‘Non-toxic’ claims could be hazardous

CAN ETHYLENE GLYCOL-BASED HEAT TRANSFER AND ANTIFREEZE FLUIDS REALLY BE ‘NON-TOXIC’? PAUL DAY, TECHNICAL DIRECTOR AT SENTINEL, SMELLS A RAT.

‘Non-toxic’ is a claim made against many substances sold within the HVAC industry – and in many cases, fair enough. But sometimes there is a fine and often shady line drawn between what is reasonable and what could be misleading. Of specific concern is the description by some manufacturers of their ethylene glycol-based heat transfer and antifreeze fluids as ‘non toxic’.

Currently, the Classification, Labelling and Packaging of substances and mixtures (CLP) Regulation ((EC) No 1272/2008) asserts that “Statements such as ‘non-toxic’… indicating that the substance or mixture is not hazardous, or any other statements that are inconsistent with its classification, should not appear on the label or packaging of any substance or mixture” (Article 28). Even if the description was permissible, can any ethylene glycol-based fluid really be ‘non-toxic’?

Despite its rather benign, clear and colourless appearance, ethylene glycol (Ethane-1,2-diol, EG), a sweet-tasting liquid, has a potent toxic effect on humans. The fluid has an LDLO (Lowest Lethal Dose) of just 786 mg/kg in humans and a NOAEL (No Observable Adverse Event Level) of 150 mg/kg body weight per day. This means that a daily intake of just 9.7g would have a toxic effect on an average 65 kg person.

When ingested, ethylene glycol is oxidised by the enzyme alcohol dehydrogenase (ADH) to form glycolic acid, which has cytotoxic properties. This substance is further oxidised to form oxalic acid, which binds with calcium ions to form insoluble crystals of calcium oxalate. These crystals can become deposited in the brain, heart, kidneys and lungs, and can have serious health implications, including blindness.

Without a doubt, ethylene glycol can be toxic. But what about ethylene glycol-based fluids, namely those which are predominantly ethylene glycol mixed with other liquids - can the same be said for those? Some manufacturers state that their heat transfer and antifreeze products, which combine a high concentration of ethylene glycol along with a small proportion of propylene glycol, are ‘non toxic’, claiming that the addition of propylene glycol has a ‘detoxifying’ effect on ethylene glycol by acting as an ADH inhibitor. However, this is erroneous.

Unlike ethanol or fomepizole, propylene glycol is not proven to act as a competitive inhibitor to ADH in humans. Any claims to the contrary, which have been made by some manufacturers, are incorrect or based solely on animal studies. These manufacturers underpin their claims based on LD50 tests which, it is affirmed, yield results that indicate such fluids are harmless. However, these tests are undertaken on rats, and therefore only suggest that such fluids are less toxic to rats. There is considerable doubt that such results would transfer to humans.

The LD50 value on rats of even plain ethylene glycol is not low enough to attract a toxic classification, but because of its very low reported lethal dose in humans, it must be labelled with a standard warning hazard symbol alongside the words “Harmful if swallowed” (H302). This instruction is tacit recognition by the authorities that the LD50 values for ethylene glycol determined on rats are simply not valid when considering toxicity to humans.

Put simply, mixing a small amount of propylene glycol into a largely ethylene glycol-based fluid is not known to diminish the toxicity of ethylene glycol beyond the effect gained from having less of the latter in the product. A product containing 95% ethylene glycol is always going to catch a STOT RE2 classification without product specific test data to the contrary, and should, therefore, carry a hazard label.

While it is true that a product with 90% ethylene glycol and 10% propylene glycol is less toxic than a 100% ethylene glycol liquid, the reduction in toxicity is not due to the presence of propylene glycol, but merely to the lower ethylene glycol content. Furthermore, claims that propylene glycol acts as an ADH inhibitor by preventing the reaction pathways that make ethylene glycol toxic in the first place, are inaccurate.

With these facts in mind, those looking for far safer heat transfer and antifreeze fluids should employ propylene glycol-based products. Propylene glycol is used widely in industry, including as an excipient in numerous medications, a base solvent in vaping products, and as an emollient/solvent in a number of personal care and veterinary hygiene products. The fluid is metabolised by ADH to form lactic acid, which is much less toxic than glycolic or oxalic acids. On the whole, employing propylene glycol-based heat transfer and antifreezes products is a far safer course of action, with users being able to rest assured of proven and time-tested safety standards.