Q. What are your thoughts on the "pet-safe" antifreeze (propylene glycol) versus the more commonly used, more deadly, ethylene glycol? I am a veterinarian, and I have seen far too many animals die of kidney failure due to ingestion of ethylene glycol. It is a very slow and painful death, and by the time an animal is diagnosed with this condition, it is often too late. As you know, antifreeze has a sweet taste, and animals will readily drink it if they happen to find a puddle (leak from a car). One teaspoon can be lethal to a cat, and a few tablespoons for a medium-size dog. I have used propylene glycol for several years. My 1995 Toyota Camry is about to hit 280K miles, and it is still in good running condition. I cannot understand why propylene glycol has not taken on more popularity. The increased safety factor seems worth the minor increase in cost.

TOM: There's not a simple, clear answer to this question, because coolant technology is complex. It's been changing, and will probably continue to change in the next 10 years. But let's start with what we DO know.

RAY: We know that propylene glycol (PG) coolants are LESS toxic to humans and animals than ethylene glycol (EG) coolants. But they're not safe. PG coolants will still kill animals or humans if they're ingested; it just takes more. But they are an improvement over EG coolants when it comes to safety.

TOM: In terms of how effective they are as coolants, they're essentially the same. So either one will do a good job of removing heat from your engine.

RAY: So, why don't manufacturers just switch to PG? There are two reasons. One is cost. It might not matter to you or me, especially if it saves the life of a cherished pet. But if you're making millions of cars a year and they all need gallons of the stuff, the added expense of PG is a consideration.

TOM: The other issue is that coolants have to do more than just transfer heat away from the engine; they also have to protect the cooling system from corrosion. There's some evidence the corrosion inhibitors are not quite as soluble in the types of PG coolants used most often today (which are called HOAT, or hybrid organic acid technology).

RAY: You can get around that simply by changing your coolant a little more often.

TOM: So far, the car manufacturers have not put their stamp of approval on PG coolants.

RAY: We have no problem with people using PG-based coolants. My own guess is the differences in corrosion protection are outweighed by the number of pets that would be saved. But there's an even better solution out there.

TOM: The reason pets lap up coolant is that it tastes sweet. So, why don't we just make it taste lousy? Seriously, several states already require that coolants contain a bitter additive that makes the stuff taste lousy to kids and animals. It adds about 3 cents to the cost of a gallon of coolant. Shouldn't this be the law of the land? If you agree, write a letter to your congressman and turn the heat up.