I think being low-cost is very important, otherwise I wouldn't consider it. I don't live near a major road, but in a small city. I don't think the air-quality is bad, but I also don't know for sure. I would not invest a significant amount of money (more than 10-15€).
I've found a lot of indoor sources of contaminants for example cheese that fell off a Pizza onto the oven floor will put you into "yellow" the next time you run the oven.
