Another indicator of a good air purifier is the CADR rating. It is a rating that can tell you the rate at which clean air is delivered. Generally, the higher the number, the better. It is also more expensive. On most air purifier boxes, it is usually measured against three contaminants namely, smoke, pollen and dust. Another important factor is the air filter. You may need to factor in the cost of replacing the air filter since it is a consumable item. Some air filters for certain brands can be expensive too. How about Best Air Purifier