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Toxicologists have long considered ethylene glycol, the active ingredient in many antifreeze and engine coolant formulas, to be a seductive and uniquely dangerous poison.

For one thing, it’s sweet. “We actually had a mechanic who developed a taste for it,” recalled Dr. Marsha Ford, director of the Carolinas Poison Center in Charlotte, N.C. “He’d pour himself a little and sip it. And he kept doing that until he got sick.”

And that’s the other danger: Ethylene glycol is a slow-acting poison. Even following a high dose, symptoms can take up to 48 hours to appear.

The country’s poison control centers record more than 5,000 ethylene glycol ingestions annually; some 2,000 cases require medical treatment. Most are accidental, but ethylene glycol also figures in hundreds of suicide attempts every year — not to mention the occasional murder. Recently an Ohio woman was convicted of killing her fiancé by spiking raspberry iced tea with antifreeze.



Poison Pen Deborah Blum writes about chemicals and the environment.

The situation for animals has been even more dangerous than for despised spouses. According to the Humane Society of the United States, as many as 90,000 pets and wild animals are poisoned annually by drinking spilled or carelessly stored products containing ethylene glycol.

Now the manufacturers of those products have determined to do something about all the carnage. They are making antifreeze taste awful — so very bitter that it will be nigh impossible to drink by accident.

Seventeen states already require manufacturers to add so-called bittering agents to ethylene glycol products. The Consumer Specialty Products Association, which represents the key manufacturers, voluntarily has agreed to require members to add these agents to all consumer products containing the compound sold nationwide. The first batches of unpalatable antifreeze started hitting store shelves last year; this year customers can buy only the bitter versions.

The action came about in part because of a surprisingly warm relationship between the Humane Society and the C.S.P.A. Both groups had earlier worked together to propose federal legislation requiring bittering agents in antifreeze. But after repeated failures, they realized that an industry agreement was a more likely resolution.

“Most people survive because they get treatment a lot faster,” said Sara Amundson, executive director of the Humane Society Legislative Fund. “We often don’t catch the problem with animals until it’s too late, partly because they often go away to some quiet place when they feel sick.”

Representatives of the two groups settled on an old-time compound, denatonium benzoate, as the best way to make antifreeze taste terrible. The compound, discovered in the late 1950s, is not considered especially toxic, is obnoxiously bitter, and has been shown not to damage engines. Even at 10 parts per million, studies have found, children in laboratory settings promptly spat out orange juice tainted with denatonium benzoate.

But not everyone is deterred. A 2008 study looking at states where bittering agents are required found that the taste appeared to make no difference to those determined to commit suicide by drinking antifreeze.

Nonetheless, researchers tracking both human and animal health are hoping that the new formulation reduces the number of inadvertent poisonings. “We’re watching it,” said Dr. Ford, who is also director of the poison control center network, which has begun analyzing its 2013 numbers. The Animal Poison Control Center, operated by the A.S.P.C.A., so far reports no change: The number of alarmed calls about antifreeze poisoning remained above 300 a year from 2011 to 2013.

Of course, no one is sure how much sweet antifreeze is still out there. For one thing, the voluntary agreement doesn’t cover professional grades of antifreeze and engine coolant, such as those used in car repair shops. And people hang onto their home supplies for years.

“I’ve got a container of antifreeze in my garage that’s probably more than a year old,” said Dr. Clark Fobian, a Missouri veterinarian and president of the American Veterinary Medical Association.

Chemical additives are only part of the safety solution, experts note. Safe storage and handling are important, as are warning labels — large ones, preferably.

“A big part of this is educating people,” said Dr. Fobian. He recalled a client who changed his engine coolant in a driveway and left the liquid sitting in a pan. A dog belonging to his in-laws drank it and died.

“He’d just never considered that it might be a risk,” Dr. Fobian said.