Pathways of Glycols and Their Toxicity

Matsuoka et al. (2005) reported a study on electro-oxidation of methanol and glycol and found that electro-oxidation of ethylene

Glyoxal

Ethylene glycol Glycol aldehyde (CH2OH)2 CH2OH(CHO)

Glyoxylate

Glyoxylate

No poisoning

No poisoning

Figure 12.9 Ethylene Glycol Oxidation Pathway in Alkaline Solution (Matsuoka et al. 2005).

Figure 12.9 Ethylene Glycol Oxidation Pathway in Alkaline Solution (Matsuoka et al. 2005).

glycol at 400mV forms glycolate, oxalate, and formate (Figure 12.9). The study further reports that glycolate is obtained by three-electron oxidation of ethylene glycol and is an electrochemically active product even at 400 mV, which leads to further oxidation of glycolate. Oxalate was found to be stable and no further oxidation was seen or termed as a non-poisoning path. The other product of glycol oxidation is called formate, which is termed as a poisoning path or CO poisoning path. A drastic difference in ethylene glycol oxidation was noted between 400 and 500 mV. The glycolate formation decreased 40-18% and formate increased 15-20%. In case of methanol oxidation, the formate was oxidized to COz, but ethylene glycol oxidation produces CO instead of C02 and follows the poisoning path over 500 mV. The glycol oxidation produces glycol aldehyde as intermediate products. As the heat increases, the CO poisoning may also increase.

Glycol ethers are known to produce toxic metabolites such as the teratogenic methoxyacetic acid during biodégradation, and the biological treatment of glycol ethers can be hazardous (Fischer and Hahn 2005). It was reported that abiotic degradation experiments with ethylene glycol showed that the by-products are monoethylether (EGME) and toxic aldehydes, e.g., methoxy acetaldehyde (MALD).

Glycol passes into the body by inhalation or through the skin. Toxicity of ethylene glycol causes depression and kidney damage (MSDS 2005). As indicated in the MSDS report, ethylene glycol in the form of dinitrate can have harmful effects when breathed in, and by passing through the skin it can irritate the skin causing a rash or burning feeling on contact. It can also cause headache, dizziness, nausea, vomiting, abdominal pain, and a fall in blood pressure. High concentration levels can interfere with the ability of the blood to carry oxygen, causing headache, dizziness, a blue color to the skin and lips (methemoglobinemia), breathing difficulties, collapse, and even death. This can damage the heart, causing pain in the chest and/or increased heart rate, or it can cause the heart to beat irregularly (arrhythmia), which can be fatal. High exposure may affect the nervous system and may damage the red blood cells, leading to anemia (low blood count). The recommended airborne exposure limit is 0.31 mg/m3 averaged over an 8-hour work shift. During a study of the carcinogenetic toxicity of propylene glycol on animals, skin tumors were observed (CERHR 2003).

Ingestion of ethylene glycol is a toxicological emergency (Glaser DS 1996). It is commonly found in a variety of commercial products including automobile antifreeze, and if ingested it will cause severe acidosis, calcium oxalate crystal formation and deposition, and other fatal organ damage (Davis et al. 1997). It is a high volume production (HPV) chemical generally used to synthesize polyethylene terephthalate (PET) resins, unsaturated polyester resins, polyester fibers, and films (SRI 2003). Moreover, ethylene glycols are a constituent in antifreeze, deicing fluids, heat transfer fluids, industrial coolants, and hydraulic fluids. Several studies have consistently demonstrated that the kidney is a primary target organ after acute or chronic exposures of ethylene glycol (NTP 1993; Cruzan et al. 2004). It has also been reported that renal toxicity, metabolic acidosis, and central nervous system (CNS) depression are reported in humans in intentional or accidental overdosing (Eder et al. 1998). Browning and Curry (1994) reported that because of widespread availability, serious health concerns have been shown for the potential toxicity of ethylene glycol ethers. From the review of these literatures, it is obvious that glycol has health and environmental problems. Hence, searching for alternative materials that have less environmental impacts is very important.

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