What a Room Air Purifier Can and Can't Do

A veteran inspector always asked one question before talking about filters: where does the fresh air in this room come from? A purifier cleans the air already in the room. It brings in nothing new. She often found bedrooms where a filter hummed through the night with every window closed, and the people sleeping there woke up groggy and unrested. A closed bedroom builds up carbon dioxide and stale air no matter how good the filter is. Ventilation is the fix for that, not filtration. And if you still wake up groggy with fresh air coming in, or you snore or wake up gasping, ask your doctor about a sleep study; sleep apnea is common, treatable and easy to miss.

Evidence: Well supported (EPA and ASHRAE guidance on ventilation and indoor air).

Current guidance: EPA advises controlling sources and ventilating first, and using air cleaning as a supplement, not a replacement.

So never let a purifier become a reason to keep windows sealed. It works best in a room closed between airings, or in a house with a mechanical supply of filtered fresh air. In her view, regular HEPA vacuuming and damp-dusting remove so much dust that many homes need no purifier at all.

• HEPA filters catch particles: dust, dander, pollen, mold spores, smoke particles. They do nothing for gases or odors.
• Activated carbon adsorbs gases and odors, up to a point. Carbon fills up, and then it can release what it held. Some sensitive people cannot tolerate the smell of the carbon itself. No filter fully removes odors; removing the source comes first.
• Never put a carbon unit in a damp basement. Mold and bacteria can grow in the carbon. The inspector took samples at purifier outlets and found some blowing out high levels of bacteria or mold. Maintain and change filters on schedule.
• A purifier does not work around corners. Its rated coverage applies only to the room it sits in.

Air changes per hour tell you how many times the unit can process the room's air. Multiply the unit's airflow in cubic feet per minute by 60, then divide by the room's volume (length times width times height). A 20 by 20 foot room with an 8 foot ceiling holds 3,200 cubic feet. A unit moving 300 cubic feet per minute gives about 5.6 changes an hour on high, but the same unit gave only about 2.4 on its medium setting. She treated 2 to 4 air changes per hour as a reasonable minimum.

Evidence: Inspector's practice (precautionary).

Purifier motors give off magnetic fields. As a precaution, the inspector advised against running one beside a sleeping person. If one is needed in a bedroom, for a child with asthma for example, plug it in across the room, or run it for several hours before bedtime and switch it off at night.

Evidence: Inspector's practice (precautionary).

Current guidance: WHO considers no health effects established from everyday magnetic-field levels below international limits. Moving the unit a few feet away costs nothing either way.

Ozone machines sold as purifiers do not kill mold at levels people can safely breathe, and ozone reacts with other chemicals indoors to form new ones, including formaldehyde. Never run one with a person, pet or plant in the house. The inspector was also wary of ionizing units, since some give off ozone and ions can settle on airway tissue, and of electronic air cleaners, which raised electric fields through the house and stopped working once coated with dust.

Evidence: Well supported (EPA).

Current guidance: EPA advises against ozone generators as air cleaners. At levels that stay within public health standards ozone does little to indoor pollutants, and at higher levels it harms the lungs.

A HEPA unit makes sense for asthma or severe allergies, in a tight house with filtered mechanical fresh air, when smoke or fine dust seeps in from next door, or carried from room to room as you clean. Handle the vacuum, the floors and the fresh air first.

This page is for education only. It is not medical, legal or engineering advice. The inspector whose work this builds on was an environmental consultant, not a physician. Talk with your doctor about health concerns, and use licensed professionals for electrical work and large mold jobs. Products, prices and labs mentioned reflect the years they were recorded and may have changed.

Source: the field work of environmental inspector May Dooley, MS, MA, CMC

Air Filters & Purifiers for MCS →