Air Purifier FAQs

An air purifier is an appliance that draws air in from a space, passes it through one or more filters or purification technologies to remove pollutants, and releases cleaner air back into the room. Most units run continuously, cycling the air in a room again and again so that pollutant levels gradually drop and stay lower over time.

An internal fan pulls air from the room into the unit, where it passes through a series of filtration stages before being released back out. A typical setup starts with a prefilter that catches larger particles like dust and pet hair, followed by a HEPA filter for finer particulates such as pollen and mold spores, and often an activated carbon filter for gases and odors. Some models add UV-C light or other technologies for additional purposes. This cycle repeats continuously, which is why placement and fan speed both matter for how well a unit performs in a given room.

It's easy to assume the air outside is the bigger concern, but for most families, indoor air deserves just as much attention. The EPA estimates that concentrations of some pollutants indoors run two to five times higher than typical outdoor levels — and Americans spend about 90% of their time indoors, which means that gap adds up over a day, a week, and a year.¹ Modern homes are also built tighter for energy efficiency, which helps with heating and cooling bills but can trap pollutants inside rather than letting them escape.

Depending on the technologies built into a given unit, an air purifier can help reduce common airborne particulates like dust, pollen, pet dander, and mold spores, along with gases and odors when activated carbon is present. Some configurations are also built to address airborne bacteria and viruses. No single technology handles everything on its own, which is why most air purifiers combine two or three filtration methods rather than relying on just one. For a closer look at exactly what air purifiers can and can't remove, see our guide on what air purifiers clean.

HEPA is short for High-Efficiency Particulate Air. The "true HEPA" designation means a filter has been tested and certified to trap at least 99.97% of particles measuring 0.3 microns — a size range that covers common airborne particulates like dust, pollen, mold spores, and pet dander. Put another way, out of every 10,000 of those particles reaching the filter, roughly three slip through. HEPA filtration is highly effective, but it's scoped to particulates: on its own, a HEPA filter doesn't capture gases or odors, which is why it's so often paired with an activated carbon filter.

You may come across two very different approaches marketed for tackling household odors and gases: activated carbon filtration and ozone generation. They work in fundamentally different ways, and health authorities have raised real concerns about the second one.

Activated carbon works through adsorption. Its surface is riddled with microscopic pores, giving it an enormous internal surface area that binds odor and gas molecules as air passes through — without releasing anything back into the room.

Ozone generators work differently: they intentionally produce ozone gas, which is a lung irritant. The EPA has found that ozone generators sold as air cleaners are often ineffective at concentrations that are safe for occupied spaces, and that the higher concentrations sometimes needed for effectiveness can pose real health risks — particularly for people with asthma or other respiratory conditions.²  For that reason, activated carbon remains the well-established, safer choice for filtering household odors and gases.

The most reliable way to size a portable air purifier is by CFM (cubic feet per minute of filtered air) and ACH (air changes per hour) for the specific room you're placing it in, rather than a single square-footage number. A unit rated for a given square footage at a low ACH will clean that room far more slowly than one rated for the same footage at a higher ACH. You can plug your room's dimensions into our ACH calculator to see what CFM and ACH would work best for your space, and our guide on choosing the right air purifier walks through the full process.

Only if it were that simple. Air purifiers work best in the area they're placed in, such as a living room or bedroom. A portable unit can't travel through walls, around corners, down hallways, or up and down flights of stairs to clean an entire living or working space.³ You'll get some residual benefit in the areas immediately surrounding it, like a hallway just outside a bedroom, but the best results come from the room the unit is actually placed in.

When comparing models, use the area you actually want to place a unit in to guide your decision, and look for a purifier certified to clean more square footage than that space — it will clean the air faster and more often. A good rule of thumb is to look for a purifier certified for at least a third more square footage than the room you're placing it in. For example, for a 600 sq ft living room, you'd want a unit certified for at least 900 sq ft to clean that space effectively. You can purchase a unit certified for equal or lesser square footage than the room, but it won't clean the air as quickly or as often.

Yes — if it's connected to your HVAC system. A whole-home air purifier installed into your existing ductwork filters the air as your system's blower distributes it, so it reaches every room your HVAC sends air to. It's often the more cost-efficient route since one unit covers multiple rooms, and it has the added benefit of filtering the ductwork itself along the way.

That said, it comes with two conditions worth knowing upfront. First, it only filters while the HVAC fan is actually running, so getting continuous coverage means running the fan continuously rather than only when heating or cooling cycles on. Second, how well it performs also depends on how efficient your existing HVAC system already is — an older or less efficient system won't move air as effectively, no matter how good the filter is.

If you'd rather not depend on the HVAC fan running around the clock, the alternative is a portable unit in each room, or at least the major ones — the room your family spends the most time in, the most contaminated room, and the bedrooms are all good priorities. Portables also clean faster and can achieve more air changes per hour in the space they're given than a whole-home system typically can. Still not sure which route makes sense for your home? Feel free to give us a call or send an email — we're happy to help you sort it out.

It depends on the filter type, how many hours a day the unit runs, and conditions in your home — pets, smoking, wildfire smoke, and higher-traffic households all shorten filter life. Prefilters typically need attention most often since they catch the bulk of larger particles first, while HEPA and carbon filters are usually rated for longer stretches before they need replacing. Always check the manufacturer guidance for your specific unit, since replacement intervals vary quite a bit from model to model.

Noise level depends on the fan speed you're running. As a rough reference point, normal conversation sits around 60 decibels, and a typical vacuum cleaner runs around 70–75 decibels.⁴ Most air purifiers on their low or medium settings fall in a similar or quieter range — comparable to a soft background hum rather than anything disruptive. Higher fan speeds, which move more air and clean a room faster, are noticeably louder and are generally best used for a quick burst of cleaning rather than around-the-clock use.

Yes, and it's not a bad thing to do. Fresh outdoor air brings its own benefits, and some outdoor pollutants like pollen will find their way into a room whether the window is open or closed — an open window just changes how much gets in. If you'd like to minimize new pollutants entering the space, keeping windows and doors closed while the unit runs will give you the most controlled result. Either way, the air purifier will keep working on whatever is in the room.

No — and this is worth being clear about. An air purifier can't resolve a mold problem growing behind a wall or in another hidden spot. An active mold issue like that should be addressed by a qualified remediation professional first.

Yes — it will help reduce the mold spores that are already airborne, so you aren't constantly breathing them in, as long as the unit is sized appropriately for the room and air is able to flow through it to be purified. Once an existing mold source has been properly resolved, running an air purifier in the space afterward can help keep airborne spore levels down and support a healthier recovery.

Not entirely, but it can make a real difference. Household dust is actually a mix of particle sizes — everything from larger, visible particles down to ultrafine particles too small to see. The larger particles tend to settle onto a surface fairly quickly, often before they ever make it to the air purifier, so that portion still needs to be swept, vacuumed, or dusted away the old-fashioned way. The upside is that since those larger particles don't linger in the air for long, they're also less likely to cause health issues from inhaling them.

Where an air purifier does its best work is with ultrafines and other airborne contaminants. Because ultrafine particles are so much lighter, they stay suspended in the air far longer than larger dust, ride air currents more easily, and get pulled into the unit's intake more readily. The bottom line is that an air purifier can capture larger particulate matter too, as long as it's still airborne and able to travel to the unit to be filtered — it comes down to weight and timing more than a strict size cutoff: lighter particles stay airborne long enough to reach the unit, while heavier ones tend to settle first. Many of our clients notice a real drop in overall dust buildup once a properly sized unit has been running for a while, even though sweeping, vacuuming, and dusting still have their place.

No — an air purifier isn't a cure, and it isn't a substitute for medical care. But many of our clients have found much needed relief by using one: reducing your exposure to airborne triggers like dust, pollen, and pet dander can noticeably ease asthma and allergy symptoms for many people. For diagnosis and treatment, that's still a conversation for your doctor.