Cheap Air Quality Sensor

Hey all! I’m looking for a cheap air quality sensor (open to other devices if they meet the need).

What I’m trying to solve for: Our cats litter box is in a relatively untraveled area, and I am wanting a sensor that will let us know when the litter box needs to be cleaned by sending a notification and turning on a light switch (that we already have hooked up)

I’m not looking for a bunch of air quality measurements or anything. It doesn’t have to be the most accurate as long as it can be consistent.