Carbon air filters are an important part of a home air purification system, but they must be used correctly.
Make sure you use enough carbon. Some filters claim to be activated carbon filters, but they only use trace amounts of carbon. These filters won't be effective because they saturate almost immediately. A rough guideline is that a good activated carbon air filter uses at least 5 pounds of carbon-of course, not all carbon is created equal, and some may have more binding sites (and saturation capacity) than others. But generally speaking, the more carbon you use, the more effective it will be because there will be more adsorption sites to stick to.
Air flow is critical. In order to remove as many pollutants as possible from the air, the air needs to spend as much time as possible passing through the carbon. In the air filter industry, this is called "dwelling time." A filter with a sufficient amount of carbon and a high dwell time will be more effective than a filter with a thin layer of carbon.
Replace the carbon filter. Once carbon is saturated, it loses all effectiveness as an air filter. When this happens, it must be replaced with a new carbon filter. The problem is that there is no perfect way to know when carbon is saturated. Sometimes you'll notice an odor; sometimes you won't. You can follow the manufacturer's guidelines and replace it regularly, although this is only a rough estimate. How saturated the carbon becomes depends on how heavily it's been used and the amount of contaminants it's been exposed to. Replacing a carbon filter is an expensive investment, but changing it monthly is a good starting point.
