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PG is a colorless, practically odorless, and rather viscous liquid. Unlike EG, it has no taste. Its physical and chemical properties are shown in Table 6-1.


PG is commonly used as a food additive, in cosmetics, and in medicinal agents. It is thought to have low toxicity and is used as a vehicle for intravenous (IV) and topical medications, as well as for cosmetics. Some medications in which notable amounts of PG are present are lorazepam (Ativan), etomidate (Amidate), diazepam, esmolol, nitroglycerin (Tridil), pentobarbital, phenytoin, phenobarbital, and trimethoprim/sulfmethoxazole (Bactrim). The percentage of PG in many injectable forms generally ranges from 40% to 80% in single-dose units. For example, each single dose of a lorazepam solution of 2 millig rams per milliliter (mg/mL) contains 0.8 mL of PG (of a 1-mL dose) as the drug vehicle, whereas phenytoin (Dilantin) contains 40% PG (see Zar et al. 2007b). The Food and Drug Administration considers PG safe for use in medication and cosmetics. It is also antibacterial, which makes it useful as a preservative and disinfectant. PG is the principal component of aircraft deicing and anti-icing fluids and of motor vehicle antifreeze. As the general weight of evidence in the toxicology literature supports the conclusion that PG-based fluids are preferable to EG-based fluids, the ISS program prefers to use PG rather than EG as a coolant.

TABLE 6-1 Physical and Chemical Properties of Propylene Glycol

Chemical formula


Chemical name

Propylene glycol


1,2-propanediol, 1,2-dihydroxypropane, methyl glycol

Molecular weight


CAS number


Water solubility

100 g/100 g of H2O at 25°C

Specific gravity


Melting point


Boiling point


Vapor pressure

0.07 mm Hg at 20°C; 0.13 mm Hg at 25°C

Concentration in air at saturation

170 ppm at 25°C (calculated from vapor pressure at that temperature)

Conversion factor

1 ppm = 3.2 mg/m3, 1 mg/m3 = 0.31 ppm

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