Environmental Engineering FOURTH EDITION

38 ENVIRONMENTAL ENGLNEEIUNG

not actual frequencies or probabilities, but do reflect, in general, the probability of each
branch of the event tree. An event tree is helpful in identifying all possible outcomes
of any situation.
In general, the risks attendant on exposure to hazardous substances are conditional
probabilities with multiple conditions; that is, the probability of cancer fatality is the
product of three probabilities: the probability of exposure, the probability that the
exposure will result in cancer, and the probability that the resulting cancer will be
the cause of death. All three of these probabilities are considerably less than unity:
“exposure,” for example, means that the carcinogen finds a sensitive target.

DOSE-RESPONSE EVALUATION

Dose-response evaluation is required both in determining exposure scenarios for the
pollutant in question and in characterizing a health effect. The response of an organism
to a pollutant always depends in some way on the amount or dose of pollutant to the
organism. The magnitude of the dose, in turn, depends on the exposure pathway. The
same substance may have a different effect depending on whether it is inhaled, ingested,
or absorbed through the skin, or whether the exposure is external. The exposure path-
way determines the biochemistry of the pollutant in the organism. In general, the human
body detoxifies an ingested pollutant more efficiently than an inhaled pollutant.
The relationship between the dose of a pollutant and the organism’s response
can be expressed in a dose-response curve, as shown in Fig. 3-2. The figure shows
four basic types of doseresponse curves possible for a dose of a specific pollutant
and the respective responses. For example, such a curve may be drawn for various
concentrations of carbon monoxide, the dose, plotted against the associated blood
concentrations of carboxylated hemoglobin, the response. Some characteristic features
of the doseresponse relationship are as follows:

1. Threshold. The existence of a threshold in health effects of pollutants has
   been debated for many years. A threshold dose is the lowest dose at which there is

Threshold @

100

Response

Detectable

response

0

0 TLV

Dose

Figure 3-2. Possible dose-response curves.