Cleaning products, air fresheners, candles, etc...
Snip...
I learned that even if you are studiously avoiding the most well-known toxic cleaners (such as the phenol-based cleaners, which usually end in the letters "sol"), some sensitive cats will have very strong reactions to seemingly innocuous cleaning products. You simply cannot be too careful when deciding what you use to clean your house. One of my cats had mysteriously developed an on-again off-again vomiting problem and I finally figured out that these episodes almost invariably occurred the day after "cleaning day" in our house.
I wonder sometimes whether people with chronically vomiting cats--who are spending a small fortune on diagnostic tests and medications to stop the vomiting--aren't inadvertently causing or aggravating the problem simply by virtue of using unsafe cleaning products around the house.
http://www.catnutrition.org/cleaning.html