On a handmade rug, the certification also claims to mean that the rug was produced without child labor. That difference can become significant during cleaning, so it's important that the cleaner be a professional who can identify the fibers. Rugs with rayon pile should always be dry-cleaned. Olefin, also called polypropylene or a branded name such as Exellan, is a plastic fiber. It wears well but is hard to clean if stained with something oily.
Labels generally warn against dry cleaning.
Olefin can be a good choice in a basement or in any moist climate because it tolerates humidity and resists mold and mildew. Although the patterns and colors are the same as those used in wool Orientals, it's fairly easy to identify low-priced olefin. It has a somewhat artificial-looking shine, and colors tend to be flat or monochromatic. More-expensive olefin looks a lot like wool. Silk rugs are fragile and can be expensive, and room-sized rugs aren't very practical.