An air-exchange rate of at least five times per hour . This is especially important for allergy sufferers. The higher the air-exchange rate, the more clean air is circulated through the room. Keep in mind that air exchange is based on fan speed, so if you can't tolerate the noise generated by high fan speed, your air-exchange rate will be lower. A monitor that lets you know when filters need to be changed .
Proper maintenance is crucial for air-filter effectiveness.
Once filters become clogged with particles, they can actually turn into air polluters, churning contaminants back into circulation. Some, but not all, monitors have visual or audio cues that prompt you to change the filter. A noise level you can accept . Look for models with different fan speeds, so you can control the noise level. Now, try to sleep with that fan on high - it sounds like a train going through your bedroom.