So I've decided to google around for "asthma safe air fresheners" and I found some ideas that could work, but I still need to do more research.
Idea 1: Coffee grounds. Apparently unused and used work, as long as you dry them if used. All you would do is place them out in the open in a bowl (maybe some vase would be pretty), or someone suggested tying them up in coffee filters (I imagine this would prevent any dusty pieces from spreading around the room). This seems really safe considering coffee doesn't seem to cause health problems in rats unless they consume it in SUPER large quantities. So overall, just keep them somewhere the rats can't reach.
Idea 2: Cooking. Yep, food smells good! Cooking with lemons or cinnamon = great smells. I think you just wouldn't want to burn anything, but most people don't like burnt food anyways. I imagine brewing tea would be an easy way to make the room smell good too! Plus, there are so many tea flavors now.
Idea 3: Vinegar (okay maybe not...) Doesn't smell great, but it sure does make itself known. I will clean with vinegar anyways, and it's pretty good at eliminating odors from plastic rat huts, trays, wheels, etc.
Overall: Most/all commercial air fresheners that use scent aren't healthy, nor can be guaranteed safe. Anything that's burned or makes a smoke isn't advised either. There are some ways to eliminate odor, like air filters and baking soda, but those don't smell 'good' because they just eliminate smell.
