Some time ago, maybe 30 years ago, winterizing anything with ethylene glycol (EG) became completely unfashionable. Politically incorrect was the new term. Everything had to switch to propylene glycol (PG) because it was lower in toxicity and thus more environmentally friendly. We had always understood that PG must be used in potable water systems because of EG's toxicity and the potential for someone to take a drink before the system was well flushed. Reasonable enough, and we assumed, right or wrong, that people weren't chemistry smart enough to keep these differences straight. But the connection between toxicity to humans and damage to the environment was never correct. It turns out EG and PG are equivalent and it is only metabolic pathways unique to mammals that make EG dangerous to people.
MSDS Information. Producers of products with any hazardous or toxic ingredients must disclose and explain these to commercial users through OSHA's MSDS program that most of us are familiar with. Though there are many products using EG and PG, these 2 sheets present representative data from the same manufacture:
Ecotoxicity in water (LC50): >5000 mg/l 24 hours [Goldfish]. >10000 mg/l 48 hours [guppy]. >10000 mg/l 48 hours [water flea].
Ecotoxicity in water (LC50): 41000 mg/l 96 hours [Fish (Trout)]. 46300 mg/l 48 hours [water flea]. 34250 mg/l 96 hours [Fish (bluegill fish)]. 34250 mg/l 72 hours [Fish (Goldfish)].
Though they have expressed the data in different terms, we can readily see that acute toxicity requires percent levels (about 3-4%) of glycols; as a practical matter, the fish will die from low dissolved oxygen before direct glycol toxicity is important. Why the great difference? EG poisons only at relatively high doses in humans, primarily by the action of specific metabolic byproducts; it should be no great surprise that marine organisms deal byproducts and salts differently. This turns out that this is true of all non-mammal organisms. I don't see this as a surprising contrast; zinc is incorporated into many bottom paints because of its high marine toxicity (0.5ppm is quite lethal) and yet the pharmacy sells zinc pills for humans as a dietary supliment. You don't feed chocolate to a dog.
Biodegradability. No important differences. Both are attacked by bacteria as readily as sugar. In fact, when glycols are used in the air conditioning systems of large buildings it is required practice to either add a biocide or to control the pH at very high levels, which is also lethal to all organisms. Both products exhibit equivalent biological oxygen demand (BOD) is water and both can cause fish kills by this mechanism if discharged in large amounts to small waterways. This is occasionally seen around airports that deice airplanes in the winter. Again, they have found no difference between EG and PG. There is a preference for PG over EG in this case because they spill large amounts--perhaps 5000 to 20000 gallons during a snow event--and for mammal wild life to drink from puddles is plausible.
From the experts: In fact, I could not find any scholarly (fact-based) work that gave a contrasting opinion.
From Risk World (European conference)
"Based upon the limited available data, no general distinction can be made between aquatic toxicities of ethylene and propylene glycol formulations."
From the World Health Organization (WHO)
"Ethylene glycol has generally low toxicity to aquatic organisms. Toxic thresholds for microorganisms are above 1,000 mg/litre. EC50s for growth in microalgae are 6,500 mg/litre or higher. Acute toxicity tests with aquatic invertebrates where a value could be determined show LC50s above 20,000 mg/litre, and those with fish show LC50s above 17,800 mg/litre. An amphibian test showed an LC50 for tadpoles at 17,000 mg/litre. A no observed-effect concentration (NOEC) for chronic tests on daphnids of 8,590 mg/litre (for reproductive endpoints) has been reported. A NOEC following short-term exposure of fish has been reported at 15,380 mg/litre for growth."
From the US EPA, study of airport deicing with EG and PG. Please, read the whole study (447 pages). Marine toxicity is never mentioned in the executive summary because they could make no case against it. They are the same. Neither is good, true enough, but they are the same. They did conclude--correctly based upon my expereince in marine toxicity testing--that the thickeners used in wing deicers are quite toxic because of their effect on gill tissues, but this is not relevant here. Many "non-toxic" cleaners, even common dish soaps such as Palmolive and Dawn, are quite toxic to marine shrimp because of this effect. I've done those studies. They also report that some of the anti-corrosion additives are toxic, it is important to note that as diluted in the formulation for use in a boat (about 3000:1) they contribute very little.
"Table 9-1 summarizes aquatic toxicity data from studies that directly compare ethylene glycol and propylene glycol under the same or similar experimental conditions. In general, the data show that ethylene glycol and propylene glycol exhibit aquatic toxicological effects at concentrations within the same order of magnitude. Although EPA does not use such a system, the U.S. Fish and Wildlife Service Classification System for Acute Exposures defines “relatively harmless” as any chemical with an LC above 1,000 mg/L (3)."
What about the corrosion inhibitors? In PG formulations common inhibitors are dipotasium phosphate and other relatively harmless salts. In engine coolants organic acids and azoles are common. 2-ethylhexanoic acid has an LD 50 for marine organisms of around 100 ppm, but multiply this by 133 for actual use concentrations (LD 50 = 13,000 ppm).
tolyltriazole (TT) has an LD 50 of about 50 ppm, but multiply this by 6000 for actual use rates (LD 50 = 300,000 ppm). Both are biodegradeable and neither is thought to bioaculmulate. So while the inhibitors do contribute to toxicity, small amounts of diluted engine coolants remain far below levels of concern.
What conclusions can we draw from this?
- EG and PG are simply no different when released into a large body of water.
- The conventional wisdom regarding EG vs PG marine pollution potential is just plain wrong. Perhaps the PG manufacturers have pushed this. Perhaps folks writing web site copy for good boating practices didn't actually research their writing; those I interviewed commonly related that their information was simply copied from another popular site.
- Both can be recycled, and this is a good practice whenever possible. But it isn't always possible and the damage of discharging a few gallons is equivalent to tossing an equivalent mass of wood chips in the water.
Why does this matter? Because PG is not so good for certain materials of construction and EG is better. Specifically neoprene and nylon don't react so well, and these are common in sanitary and engine systems. That's why.