The wool is totally untreated and is naturally antibacterial thus doesn't retain odors. (greenmountaindiapers.com)
In addition, it does not soil or retain odors easily, so it does not need to be washed very frequently. (littlespruceorganics.com)
When in use for long enough, plastic trays will start to absorb and retain odors no matter how often and how thoroughly they are cleaned. (best-cat-art.com)