Are diffusers cleaner than air fresheners?
Not really. Diffusers look like the natural option next to plug-in air fresheners, but both release terpenes, the scent molecules in essential oils, and indoors those react with ozone to form formaldehyde and ultrafine particles. Source 3
What about cats?
Keep diffusers out of the rooms your cat uses. Cats are especially sensitive to essential oils, and some oils are dangerous for them. Source 2 The droplets settle on fur, and cats lick them off. Source 1 Which oils to avoid, for dogs too, and what to do if your pet is exposed: Are essential oils safe for cats and dogs?
What can you use instead?
Open a window or run the fan; it costs nothing. In a room that stays closed, a charcoal bag soaks up smells without adding a scent. Have a cat? Keep diffusers out of the rooms it uses.
Set the bag in the sun for an hour or two each month to refresh it, as the makers suggest. Essential-oil diffusers aren’t scent-free either: they release the same kind of terpenes. Have a cat? Keep diffusers out of the rooms it uses: many essential oils can be toxic to cats, and droplets settle on fur that cats lick off when they groom. More on essential oils, cats and dogs.