fill power, an ounce of down is placed into a graduated cylinder, and a small weight is dropped in on top of it; the volume below the weight indicates the fill power. Eiderdown has the highest fill power, at 1200. However, even down with a fill power as low as 550 still provides reasonably good insulation. Higher fill-power downs will thus insulate better than lower fill-power downs of the same weight. Insulation in most outdoor equipment ranges from about 400 to 900 in3/oz (230–520 cm3/g). Down rated 500–650 in3/oz (290–375 cm3/g) is warm enough and light enough for most conditions, and 800–900 in3/oz (460–520 cm3/g) fill is used for very lightweight and/or very cold-weather gear. Down is warm, lightweight and packable. If well cared for, it retains its loft up to three times longer than do most synthetics. However, when it is wet, the thermal properties of down are virtually eliminated. Down forms clumps if exposed to dampness or moisture, and will mildew if left damp. In addition, it will absorb and retain odors. Down feathers tend to become more allergenic as they age, as they become contaminated with molds and dust mites. Pillows provide the most common source of exposure, though mattresses, comforters, outerwear and upholstery can also cause problems. A Finnish study has shown that true feather allergies are rare, with most issues caused by dust mites. Non-avian dinosaurs Feathers found in amber in western France, Canada and northeastern China suggest that some non-avian dinosaurs may have had primitive, down-like feathers. See also International Down and Feather Testing Laboratory References External links BBC article showing Tawny Owl powder down imprint on window Video showing rose-ringed parakeet shedding feather dust as it preens Bird products Feathers Insulators