# Thallium Poisoning: Case Report and Scoping Review on Diagnostic Delay and Therapeutic Outcome
## 1 Introduction  
Tasteless, colorless, odorless, and soluble in water, thallium is a chemical element that has been known to cause accidental intoxication as well as intentional poisoning. In Western countries, thallium zinc sulfate was used in the 1950s as rat and ant poison [1] but was banned, firstly, in American homes in 1965 and then, commercially in 1975. However, thallium is still utilized in rodenticides and insecticides in several countries where, on account of its odorless and tasteless properties, it has led to both unintentional poisoning and, in some cases, illegal use. In medicine, thallium salts were previously employed in the treatment of dermatophytosis [2]. Prior to the adoption of technetium-99m in nuclear medicine, thallium-201, a radioactive isotope, was the primary agent [3] and continues to be utilized in neuroradiology [4], radiotherapy [5], and scintigraphy [6], and for stresstests assessing coronary artery disease [7]. This kind of thallium is around 4000 times less powerful than zinc sulfate. In industry, thallium is a vital element commonly used in high-technology industries where there is currently a growing demand for semiconductors. The occupational exposure limit for thallium is 0.1 milligrams (mg) per cubic meter for no longer than eight hours a day. Acutely hazardous levels are those reaching 15 mg/m\({}^{3}\) and over. Thallium can easily enter the body via skin absorption and inhalation. Cases of occupational thallium intoxication have been reported, mainly in Asian countries [8; 9; 10]. Wastewater discharges from manufacturing plants or metal-mining activities may result in elevated levels of thallium in the receiving water. For this reason, it is a priority pollutant regulated by the American Environmental Protection Agency (US EPA) [11; 12; 13; 14]. Thallium has been utilized as a pesticide in Africa, where it has led to food contamination. Chinese herbal medicines have also been compromised. Intoxication results from cumulative absorption via dermal, respiratory, and gastrointestinal pathways. Instances of inadvertent snorting by cocaine users [15], unintentional injection by heroin users [16; 17; 18], and dermal absorption through protective gloves have been documented [9]. Thallium contamination has cumulative effects. The chronic exposure of populations to this pollutant has been associated with many pathologies. Thallium urinary levels have been associated with a reduction in cognitive performance in older US adults [19], with Parkinson's disease [20], and also with a risk of gestational diabetes [21]. Plasma thallium levels have been associated with decreased renal function [22], central obesity [23; 24], ischemic stroke [25] and increased all-cause and cardiovascular mortality in the general population in China [26]. Studies with DNA methylation probes in exposed populations indicated alterations that could be implicated in cancer progression and respiratory diseases [27]. A longitudinal study showed that thallium exposure at low concentration leads to the early damage of multiple organs in children [28]. Thallium (Tl+), one of the most toxic heavy metals, shares an ionic ratio similar to potassium (K+), allowing it to replace K+ in enzymatic processes. Its primary target within cells is the mitochondria, where it disrupts intrinsic pathways, affects antiapoptotic and proapoptotic proteins, and activates oxidative stress mechanisms. Tl+ exposure leads to increased reactive oxygen species (ROS) and lipid peroxidation, causing cellular damage and triggering antioxidant responses. In humans, Tl+ is absorbed through skin and mucous membranes, distributing across organs like bones, kidneys, liver, and the central nervous system, with neurotoxic effects that are now recognized as a significant global health concern due to rising reports of Tl+ pollution [29]. The establishment of surveillance protocols to monitor toxic substances like thallium is essential, particularly in high-risk environments and workplaces.