The promise of an air purifier is an enticing one: An appliance designed to cleanse the air in your home, getting rid of all the impurities including odors, smoke, dust, and pet dander. Given the fact that indoor air can have levels of certain pollutants up to five times higher than outdoor air, we get it. But in reality, not all air purifiers necessarily live up to the marketing hype. Air purifiers usually consist of a filter, or multiple filters, and a fan that sucks in and circulates air. As air moves through the filter, pollutants and particles are captured and the clean air is pushed back out into the living space. Typically, filters are made of paper, fiber (often fiberglass), or mesh, and require regular replacement to maintain efficiency. That means, in addition to the purchase price of an air purifier, you should also factor in operating costs and filter replacement costs. Operational costs can easily amount to $50 annually, since you should be running air purifiers near constantly to garner the benefits. Filter replacements can run upwards of $100 a year all told. How frequently you will have to change filters varies based upon the purifier type and usage. Some filter...