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A well understood substance

Chemically, propylene glycol has a very simple structure:

The two parts of the molecule, the alcohol groups ("OH", likes water) and the hydrocarbon backbone ("CH", likes oil), represent fundamental chemical characteristics. Their behaviour both in industrial processes, as well as in the human body or in the environment, is well understood.

Humans are exposed to propylene glycol primarily through oral intake or skin contact. It is expected to be readily absorbed by oral exposure whereas uptake through the skin is very low. Once in the body, under conditions of normal low exposure, propylene glycol is quickly metabolised and excreted.The half-life of propylene glycol in the blood stream is about 2-4 hours. It is primarily metabolised to lactate which is further metabolized to pyruvate, carbon dioxide and water. Lactate also contributes to glucose formation through gluconeogenic pathways. Very large exposures to propylene glycol may result in lactic acidosis and hyperosmotic changes in the blood.

Studies show that propylene glycol has a very low degree of toxicity.  It is a low concern for acute toxicity by ingestion, skin contact, and inhalation, although transient signs of altered nervous system function (commonly observed with short-chain glycol exposure) are observed with oral exposure to high levels. Additionally blood pH and osmotic changes may occur with high levels of propylene glycol that over-load normal metabolism. Propylene glycol is non-irritating to the skin and eye, and exhibits a very low skin sensitization potential. Long-term studies in rodents conducted with high oral doses found no evidence of adverse effects. Ingestion by cats, however, results in species-specific haematological changes. High aerosol concentrations inhaled by rats caused minor nasal and ocular signs that may have been due to mild irritation or drying effects of propylene glycol on mucous membranes. Tests conducted in bacteria, mammalian cells, and in animals demonstrate propylene glycol is not utagenic/ genotoxic. Long term toxicity studies conducted in rodents and dogs demonstrate that this substance is not a carcinogen. Studies conducted in rodents with high oral doses indicate monopropylene glycol is not toxic to reproduction or development.

In the environment, tests have shown that propylene glycol is a low concern. The Log Kow value for propylene glycol is low, indicating a tendency to partition to aqueous phases and a low potential for bioaccumulation.  In addition, propylene glycol is readily biodegradable in both fresh water and seawater and is not expected to be persistent in the environment. Several acute aquatic toxicity studies covering three trophic levels (including marine organisms) plus chronic test data (fresh water invertebrates) are available and together these indicate that propylene glycol is of low
concern for ecotoxicological hazard.