Oh, what a difference a single carbon atom makes! It can be the difference between life and death! Just 2 tablespoons of ethylene glycol can kill a child, while the same amount of propylene glycol is practically harmless. These compounds are not esoteric substances that are only of theoretical interest; they are commonly encountered in various antifreeze preparations. Ethylene glycol is found in virtually every automobile radiator and is responsible for a couple of dozen deaths every year in North America, along with thousands of cases of poisoning. Since it can cause inebriation just like ethanol, some desperate alcoholics resort to ethylene glycol with the mistaken belief that it is no more toxic than the usual alcohol they drink. But it is! Other poisoning cases involve suicides or the accidental ingestion of the sweet-tasting glycol by children. There are also innumerable cases of poisonings of pets that love to lap up little puddles of the fluid that leak from a car’s radiator.
Antifreeze formulated with propylene glycol (Sierra brand) is available, albeit more expensive than the conventional variety. Many zoos have converted to using propylene glycol in the radiators of their vehicles to reduce the risk to animals that might ingest the fluid after radiator leaks or boil overs. The switch to the safer antifreeze received a big boost when a rare California condor died after drinking from an ethylene glycol spill. So how could just one little extra carbon atom in a molecule cause such a difference in toxicity? It all comes down to the manner in which ethylene glycol and propylene glycol are metabolized in the body.
The liver is the body’s main detoxicating organ, and it is here that an enzyme, alcohol dehydrogenase, reacts with glycol of either kind. But the products of the reaction are quite different! Propylene glycol is converted into lactic acid, while ethylene glycol mainly forms glycolic and oxalic acids. Any excess acid in the blood is a problem, but oxalic acid presents a further complication. It reacts with calcium ions in the blood to form microscopic crystals of calcium oxalate, which can interfere with blood flow and cause damage to the brain, heart, and lungs. But the most serious effect is blockage of the tiny tubes that make up the blood-filtering structure of the kidneys. Death from ethylene glycol poisoning is usually due to kidney failure.
If ethylene glycol poisoning is diagnosed quickly, it can be treated effectively. A stomach wash with saline solution and activated carbon (a form of charcoal that absorbs toxins) can remove some of the glycol before it is absorbed. Intravenous sodium bicarbonate can counter the metabolic acidosis. And then comes the most interesting part of the treatment. Oral and intravenous administration of alcohol! Since alcohol dehydrogenase has a greater affinity for alcohol than for ethylene glycol, the enzyme goes to work on alcohol and leaves the glycol alone. By the time the alcohol has been metabolized, the ethylene glycol has hopefully been eliminated in the urine. The patient ends up very drunk, but very much alive! A drug called fomepizole (4-methylpyrazole) can also be used to inactivate alcohol dehydrogenase without the adverse effects of alcohol administration.
With this background we are now ready to tackle the “Swiffer WetJet” scare story that has been making the rounds on the Internet. This mop-like device uses a premoistened cloth to clean stains on floors. A circulating e-mail claims that the solvent used on the cloth is dangerous to pets, and tells of a dog and two cats that perished just from having walked on a freshly mopped floor.
I’m going to come to the aid of Procter & Gamble, the makers of the Swiffer WetJet. Not because I’m a great supporter of giant corporations, but because I’m a great supporter of good science. And the allegations against the Swiffer WetJet are just plain bad science. The e-mail does not document where the supposed death of the dog occurred, so it is impossible to check out the story or the claim that an autopsy was performed on the dead animal and liver failure was diagnosed. In any case, according to the author, the toxic culprit is “propylene glycol n-propyl ether,” the solvent used in the Swiffer product. This, he suggests, is “one molecule away from antifreeze,” which is known to be deadly to animals.
First of all, the antifreeze that really is dangerous to dogs, as we have seen, is ethylene glycol, but it causes kidney and not liver failure. That, though, is hardly the point. The statement of propylene glycol n-propyl ether being one molecule away from antifreeze is absurd. This is where a little learning becomes a truly dangerous thing. Propylene glycol is close in structure to ethylene glycol. It isn’t “one molecule away,” it just has an extra carbon and three hydrogens. That difference, though, is what makes it into a safer product! That’s why propylene glycol has replaced ethylene glycol in many products where toxicity is an issue.
But we are still not done. Swiffer doesn’t contain propylene glycol; it contains propylene glycol n-propyl ether, a completely different substance with very low toxicity. Furthermore, the solvent on the mop is mostly water with the propylene glycol n-propyl ether being present to a maximum of 4 percent. And finally, the solvent is reabsorbed into the mop, so essentially nothing is left on the floor. The only thing left after using a Swiffer WetJet is the Internet nonsense that is going around. I wish we could swiff that away as easily as the Swiffer WetJet swiffs away stains on floors.