An American living and working in Italy e-mailed me, perplexed about olive oil.

Italian food authorities, magazines and cookbooks, he says, claim that olive oil is best for deep-frying because it has a higher punto di fumo, or smoke point, than other oils. But culinary experts in the United States typically say that olive oil's smoke point is too low for such use and that peanut oil stands up better to the high temperatures of deep-frying.

"Who is right?" he asks.

Well, both and neither, actually. Perhaps, as my cynical correspondent suspects, Italy promotes olive oil because it produces one-fifth of the world's supply, whereas the United States is big in peanuts. That is, we cook with the oil we have, not with the oil we wish we had.

But the fact is, one cannot make a blanket assertion about the smoke points of individual oils.

When heated, most liquids eventually reach a temperature at which they boil — that is, turn into vapor. (That standard temperature for water, of course, is 212 degrees.) But long before a vegetable oil can boil, it begins to break down or decompose, giving off smoke and acrid, toxic fumes.

At what temperature does a given oil begin to smoke? That depends on how diligently we hunt for the smoke: Are we looking for a wisp or a cloud? The breakdown occurs gradually, not at a precise temperature.

More problematically, the smoke point of any given bottle of cooking oil depends on its cultivation conditions and the particulars of its refining. That provides lots of leeway for producers to brag about the high smoke points of their pet oils.

Those tables of exact smoke-point temperatures you'll find in cookbooks and food reference books? Don't believe them. The wide variations from chart to chart show they're no more than shots in the dark. There is no such thing as a typical olive or peanut oil. Manufacturers of extra-virgin olive oil, in fact, list their smoke points as anywhere from "just under 200" to well over 400 degrees.

But there are some useful generalizations. Oils with a higher percentage of free fatty acids — fragments of whole fat molecules — tend to have lower smoke points, because such acids burn easily. Conversely, highly refined oils, which generally have reduced amounts of free fatty acids, have higher smoke points. Thus, the highest-smoke-point olive oils are the "light" ones, which have been so thoroughly refined as to be virtually tasteless and colorless.

But note that the longer an oil is heated, the more free fatty acids are formed, and the lower its smoke point will be. That is one reason oil should not be used for deep-frying more than once or twice. Another reason is that prolonged heating breaks apart unsaturated fatty acids and produces acrid and toxic compounds.

The relatively low price of "light" olive oils may shift them into the affordable range for deep frying. In fact, for health reasons, you might wish to fry in olive oil all the time — even the cheapest you can find. Cooks in Mediterranean cultures often fry in inexpensive "refined olive oil" (formerly labeled "pure olive oil") or a low-grade olive pomace oil, extracted from the pulp remaining after the pressing of the fruit.

At the other end of the quality and price spectrum are the extra-virgin and virgin olive oils, which have been mechanically expressed and bottled with no processing other than the removal of solids by filtration or centrifugation. Unfortunately, they often have the lowest smoke points. But most chefs would not want to waste the fine flavor of virgin oils by deep-frying in it, anyway. (No generalizations can be made about the relative smoke points of the two varieties.)

What about all those other oils on your supermarket shelf: the canola, corn, peanut, safflower, soybean and sunflower oils, among others? Generally, they have all been substantially refined to raise their smoke points and lengthen their shelf lives. Refining crude peanut oil, for example, can raise its smoke point by 100 degrees. If used beneath your range hood at a reasonable deep-frying temperature of 350 to 375 degrees, none of the supermarket seed oils should set off your smoke alarm.

Except with butter, whose proteins burn at 250 to 300 degrees, smoke from the small amounts of fat used in pan frying or sauteing shouldn't be much of a problem with any oil.

For those of you who insist, here are ballpark smoke points of some common refined cooking oils: canola, 400 degrees; cottonseed, 420; sunflower, 440; corn, 450; peanut, 450; olive pomace, 460; soy, 460; extra-light olive, 468; safflower, 510.

But if you quote me, I'll deny it.

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh and author of "What Einstein Told His Cook: Kitchen Science Explained" (W.W. Norton, 2002).