In other words, a drug with a large difference between the lethal and therapeutic dose is safer than one with a smaller difference. If a threshold dose of a chemical exists, then a concentration of that chemical in the environment below the threshold is safe. If there is no threshold dose, then even the smallest amount of the chemical has some negative effect. Whether or not there is a threshold for environmental toxins is an important environmental issue. For example, the U.S. Federal Clean Water Act originally stated a goal to reduce to zero the discharge of pollutants into water.
The goal implies there is no such thing as a threshold that no level of toxin will be legally permitted. However, it is un- realistic to believe that zero discharge of a water pollutant can be achieved or that we can reduce to zero the concentration of chemicals shown to be carcinogenic. A problem in evaluating thresholds for toxic pollutants is that it is difficult to account for synergistic effects. Little is known about whether or how thresholds might change if an organism is exposed to more than one toxin at the same time or to a combination of toxins and other chemicals, some of which are beneficial.
Exposures of people to chemicals in the environment are complex, and we are only beginning to understand and conduct research on the possible interactions and consequences of multiple exposures. Dose response differs among species. For example, the kinds of vegetation that can live nearest to a toxic source are often small plants with relatively short lifetimes (grasses, sedges, and weedy species usually regarded as pests) that are adapted to harsh and highly variable environments.