McKay, Donald. "Front matter" Multimedia Environmental Models ...

8.14.1 Introduction
©2001 CRC Press LLC
8.14 HUMAN EXPOSURE TO CHEMICALS
The multimedia environmental models described in this chapter lead to estimates
of fugacities and concentrations in air, water, soil, and sediments. These abiotic
fugacities can be used to deduce fugacities and concentrations in fish, and possibly
in other animals and plants. The primary weakness is probably that they do not yet
adequately quantify partitioning into the variety of vegetable matter that is consumed
by animals and humans. For some compounds, such as the dioxins, the air-grasscow-milk-dairy
product-human route of transfer is critical. In this chapter, we discuss
briefly the principles by which these concentration data can be used to assess the
impact of chemicals on humans and other organisms. The reader is directed to
reviews such as that by Paustenbach (2000) for a detailed treatment of exposure and
risk assessment.
The first obvious use of these abiotic and biotic concentrations is to compare
them with concentration levels that are believed to cause adverse effects. These
levels are usually developed by regulatory agencies and published as guidelines,
objectives, or effect-concentrations of various types. Target or objective concentrations
can be defined for most media. For example, from considerations of toxicity
or aesthetics, it may be possible to suggest that water concentrations should be
maintained below 1 mg/m 3 , air below 1 mg/m 3 , and fish below 1 mg/kg. These
concentrations can be compared as a ratio or quotient to the estimated environmental
concentrations. A hypothetical example is given in Table 8.4, illustrating the quotient
method. In this example, the primary concern is with air inhalation and fish ingestion.
The proximities of the estimated prevailing concentrations to the targets are
expressed as quotients, which can be regarded as safety factors. A large quotient
implies a large safety factor and low risk. The high-risk situations correspond to low
quotients. This quotient is also called a toxicity/exposure ratio or TER. The concentration
level in fish may not be directly toxic to fish but may pose a threat to humans
if the fish is consumed on a regular basis. The reciprocal ratio is also used in the
form of a PEC/PNEC ratio, i.e., predicted environmental concentration/predicted no
effect concentration. In this case, a high value implies high risk.
Table 8.4 Comparison of Predicted or Measured Environmental Concentrations with
Concentrations Producing a Specified Effect, or No Effect
Concentrations
Medium Predicted Level Effect Level Quotient
Air (m/m3 ) 3 60 20
Water (mg/L) 10 3000 300
Fish (mg/g) 2 10 5
Soil (mg/g) 1 100 100
Difficulties are encountered when suggesting target concentrations in soil and
sediment, because these media are not normally consumed directly by organisms.