Most vacuum cleaners come with an effective filtration system that prevents dust from entering back into the room. However, it doesn't clean the air. Rather, it cleans various surfaces and prevents dust from those surfaces contaminating the air. A vacuum cleaner cannot clean the impurities already present in the air. For that, you need an air purifier. That being said, a good vacuum cleaner with HEPA filter prevents settled dust which vacuum cleaner captures from entering back into the room. Something which cheaper vacuum cleaners fail to do.