People are high on down -- both for clothing and home furnishings -- and for good reason.

For lightweight warmth, both indoors and out, it's hard to beat quilted-down clothes. And the informality of comforters, plus the fact they're becoming more attractive, no doubt accounts for the almost double volume of quilt sales this year over last.

There are two types of filling for quilted items: the natural down or down-blend variety, and the synthetic fiberfill(usually polyester).

To be legitimately called down-filled, a jacket, for example, must contain at least 80 percent down and plumules (underdeveloped feathers) and downfiber (broken pieces of the down pod), with not more than 10 percent fiber in that 80 percent. Natural down has the advantage of usually being lighter, natural, of course, and it utilizes your own body heat, returned to you as warmth.

The synthetic filling may be chopped fibers or, what many feel is far better, a continuous piece of fiber-fill crimped to give loft and warmth. The synthetics have it over real down in terms of easy care. And, of course, they are the essential alternative for those allergic to feathers.

The quality of the fill -- and there are several qualities in both natural and synthetic -- the type and quality of the cover, and the construction for keeping the fill in place all affect price. You can count, however, on real down being more expensive than the synthetic because of its limited quantity on the world market -- even though the synthetic is petroleum-based.

The best, hands down, of all the fills generally available is genuine goose down. When its outershell wears out it can be recycled into a new item. An authority at one store suggests a 50-year lifespan for white goose down.

The biggest letdown in down, and fiberfill, may come at cleaning time (usually once a year). The pros all agree that the best cleaning techniques should be determined by both the fill and the shell. But after that, opinions differ.

These general suggestion, however, were cleaned from conversations with outerwear specialists, home-furnishing pros and industry representatives. Down

Wash in mild detergents, such as Woolite or special down soaps.

Tumble-dry with sneakers or tennis balls to break up clumps.

Dry cleaning must be done by a professional dry cleaner familiar with cleaning down garments. (It is essential that clean petro-chemical solvents are used.) Synthetics

Machine-wash; do not dry clean.

Tumble dry on air setting. Heat above 140 degrees is destructive, "straightening" crimp in fill and thus removing some insulating qualities

According to Henry (Hank) Cohan, general manager of Hudson Bay Outfitters, the heat accumulated when a synthetic-filled parka is left in the trunk of a car on a hot day can damage the garment. "When it gets heated, it straightens, which lessens the protective qualities of the jacket or vest."

The contrary for down: Bloomingdale's advises drying of a down comforter first at the temperature suitable to the cover. To dry the down, the comforter should then be put in the dryer for a few minutes at a high temperature.

Other care suggestions for top-quality comforters: Turn about once a month to redistribute the down filler. Use a duvet cover, a broadly accepted comforter cover, usually in high-quality cotton or silk, with large openings (like armholes) at each end, permitting the maximum air circulation possible. (Bloomindale's now makes duvet convers to order.)

For choosing down-filled items, Ellen Stark of the Feather and Down Association recommends checking the loft-depth and thickness of filling. "The more left, the more warmth the item will provide." Naturally she recommends down over the sythetics. . . "Polyester fibers are two-dimensional," she says, "and down is three-dimensional, so down can withstand an infinite number of compressions and expansions without matting." By her calculations 50 percent more filling by weight of synthetic is needed to get the same warmth as down.