Down comforters are filled with small, fluffy down feathers in place of polyester, wool or cotton fillings. Down has been used for many years to keep people warm in their beds, especially in colder climates. A down comforter may smell depending on how it was manufactured, the quality of down used in the comforter and the way the comforter has been cared for. The fabric surrounding the down may also have an odor, especially when new.
The down used in down comforters varies greatly in quality, depending on the country it originated from, the type of fowl that it was harvested from and the procedures used for gathering, harvesting and cleaning the down. Some countries may not have the same quality standards found elsewhere during production of the bedding. Some fowl down such as duck down has a distinctive odor different from the goose down used in quality down products, which may also cause an unusual smell.
Down comforters are fragile, as far as bedding goes. Frequent washing can damage a down comforter, so it is generally recommended to keep down comforters inside protective duvet covers, which can be removed and washed whenever necessary without requiring the entire comforter to be washed. If a down comforter is washed and not dried properly, the feathers inside of it can mildew or mold, causing bad smells and potentially dangerous contaminants. The bedding may also absorb sweat and moisture as you use it, which can also cause odor over time if the bedding is used without a duvet cover.
Virtually everything has some kind of smell to it. Smells on a brand new comforter could be caused by the location where it was stored, what it was stored in or protectants and treatments that were used on it during the manufacturing process. Over time, comforters are exposed to human bodies, pets, smells inside homes that the fabric and feathers can absorb over time.
If your down comforter has an odor that you find unpleasant, cleaning it or airing it out may do the trick. Take it to a cleaner that specializes in down-filled materials -- regular dry cleaning may leave a chemical odor in the comforter. Airing the comforter out for a day or two during non-humid weather may also help reduce odors and will also reduce moisture, making the comforter fluffy once again.