## 4 Discussion
### Toxicological Parameters  
During the experiment, no death of the experimental animals was recorded in any of the experimental groups (Cd, Hg or K). Thus, the survival was 100% and at the same time it was confirmed that the applied dose is low chronic dose. In a lifelong experiment [9] evaluating the survival of animals exposed to cadmium, more individuals survived (one-year survival was also 100% as confirmed by the experiments) compared to other heavy metals. This may be because cadmium in low doses may function as an essential element [10; 11; 12]. [PERSON] et al. [13] also did not report any death of exposed animals during a chronic experiment in rats lasting 90 days, administering CdCl\({}_{2}\) in drinking water per os with a predicted daily dose of cadmium of 2.5 mg\(\cdot\)kg\({}^{-1}\), similar to this study. A second possible explanation may be that rats received only 47.16% of LD\({}_{50}\) for a single administration per os throughout the experiment, which may also have resulted in no mortality. As reported by [PERSON] and [PERSON] [14], the LD\({}_{50}\) for single intoxication of Cd ingested as CdCl\({}_{2}\) per os in rats is 225 mg\(\cdot\)kg\({}^{-1}\) of live weight. The fact that no death of experimental animals was recorded, even in the group exposed to mercury, may be related to the low total dose received, the LD\({}_{50}\) for Hg taken in the form of HgCl\({}_{2}\) per os for single intoxication is a dose of about 37 mg\(\cdot\)kg\({}^{-1}\) of live weight [15]. In the experiments, rats received only 55.51% of LD\({}_{50}\) for a single oral administration throughout the experiment.  
At lifetime exposure to cadmium and mercury (156 days) in rats [16], HgCl\({}_{2}\) was administered at a concentration of 1 \(\upmu\)mol\(\cdot\)L\({}^{-1}\), i.e., 0.2 mg of Hg per 1 L of drinking water, and CdCl\({}_{2}\) in a concentration of 20 \(\upmu\)mol\(\cdot\)L\({}^{-1}\), i.e., 2.0 mg of Cd per 1 L of drinking water. In the group exposed to Cd, 80% of animals survived; in the group exposed to Hg, only 40% of animals survived. [PERSON] et al. [17] stated that, at a concentration of 20 \(\upmu\)mol\(\cdot\)L\({}^{-1}\) CdCl\({}_{2}\) in drinking water, the survival of rats in the cadmium-exposed group was 10% lower than in the control group. In all the above works, however, no mortality was found in exposed animals for a period of one year, which is also confirmed by the experiments presented in this work. The mortality of animals in experiments with a similar focus can be significantly affected by breeding, handling conditions and overall welfare of the animals. For these reasons, in long-term experiments, it is necessary to adapt the conditions (experimental design) as much as possible to the natural method and the needs of the experimental animals. Ensuring adequate physical activity, daily physiological needs (e.g., shelter and cleaning) and well-being plays an important role and can fundamentally affect the results of an animal experiment. In the experiment, it is important to consider not only the duration of exposure but also other factors, e.g., concentration of toxic substance, light regime, handling and sampling time, heat stress, psychologically induced stress, etc. [18].