Environmental Engineering FOURTH EDITION

42 ENVRONMENTAL ENGINEERING

This frequency is often called a probability P, and written without units. Because of
this common (though confusing) usage, we will refer to “probability” throughout the
remainder of this chapter.
If the adverse effect is death from cancer, and the cancer occurs after a long latency
period, the adverse health effects are called latent cancer fatalities, or LCF.
ReZative risk is the ratio of the probabilities that an adverse effect will occur in two
different populations. For example, the relative risk of fatal lung cancer in smokers
may be expressed as

where

Ps = probability of fatal lung cancer in smokers,

Pn = probability of fatal lung cancer in nonsmokers,

Xs = fatal lung cancers in smokers,

Xn = fatal lung cancers in nonsmokers,

Ns = total number of smokers, and

Nn = total number of nonsmokers.

Relative risk of death is also called the standard mortality ratio (SMR), which is also

written as

where

Ds = observed lung cancer deaths in a population of habitual smokers, and

Dn = expected lung cancer deaths in a nonsmoking population of the same size.

In this particular instance, the SMR is approximately 11/1, and is significantly

Three important characteristics of epidemiological reasoning are illustrated by this

greater than

example:

0 Everyone who smokes heavily will not die of lung cancer.

0 Some nonsmokers die of lung cancer.

3De.terminati0n of statistical significance is beyond the scope of this text. In determining statistical

significance, a test of significance (like the Fisher’s test or the Student’s t-test) appropriate to the population

under consideration is applied.