk =

Unit lifetime risk =

Unit lifetime occupational risk =

LCF/year

10-6 g/m3

LCF

( 10m6 g/m3)(70 yrs)

(3.5a)

(3.5b)

LCF

( g/m3)(47 yrs)(2000/8760).

(3.5c)

EXAMPLE 3.2. The EPA has calculated that unit lifetime risk from exposure to eth-
ylene dibromide (EDB) in drinking water is 0.85 LCF per lo5 persons. What risk is
experienced by drinking water with an average EDB concentration of 5 pg/L for five
years?
The risk may be estimated using either unit annual risk or unit lifetime risk. Since
the unit lifetime risk is given, we may write

Risk = (5 x g/L) (0.85 LCF) (5 yrs) = 3.0 x 10-~ LCF.

(105) (10-9 g/~) (70 yrs)

The estimated risk is that about three fatal cancers would be expected in a population
of a billion people who drink water containing 5 pgL EDB for five years. Although
there is a popular tendency to translate this to an “individual risk” of “a chance of three
in a billion of having a fatal cancer,” this statement of risk is less meaningful than the
statement of population risk.

RISK PERCEPTION

Since 1983, a great deal has been written about risk perception, and there has been

considerable research and speculation as to why risks are perceived very differently

from their estimates. For example, the risks attendant on transporting radioactive waste

are perceived by some people as far greater than the risks attendant on transporting

gasoline in tank trucks, when in fact the latter are considerably larger. The reader is

referred to the works of Paul Slovic and Hank Jenkins-Smith, listed in the bibliog-

raphy, for some detailed discussions of risk perception. In general, the factors that

appear to influence risk perception are the familiarity of the risk, knowledge about the

risk, whether the risk is undertaken (or perceived as undertaken) voluntarily, and the

estimated and perceived benefits of the activity that offsets the attendant risk. While

engineers deal with mitigation of estimated risks, they must often take risk perception

into account.