Q: The other day a visiting friend noticed that the lining on one of my copper pans had worn through, leaving the copper exposed. I have taken the pan to be retinned. Did using it for the week or so before, with the copper exposed, put me in any danger?

A: No. Copper can be toxic, but not in the amount you were getting from a small spot in your pan. No one knows the exact level at which it becomes harmful, but it is estimated that man can tolerate at least 20 times his normal intake over a prolonged period before toxicity develops. Clearly that far surpasses what you might have ingested.

In earlier times, copper poisoning was common. The problem was so widespread that in 1753 the Swedish senate prohibited the fleets and armies from using cooking utensils made from anything but iron. Nowadays, copper cooking pots are lined either with tin or nickel. Food does not come into direct contact with the metal, except in circumstances such as you describe, or unless you are one of those cooks who prefers to whip egg whites in a copper pan.

All of us do need some copper in our systems. It is part of essential enzymes involved in reactions throughout the body's highways and byways. For instance, it plays a role in the utilization of iron and the synthesis of hemoglobin, and in the metabolism of glucose. But the quantity we need for these and other functions is tiny. The copper in a single penny would supply us for four years.

There is little problem with getting all we require from a varied diet. Copper is available in many foods, especially organ meats, shellfish, nuts, dried beans and cocoa. It may also find its way into the food supply via milk which has been exposed to copper rollers during pasteurization or in drinking water which has traveled through copper pipes.

Q: I have been told by friends who rely heavily on health foods that products such as flour-based cakes and cookies have a tendency to form mucus in the body. These people say that excess mucus creates a fertile environment for some diseases, and that a mucusless diet can be marvelously therapeutic. Another friend, who is a doctor, says this is all nonsense. What is the truth?

A: The idea that certain food produce "mucus," and that mucus is a breeding ground for disease, is no more true today than when it was first propounded over 60 years ago.

The originator of the Mucusless Diet Healing System was Arnold Ehret, who believed he had cured himself of diabetes on a diet largely composed of fruit. His theory drew on a heady mixture of the writings of Socrates, the Book of Genesis, unidentified Egyptian philosophers, and the late 19th-century food faddist Horace Fletcher.

How was one to decide if the mucusless diet would be effective against a particular ailment? As a first step, the regimen prescribed a three-day fast. Dizziness or headaches -- common enough symptoms after three days without food -- supposedly indicated clogging with mucus and toxemias.

According to Ehret, that meant it was time for a mucusless diet. If the diet failed to alleviate the symptoms, Ehret prescribed a purgative concoction containing buckthorn bark, ground psyllium seed husks and ground dark fennel seeds.

Since the 1920s, when this diet emerged, research has uncovered nothing to support it. Your doctor friend is correct.

Q: My doctor advised me to cut down on sodium. In reading pamphlets about controlling sodium intake, I've noticed that monosodium glutamate is often listed among the ingredients to watch. It appears in so many foods that I would like to know how its sodium content compares to that of salt.

A: Measure for measure, monosodium glutamate (MSG) contains considerably less sodium than table salt.

By weight, salt is 40 percent sodium; MSG, only 12 percent. Thus a quarter-teaspoon of salt contains about 550 mg. of sodium. The same amount of MSG has only 165 mg. The contribution of MSG to the sodium content of a particular food may be small, both because it contains far less sodium than salt and because only limited amounts are used.

But for those interested in curbing sodium intake, here is a point to consider: MSG is often added to foods which are already generously salted. Moreover, salt and MSG may not be the only sources of sodium in a processed food. To get a firm idea of how much sodium a serving of a particular processed food contains, check the sodium-labeling information.

If it is not present, you have two alternatives: Write to the manufacturer and request the information, or buy a comparable product where the information is readily available.

Q: Recently you answered a question about how coffee is decaffeinated. Can you tell me which brands use which process? And is it really true that the methylene chloride method is safe?

A: Methylene chloride is used to extract caffeine from Chock Full O'Nuts, Mellita, Maxwell House, Urban and Brim (the latter three are all made by Maxwell House). Folger and High Point, both made by Folger, use ethyl acetate, a compound normally found in many fruits. Hills Brothers and Nescafe use water and coffee-bean oil; and Sanka and Private Collection, both of which are made by Maxwell House, use carbon dioxide under pressure.

On a fact-finding trip to the supermarket, we were delighted to find that several companies were responding to consumers' desire for product information.

The process used in making Nescafe was described right on the label, and two other companies, Maxwell House and Folger Brothers, provided toll-free numbers so that consumers could call in questions about their products. We found the services prompt and courteous.

As for your query about methylene chloride: In a recent review, the Expert Panel on Food Safety and Nutrition of the Institute of Food Technologists described a considerable body of evidence indicating that using it for decaffeination is safe. In long-term animal studies sponsored by the National Coffee Association, rats did not develop cancer on doses equivalent to as many as 6 million cups of decaffeinated coffee.

No toxic effects were observed in rats receiving the equivalent of 120,000 cups a day, or in mice consuming the human equivalent of 4.4 million cups a day. Studies of humans have failed to find an increased risk of cancer or other serious disease associated with exposure to methylene chloride used in manufacturing.

In making a decision on the safety of methylene chloride, the FDA in 1985 concluded that while methylene chloride does cause cancer in animals when given in inhalation, the risk from consuming coffee that has been decaffeinated with a process using it is so low to be nonexistent.