Occupational exposure to arsenic and leukopenia risk: Toxicological alert

Exposure and toxicokinetic

Due to arsenic dispersion in the environment because of biological processes, volcanic activity, movement of dust, volatilization, and anthropogenic activities, humans can be exposed to arsenic by ingestion, inhalation, mucous membrane contact, and skin absorption, being dermal exposure not considered a primary exposure route. Accumulation occurs in the hair, skin, and nails (ATSDR, 2023; Ergün et al., 2017; Ganie et al., 2024; Genchi et al., 2022; Palma-Lara et al., 2020; Public Health England, 2019).

For the general population, the main contamination source is underground water (aquifers) (Ganie et al., 2024; Palma-Lara et al., 2020). However, anthropogenic activities are the principal exposure sources in countries without contaminated aquifers. These activities involve metallurgical, smelting, coal burning, production and use of pesticides, and mining for gold, silver, and other metals, with mining being the predominant source of arsenic environment dispersion (Ganie et al., 2024; Palma-Lara et al., 2020; Public Health England, 2019). Besides this environmental exposure, the population can be exposed to arsenic by using traditional medicine and herbal supplements (Public Health England, 2019).

In workplaces, the main source of workers’ exposure is the contaminated air by the inorganic arsenic in mines, arsenic or copper smelters industries, and chemical factories, such as wood preservation, vineyard spraying, nonferrous metal alloys, glass production, and electronic semiconductor production (Nurchi et al., 2020). The exposure can result from raw materials, intermediate and final products of industrial fungicides, and arsenic pesticides (Ergün et al., 2017; Palma-Lara et al., 2020).

Absorption depends on chemical form, time and type of exposure, solubility and pKa of the compound, and size of particles (Palma-Lara et al., 2020; Public Health England, 2019). Arsenic gas (arsine, AsH₃) is the most toxic compound with 30% to 60% of the inhaled arsenic form deposited and absorbed in the lungs, but inorganic arsenic (As³⁺ and As⁵⁺ forms) is easily absorbed in the gastrointestinal tract. Ninety-five to 99% of arsenic absorbed is transported in erythrocytes, and the distribution phase occurs in different organs with inorganic forms accumulating in muscles, nerve tissue, kidneys, bones, lungs, bladder, and, mainly, in the liver (Bissen and Frimmel, 2003; Ganie et al., 2024; Palma-Lara et al., 2020; Public Health England, 2019).

The main route of arsenic metabolism is hepatic and involves two metabolic processes: oxidation and reduction reactions to interconvert arsenate and arsenite; and non-enzymatic methylation to complex glutathione with arsenite-originating monomethyl arsine (MAI), monomethylarsonate (MMA^V), dimethylamine (DMA^III), dimethyl arsenate (DMA^V), trimethyl arsenic acid (TMA), and trimethylarsine oxide (TMAO). The excretion occurs primarily and rapidly via the kidneys followed by exhalation in the forms of MMA and DMA. Only 25% of the excretion forms are composed of inorganic arsenic (Bissen and Frimmel, 2003; Ganie et al., 2024; Palma-Lara et al., 2020; Public Health England, 2019). As kidney elimination is the main route of excretion, urinary arsenic concentration is a good indication of inorganic arsenic acute exposure (Public Health England, 2019).

Toxicodynamic and effects

The mechanism of arsenic toxicity is a complex and not fully understood interplay probably initiated by the increase of reactive oxygen species (ROS), which is one of the central processes of this intoxication. Due to the large quantity of ROS produced, there is an unbalance between the production and detoxification of ROS, which leads to the accumulation of this species and the initiation of oxidative stress. The excess ROS produced interferes with several signaling pathways involving the oxidative adaptations of biomolecules. These adaptations are related to protein loss function, organelle damage, and cell death (Ganie et al., 2024).

Higher levels of exposure have been associated with multiple health effects in different organ systems, including cardiac failure, peripheral neuropathy, vascular diseases, neurological, respiratory, gastrointestinal, and genitourinary, as well as hematopoietic systems, like anemia and leukopenia. This may lead to death (Cheng et al., 2004; Mazumder, 2015; Islam et al., 2004; Kuivenhoven and Mason, 2024; Tchounwou et al., 2003). In contrast, chronic exposure may cause a range of cancers as well as liver injury, neuropathy, cardiovascular lesions, and other diseases. The non-carcinogenic arsenic effects are related to known action mechanisms, such as cellular respiration inhibition, changes in heme synthesis and porphyrin metabolism, and changes in stress protein gene expression. These effects may be due to direct, cytotoxic, or hemolytic impact on the blood cells and erythropoiesis suppression (Cheng et al., 2004; Islam et al., 2004; Mazumder, 2015).

Hematopoietic and immune interference

The interference in the hematopoietic system occurs in acute and chronic poisoning, with the most common complications including hemolysis, leukopenia, granulocytopenia, thrombocytopenia, mild eosinophilia, profound anemia ranging from normochromic, normocytic to hypochromic, microcytic and hemolytic, and a reversible bone marrow depression with also reversible pancytopenia (ATSDR, 2023; Feussner et al., 1979; Harkins et al., 2000; Kuivenhoven and Mason, 2024; Lee et al., 2004; Shameem et al., 2015; Tchounwou et al., 2003).

One important mechanism of arsenic that interferes in the development of the hematopoietic and immune systems is inhibiting the Janus kinase (JAK)-signal transducer and the activation of the transcription (STAT) pathway. JAK-STAT is a widely expressed and important intracellular signal transduction pathway responsible for the mediating normal functions of several cytokines related to cell proliferation, differentiation, apoptosis, and hematopoietic and immune systems development and regulation (Cheng et al., 2004; Xin et al., 2020). Arsenic, in the form of sodium salt, inactivates JAK-STAT by inhibition of JAK tyrosine kinase and using the interferon γ (IFNγ) pathway (Figure 1) (Cheng et al., 2004; Dillon, 2021).

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Figure 1

Pathway of Janus Kinase (JAK)-signal. (a) JAK-signal transducer and activator of the transcription of hematopoietic and immune systems. (b) Inhibition of JAK-signal in the presence of sodium salt of arsenic. Source: based in Dillon (2021).

The intravascular hemolysis starts after the absorption phase when arsenic binds to the globin of hemoglobin in erythrocytes and is transported into the body, which increases the oxidation of sulfhydryl groups in hemoglobin. Added to this, there is a reduction in oxygen uptake by cells due to a decrease in intracellular glutathione, thus reducing the useful life of erythrocytes, which leads to anemia (Shameem et al., 2015).

Erythropoiesis inhibition is one of the arsenic effects and it is related to regulatory factor GATA-1. This factor is necessary for the normal differentiation of early erythroid progenitors. The zinc finger (ZF) transcription factor GATA-1 highly regulates normal erythropoiesis (Zhou et al., 2020). The arsenic inhibitory effect on erythropoiesis is related to (a) inhibition of Stat5 activation, with a consequent reduction in the expression of target genes Bcl-XL and glycophorin-A; (b) the activation of an apoptotic mechanism that leads to the cleavage of the erythroid transcription factors Tal-1 and GATA-1, the integrity of which is required for erythroid cell survival and differentiation (Saulle et al., 2006). Arsenic interacts with the N- and C-terminal zinc fingers of GATA-1, causing loss of zinc and inhibition of DNA and protein binding activities, leading to dyserythropoiesis and imbalance of hematopoietic differentiation. Arsenic-induced inhibition of erythropoiesis may be attributed to loss of GATA-1 function, resulting from selective interaction of As³⁺ with the ZFs on GATA-1, which are 4-cysteines configuration (Zhou et al., 2020).

The toxicity relationship between arsenic and erythropoiesis is well-documented, but only a few studies report leukopenia as a toxicity consequence of arsenic occupational exposure (Selgrade, 2007; Vahter, 2008). As well as anemia, leukopenia is common in chronic arsenic toxicity and is frequently associated with mild eosinophilia and thrombocytopenia (Harkins et al., 2000). Arsenic actions in the immune system are dose-dependent and can inhibit or induce immune cell proliferation, with macrophages being the major target of this toxicity (Shameem et al., 2015). The key mechanism by which arsenic promotes toxicity in macrophage function involves a failure in the signaling pathway in unfolded protein response, which is a homeostatic signaling network necessary to maintain macrophage viability and function by promoting recovery in the endoplasmatic reticulum (ER) when different agents, arsenic included, cause ER stress (Ramirez et al., 2019; Shameem et al., 2015).

When exposed to arsenic, macrophages quickly become rounded and lose their adhesion capability, affecting the markers’ expression on their surface, and leading to altered endocytosis and phagocytosis. Moreover, arsenic inhibits factors that stimulate monocyte maturation into macrophages. These effects combined reduce innate immune response, increasing susceptibility to infections (Shameem et al., 2015).

In the studies about leukopenia caused by arsenic, there is a demonstration that minimum doses of 50 μg/kg per day can promote those hematopoietic symptoms, mainly anemia and leukopenia (Islam et al., 2004). According to Luo et al. (2002), leukopenia is a potential toxicity reaction on the health of semiconductor industry workers exposed to multiple agents, including arsenic. These findings suggest that exposed workers have lower white blood cell counts than unexposed.

Occupational exposure limits

The U.S. Environmental Protection Agency (EPA) and World Health Organization (WHO) have set 10 ppb as the allowable level for arsenic in drinking water (maximum contaminant level) (ATSDR, 2023). The relationship of inorganic arsenic in drinking water to various cancers is well-documented at high exposure levels, generally >200 mg/L. The cancer risk assessment has usually involved a linear, non-threshold dose-response, indicating risks over regulatory guidelines even for drinking water exposures of 10 mg/L and lower (Tsuji et al., 2019).

The Occupational Safety and Health Administration (OSHA) mandates permissible limits for occupational exposures to inorganic arsenic in the air, being the permissible exposure limit of 10 μg/m³, averaged over any 8-h period for a 40 h workweek (ATSDR, 2023). The recommended exposure limit set by the National Institute for Occupational Safety and Health (NIOSH) is 2 μg/m³ for no more than 15 min, based on the classification of arsenic as a potential human carcinogen (ATSDR, 2023; NIOSH, 2014). Based on animal acute inhalation toxicity data, the revised Immediately Dangerous to Life or Health (IDLH) for inorganic arsenic compounds is 5 mg As/m³. OSHA currently requires that workers be provided with and required to wear and use the “most protective” respirators in concentrations exceeding 20 mg As/m³ (NIOSH, 2014). Table 1 resumes those limits for inorganic arsenic.

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Table 1.

Standards and regulations for inorganic arsenic.

Agency	Focus	Level	Comments
ACGIH	Air-workplace	10 µg/m3	Advisory; TLV/TWA
NIOSH	Air-workplace	2 µg/m3	Advisory; 15-min ceiling limit
OSHA	Air-workplace	10 µg/m3	Regulation; PEL over an 8-h day
EPA	Water-drinking	10 ppb	Regulation; maximum contaminant level in public drinking water supplies

According to some estimates, the lethal dose of inorganic arsenic is 0.6 mg/kg and death occurs 1 to 4 days after ingestion (Kuivenhoven and Mason, 2024). So, based on the high degree of toxicity to humans, and the non-threshold dose-response assumption, a zero-level exposure is recommended for arsenic, even though this level is practically non-attainable (Tchounwou et al., 1999).