I = 1 - S for S > 1.0 (less than additive interaction) (19.23b)

The resulting index produces a I value of 0.0 for additive toxicity, greater than 0.0 for positive interaction, and less than 0.0 for negative interaction.


The picture of dose-response relationships we have been describing so far is a static one. It assumes that all measurements are taken at a fixed interval after the first exposure. Inclusion of time adds another dimension to the response and is an important qualifier in any toxicity data. A 96-hour LD50 may seriously underestimate the risk if the situation being studied involves chronic, ongoing exposure.

The progress of a toxicity test can be followed by examining the toxicity curve, made by plotting the logarithm of time vs. the logarithm of the LD50. The toxicity curve indicates when acute lethality has ceased by leveling off parallel to the time axis (Figure 19.5). The LD50 associated with the plateau region is called the threshold LD50 (denoted as dt on Figure 19.5), also called asymptotic or incipient LD50. If a plateau is not reached, the LD50 computed will be sensitive to the experimental interval selected, and it would be advisable to extend the test. The toxicity curve may assume other shapes as well. This can provide the experimenter with information about how the compound acts or may indicate the presence of multiple toxic agents in a tested mixture. In Figure 19.5 the threshold LD50 is 5 mg/L. The figure also indicates that the LD50 would be 13.6 if the organisms were exposed for just 24 hours.


Bioassay results are used to compute a variety of parameters related to the toxicity of an agent. Table 19.6 summarizes some of the more common ones, along with acronyms used

Time (hrs)

Figure 19.5 Hypothetical time-toxicity curve.

Time (hrs)

Figure 19.5 Hypothetical time-toxicity curve.

TABLE 19.6 Acronyms Describing Toxic Doses or Concentrations

LDx, LCx

Lethal dose (or concentration)

Dose or concentration resulting in mor-

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