What Are the Toxicological Effects of Carbon Tetrachloride Exposure?

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• Describe the toxicological effects associated with carbon tetrachloride (CCl₄) exposure.

The primary effects of CCl₄ in humans are on the central nervous system (CNS), liver, and kidneys. Symptoms of acute inhalation or ingestion of CCl₄ include

• Headache,
• Weakness,
• Nausea, and
• Vomiting.

Chronic exposure to carbon tetrachloride – and sometimes acute exposure to very high concentrations – produces liver and kidney damage [EPA 2000].

Acute Efects

The immediate effect of acute CCl₄ exposure by all routes is CNS depression [Laine and Riihimaki 1986].

The intensity of the effects is proportional to exposure dose. Symptoms can include initial euphoria and disinhibition, followed by

• Dizziness,
• Nausea and vomiting,
• Incoordination,
• Paresthesia,
• Increased salivation, and
• Tachycardia.

The symptoms are generally transient, disappearing quickly after the exposure ends. Higher-dose exposures, however, can lead to

• Respiratory depression (as a result of CNS depression),
• Seizures,
• Coma, and
• Death [Rom and Markowitz 2007].

In fatal cases, autopsies reveal permanent damage to nerve cells, with focal areas of fatty degeneration and necrosis [Stevens et al. 1953; Cohen 1957].

Chronic Effects

The CNS effects of chronic exposure are open to question. Many of the impairments observed in workers chronically exposed to solvents could be attributable to other causes [Rom and Markowitz 2007], such as

• Chronic alcohol abuse,
• Other neurologic disorders, or
• Injuries.

Chronic neurologic effects can be classified as follows:

• Mild – consisting mainly of affective changes and loss of concentration,
• Moderate – with some impairment of concentration and memory, or
• Severe – with significant loss of intellectual functioning [WHO 1985].

Sensorimotor neuropathy and abnormalities of vision have been reported, but epidemiological data are insufficient to support an association between CCl₄ exposure and these effects [O’Donoghue 2000].

Liver damage occurs more often from swallowing liquid CCl₄ than from inhaling CCl₄ vapor [Hathaway et al. 1991]. Direct skin exposure to high doses of CCl₄ can also cause hepatic effects in humans [ATSDR 2010; Gummin 2015].

Acute hepatic injury usually manifests after CNS effects have subsided, typically 1 to 4 days after acute exposure. Chronic hepatic injury (cirrhosis) takes longer to develop.

The typical signs of liver injury are nonspecific and include

• Swollen and tender liver (acute);
• Elevated levels of hepatic enzymes (e.g., AST and ALT);
• Elevated serum bilirubin levels, with or without jaundice;
• Decreased serum levels of proteins, such as albumin and fibrinogen; and
• Elevated prothrombin time (PT) or international normalized ratio (INR).

Acute exposure to CCl₄ causes a hepatocellular pattern of injury, with elevated AST and ALT and primarily centrilobular (zone 3) damage. This occurs because CYP2E1 enzymes are concentrated in the perivenous (zone 3) region of the hepatic acinus, and accordingly, the highest concentrations of CCl₃ are produced in this region first. In cases of lethal exposures, this histologic variation in injury is lost, and pronounced diffuse hepatic steatosis and frank liver necrosis can occur.

In the case of chronic exposure, ongoing subclinical cell death promotes activation of stellate cells and collagen deposition. Over time, this results in fibrosis and cirrhosis. As hepatic synthetic function fails, a decrease in clotting factors might predispose the patient to hemorrhage [Zimmerman and Ishak 2002; ATSDR 2005]. Additionally, continual oxidative stress from CCl₃ causes DNA damage, protein malfunction, and disrupted calcium homeostasis. These mechanisms, in addition to increased cell death and turnover, are thought to lead to steatosis and carcinogenesis [Palmer and Phillips 2007].

Persons at increased risk for CCl₄ induced hepatotoxicity

The toxic metabolites of CCl₄ are produced from reactions catalyzed by cytochrome P450 enzymes, particularly CYP2E1 and CPY3A4 [Zimmerman 1986; Recknagel et al. 1989; Weber et al. 2003; Manibusan et al. 2007]. Although no human data clearly define the relationship between CYP2E1 or CYP3A4 activity and CCl₄ toxicity, animal studies have shown that CYP2E1 activity is positively correlated with the degree of CCl₄-induced hepatotoxicity [Wong et al. 1998; Dai et al. 2014]. Thus, patients with a history of chronic, heavy alcohol intake (which induces CYP2E1) or patients who are on medications known to induce CYP3A4 (e.g., barbiturates, protease inhibitors) might be at increased risk for free radical damage resulting from increased bioactivation of CCl₄.

Nephritis and nephrosis are common following CCl₄ exposure. A number of derangements might appear within hours to days after exposure:

• Proteinuria,
• Hemoglobinuria,
• Glucosuria,
• Oliguria, and/or
• Anuria.

The mechanism of nephrotoxicity is thought to be similar to the pathophysiology of liver toxicity: bioactivation by cytochrome P450 enzymes to the CCl₃ radical, with resulting oxidative injury [Abraham et al. 1999; Ozturk et al. 2003]. The intracellular and cell membrane damage is seen as proximal tubule cell edema and vacuolization, protein leakage into the tubule lumen, glomerular necrosis, and interstitial hemorrhage [Elmubarak 2015; Yoshioka et al. 2016; Yoshioka et al. 2016].

Pulmonary effects can occur through various means of exposure:

• Inhalation [Kirkpatrick and Sutherland 1956; Love and Miller 1951],
• Oral [Ruprah 1985], and/or
• Parenteral [Das et al. 2014; Ferrari et al. 2012; Zhang et al. 2014].

Signs of respiratory damage appear to be exceedingly rare in human case reports. However, autopsies of humans and pathological evaluations in animal studies consistently show numerous gross and histological findings, including:

• Pulmonary edema,
• Interstitial and alveolar hemorrhage,
• Epithelial cell damage and death,
• Alveolar infiltrates,
• Damage to pulmonary vasculature,
• Alveolar wall thickening, and
• Fibrosis [Naz et al. 2014; Taslidere et al. 2014].

The mechanism of pulmonary toxicity is multifactorial and includes:

• Direct bioactivation of CCl₄ to CCl₃ and other free radicals by cytochrome P450 enzymes in the lung parenchyma [Boyd 1980], specifically by CYP2E1 [Gundert-Remy et al. 2014],
• Hepatopulmonary syndrome [Zhang et al. 2014], and
• Direct and indirect oxidative stress [Das et al. 2014; Ferrari et al. 2012].

A number of epidemiological studies have evaluated the association between occupational exposure to CCl₄ and cancer risk. Evidence remains inadequate to make definitive conclusions about the carcinogenicity of carbon tetrachloride in humans [IARC 1999; ATSDR 2005; NTP 2016].

In animal studies [IARC 1999; Manibusan et al. 2007; Nagano et al. 2007], CCl₄ has induced hepatocellular carcinomas in rodents via all exposure routes. With sufficient evidence of carcinogenicity in experimental animals such as this, and limited evidence of carcinogenicity in humans, various agencies have concerns about the risks for cancer from CCl₄ exposure.

• The International Agency for Research on Cancer [IARC 1999] has determined that CCl₄ is possibly carcinogenic to humans (Group 2B).
• The National Institute for Occupational Safety and Health (NIOSH) has identified CCl₄ as a potential occupational human carcinogen [NIOSH 2005].
• The American Conference of Governmental Industrial Hygienists (ACGIH) considers CCl₄ to be a suspected human carcinogen [ACGIH 2016].
• The National Toxicology Program reports that CCl₄ is reasonably anticipated to be a human carcinogen [NTP 2016].

CCl₄ has been extensively studied for its genotoxic and mutagenic effects, with largely negative results. When such changes have been seen, they have generally been related to hepatic cytotoxicity. Mutagenic effects, if they occur, would likely be generated through indirect mechanisms resulting from oxidative and lipid peroxidative damage [ATSDR 2005; Manibusan et al. 2007].

No information is available on the reproductive effects of CCl₄ in humans. Epidemiological studies have investigated possible associations between oral exposure to carbon tetrachloride and various adverse birth outcomes [Croen et al. 1997; Bove et al. 1995, 1992a, 1992b]. Because of multiple chemical exposures and insufficient power, these studies are considered limited and insufficient to determine whether carbon tetrachloride exposure and adverse birth outcomes are associated [EPA 2010]. CCl₄ can induce embryotoxic and embryo lethal effects, but only at doses that are toxic to the mother, as observed in the inhalation studies in rats and mice. No adequate reproductive toxicity studies have been conducted in animals exposed by the oral route. Teratogenicity has not been observed in the offspring of rats orally exposed to CCl₄ [ATSDR 2005; WHO 1999; EPA 2010].

Most case reports of human CCl₄ toxicity do not have cardiovascular injury as a predominant feature. However, volatile hydrocarbons, particularly the halogenated hydrocarbons, are associated with cardiac dysrhythmias and sudden sniffing death [Adgey 1995; Williams and Cole 1998]. The mechanism is not fully known, but is thought to involve “sensitization” of the myocardium to the effects of catecholamines, increasing heart rate, QT dispersion, and the rate of after-depolarization. A large acute exposure to CCl₄ could possibly produce similar clinical effects.

Case reports have described cardiomegaly, congestive heart failure, and cardiac fibrosis after exposure to CCl₄. The cardiac effects are thought to occur largely secondary to the fluid overload caused by hepatic and renal damage.

More recently, multiple cytochrome P450 enzymes have been found in cardiac tissue, including the CYP 2E family of isoenzymes [Chaudhary et al. 2009]. Though the exact role of each CYP isoform in myocardial cells is not yet clear, many of these appear to be involved in lipid and drug metabolism. Rodents exposed to CCl₄ demonstrate increased markers of inflammation, myocyte injury (troponin, CK-MB) [Al-Rasheed et al. 2014], and oxidative stress in the heart [Manna et al. 2007; Jayakumar et al. 2008]. These biochemical changes might reflect a systemic increase in oxidative stress caused by CCl₄, local bioactivation of CCl₄ with resulting myocardial damage, or both.

Common gastrointestinal effects of exposure to CCl₄ include

• Nausea,
• Vomiting, and
• Abdominal pain.

Prolonged or repeated contact of skin with liquid CCl₄ can cause defatting of the skin and dermatitis, with redness and blister formation [NIOSH 1995].

• The immediate effect of acute CCl₄  exposure by all routes is CNS depression.
• The NTP classified CCl₄ as “reasonably anticipated to be a human carcinogen based on sufficient evidence of carcinogenicity from studies in experimental animals” [NTP 2016].