Covered vs open litter box: which is better for smell?

Hoods hide the mess from people. For odor, though, the guidance is fairly clear:

• OSU: "covered litter boxes will concentrate odors more than uncovered boxes. Your cat may refuse to use a covered box if it is not kept very clean."
• Cornell: "from your cat's point of view, covers hold odors in." Most cats prefer an uncovered box with a 360-degree view.
• AAFP/ISFM: research found cats "equally divided" between large covered and uncovered boxes, and preference "may be influenced more by regular cleaning." Open boxes are recommended "whenever possible to facilitate owner monitoring and frequent scooping."

Bottom line

An open, large box that you scoop often is the vet-recommended default. If you need a cover (for example, to keep a dog out), make it large and keep it very clean. Cornell also suggests making sure the box has more than one exit route in multi-pet homes.