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- Quick answer
- What an air purifier does
- Steps 1–2: Measure your room, understand CADR
- How much CADR you need (table)
- ACH and why square-foot claims mislead
- HEPA, True HEPA, HEPA-type and H13
- Activated carbon
- Choosing by problem and by room
- Noise, sensors and smart features
- Filter costs and energy use
- Placement, mistakes, checklist, FAQ
Shopping for an air purifier for the first time is confusing. One box says “covers 1,700 sq ft,” another says “H13 True HEPA,” and a third says “22 dB.” They don’t tell you which numbers matter.
This air purifier buying guide is for people who have never bought one. It shows how to check a purifier against your room, your main problem, your noise tolerance and your budget. It uses three kinds of information, and I label each:
- EPA guidance (U.S. government)
- Manufacturer specifications (claims, not independent proof)
- LifeCareBalance calculations (our own arithmetic, shown so you can check it)
By the end you’ll know which specifications to compare before you buy.
Quick Answer: How to Choose an Air Purifier
- Measure your room. Multiply length by width, and note your ceiling height.
- Use CADR as your sizing number. Match the purifier’s smoke CADR to your room size using the table below. Go higher for tall ceilings, open plans or smoke.
- Check the air-changes-per-hour (ACH) assumption behind any “covers X sq ft” claim. A big number often means slow cleaning.
- For particles, look for a HEPA-grade filter and a tested CADR. For odors and gases, look for a substantial activated carbon filter.
- Judge noise at the fan speed you’ll actually run, not the quietest number on the box.
- Price the replacement filters before you buy the purifier. Avoid devices that intentionally produce ozone.
What Does an Air Purifier Actually Do?
A portable air purifier (the EPA calls it an air cleaner) pulls room air through a filter and pushes it back out. The EPA notes that indoor pollutants can be particles or gases, including volatile organic compounds (VOCs), and that portable air cleaners can reduce them but cannot remove everything. epa
Particles are tiny solid or liquid bits floating in the air:
- Household dust
- Pollen
- Pet dander
- Smoke particles (wildfire, cooking, tobacco)
- Mold fragments
Fine particles below 2.5 micrometers are called PM2.5. The EPA lists PM2.5, formaldehyde, mold and pollen among common indoor contaminants. epa
Gases and odors are molecules, not particles. Cooking smells, some smoke components, and VOCs from paints, cleaners and furnishings fall here. A particle filter alone does not address them.
One point beginners often miss: the EPA says the most effective ways to improve indoor air are to reduce pollution sources and ventilate with clean outdoor air. Filtration is a supplement to those steps, not a replacement. epa
On health, be realistic. The EPA says several studies with portable HEPA air cleaners found small improvements in cardiovascular and respiratory health, and that allergy or asthma symptom improvements were not always major or seen in every symptom. A purifier may help reduce airborne particles. It is not a treatment for allergies, asthma or illness. epa
Image suggestion (alt text): “Diagram comparing airborne particles like pollen, dust, and smoke with gases and odors that a particle filter does not capture.”
Step 1: Measure Your Room Size
The EPA’s method is to multiply the room’s length by its width to get square feet, then compare that with the maximum room size on the packaging. epa
- A 12 ft × 15 ft bedroom is 180 sq ft.
- A 14 ft × 20 ft living room is 280 sq ft.
Open-plan spaces. If your floor plan is open, the EPA says to consider the whole space the purifier will serve, and that a space larger than any single unit can handle may need more than one air cleaner. A living room that flows into a kitchen is one big room to the purifier, not two. epa
Ceiling height matters. The EPA’s sizing chart assumes 8-foot ceilings and says higher ceilings call for a higher CADR. Air purifiers clean volume, not floor area. epa
- 200 sq ft × 8 ft = 1,600 cubic feet
- 200 sq ft × 10 ft = 2,000 cubic feet, which is 25% more air to clean
Step 2: Understand CADR (Clean Air Delivery Rate)
CADR in plain English: it is how much clean air a purifier delivers per minute, measured in cubic feet per minute (CFM). A higher number means the purifier cleans the air in a room faster.
Think of a faucet filling a bathtub. CADR is the flow rate. A stronger flow fills the tub faster, and a stronger CADR cleans a room faster.
The EPA says a higher CADR means the unit can filter more particles and serve a larger area, and that CADR is typically measured at the highest fan speed. That second point matters because you may not run your unit at its highest speed. epa
Smoke CADR, Dust CADR and Pollen CADR
Most CADR labels list three numbers. The EPA explains that tobacco smoke, dust and pollen stand in for small, medium and large particles, and that buyers who want to remove the smallest particles (PM2.5) should look for a high smoke CADR sized to their room. epa
| CADR number | Represents | When to focus on it |
|---|---|---|
| Smoke CADR | Smallest particles | Best default sizing number; wildfire or cooking smoke; fine particles |
| Dust CADR | Medium particles | Dusty homes, general household particles |
| Pollen CADR | Larger particles | Seasonal pollen; larger particles are generally easier to capture |
CADR is a particle rating only
The EPA points out that the CADR system covers particles only, and that there is no widely used performance rating for gas-removing filters. A high CADR says nothing about odors or VOCs. epa
Look for independent CADR verification
AHAM (the Association of Home Appliance Manufacturers) runs the CADR program, and its Verifide mark on packaging indicates the CADR figures went through the AHAM program. AHAM also offers a searchable directory where you can look up certified models by CADR and suggested room size. If a model isn’t listed, ask how its CADR was measured. AHAM VerifideAHAM Verifide
How Much CADR Do You Need?
Use this table as a starting point, not a guarantee.
Approximate guidance. The middle column is the EPA’s sizing chart. The EPA calls it an estimation tool, built on an 8-foot ceiling. The right column reflects AHAM’s suggestion for wildfire smoke: a smoke CADR roughly equal to the room’s square footage. epaAHAM Verifide
| Room area | Example room | EPA approx. minimum CADR (8-ft ceiling) | Higher target for smoke or heavy use (CADR ≈ room area) |
|---|---|---|---|
| 100 sq ft | 10 × 10 | 65 CFM | ~100 CFM |
| 200 sq ft | 10 × 20 | 130 CFM | ~200 CFM |
| 300 sq ft | 15 × 20 | 195 CFM | ~300 CFM |
| 400 sq ft | 20 × 20 | 260 CFM | ~400 CFM |
| 500 sq ft | 20 × 25 | 325 CFM | ~500 CFM |
| 600 sq ft | 20 × 30 | 390 CFM | ~600 CFM |
Reading the table
- For a 12 × 15 bedroom (180 sq ft), the EPA ratio works out to roughly 115–120 CFM. That is our arithmetic, not an EPA figure for that exact size.
- Tall ceilings. Scale the EPA number by ceiling height ÷ 8. A 300 sq ft room with 10-foot ceilings needs about 195 × 1.25 ≈ 245 CFM (LifeCareBalance calculation).
- Buy a little above the minimum. A purifier that comfortably exceeds your room’s needs can run at a lower, quieter speed.
- Don’t oversize wildly. AHAM’s consumer guidance warns that the highest-numbered unit may be too large for the room and use more energy than necessary (see the AHAM guidance in our sources).
What Is ACH (Air Changes per Hour)?
ACH tells you how many times per hour a purifier can theoretically filter the entire volume of air in a room.
The formula (LifeCareBalance calculation):
ACH = (CADR × 60) ÷ (room area × ceiling height)
Example: a 200 sq ft room with 8-foot ceilings holds 1,600 cubic feet.
| Purifier CADR | ACH in that room | What it means |
|---|---|---|
| 65 CFM | ~2.4 | Slow cleaning for this room |
| 130 CFM | ~4.9 | Matches the EPA table’s row for 200 sq ft |
| 260 CFM | ~9.8 | Fast cleaning; can run on a lower speed |
The EPA’s sizing table works out to roughly 4.8–5 ACH at every row, which is where the “4.8 ACH” figure in many spec sheets comes from. More ACH means faster cleanup after a dusty or smoky event. Nothing here suggests a health threshold you must hit.
Don’t Trust Square-Foot Coverage Alone
A “covers up to 1,500 sq ft” claim means nothing until you know the ACH assumption behind it. The same purifier can be described with very different coverage numbers:
Coverage = (CADR × 60) ÷ (ACH × ceiling height)
For a hypothetical purifier with a 250 CFM CADR and 8-foot ceilings (LifeCareBalance calculation):
| Assumed ACH | “Coverage” the same purifier could claim |
|---|---|
| 1 ACH | ~1,875 sq ft |
| 2 ACH | ~940 sq ft |
| 4 ACH | ~470 sq ft |
| 4.8 ACH | ~390 sq ft |
| 6 ACH | ~315 sq ft |
Real manufacturer listings show this pattern. These are manufacturer specifications, not independent test results, and we show them only to illustrate the ACH effect. This is not a ranking.
| Product (manufacturer figures) | Smoke CADR | Coverage at 1 ACH | Coverage at higher ACH | EPA-table-style estimate (CADR ÷ 0.65) |
|---|---|---|---|---|
| Levoit Core 400S-P | 231 CFM | 1,718 sq ft | 358 sq ft at 4.8 ACH | ~355 sq ft |
| Coway Airmega 250 (AP-1720H) | 249 CFM | 1,860 sq ft | 930 sq ft at 2 ACH; 465 sq ft at 4 ACH | ~385 sq ft |
Sources: Levoit’s official product page lists 1,718 sq ft at 1 ACH, 358 sq ft at 4.8 ACH, and a 231 CFM CADR. Coway’s official page lists 249/261/230 CFM (smoke/dust/pollen) and coverage of 1,860 sq ft at 1 ACH, 930 sq ft at 2 ACH and 465 sq ft at 4 ACH. To their credit, both brands show the ACH assumption. The takeaway for you: read the ACH note before trusting the headline number. LevoitCoway
Two more cautions:
- Specs vary by version and retailer. Levoit’s Amazon listing says up to 1,733 sq ft, a slightly different figure than the brand site’s 1,718. Older reviews of the original Core 400S cite 260 CFM and about 403 sq ft, which don’t match today’s 400S-P figures. Coway’s separate “Airmega 250 Art” listing shows different CADR numbers (235 smoke, 240 dust, 293 pollen) than the standard 250. Always confirm the exact model number. Amazon.com: LEVOIT Smart Air Purifiers for Home Large Room Up to 1733 Ft² Core 400S-P | HEPA Sleep Mode, AHAM VERIFIDE, Auto Mode, Air Quality Monitor, 3-in-1 Filter For Pet Allergy, Smoke, Dust, ChildLock, White : Home & Kitchen +2
- Do your own math. For a like-for-like comparison, read the smoke CADR and use the formula above. If you want to see two of these models compared in detail, see our Levoit Core 400S vs Coway Airmega 250 comparison.
HEPA vs True HEPA vs HEPA-Type vs H13
This is where marketing language causes the most confusion. Here is how to read the terms carefully.
| Label | What it usually means | What to check |
|---|---|---|
| HEPA | High-efficiency particulate air filter. The commonly cited benchmark is capturing 99.97% of 0.3-micron particles. | Whether the spec states an efficiency figure |
| True HEPA | A marketing term brands use for a filter that meets the HEPA benchmark. It is not a separate, regulated tier. | Whether the brand states the tested efficiency |
| HEPA-type / HEPA-like / “HEPA-style” | Wording that generally does not promise the full HEPA benchmark. It may be close or may not be. | Ask for a number or a tested CADR. If neither exists, treat it as unverified. |
| H13 / H14 | Classes from the European EN 1822 system. H13 is rated at 99.95% at the most penetrating particle size. | It’s a filter grade, not a whole-purifier score |
What we can and can’t say
- Coway describes its filter as capturing 99.97% of airborne allergens as small as 0.3 microns. That is a manufacturer claim. Coway
- No label automatically guarantees better real-world performance. A great filter in a poorly sealed housing, or with weak airflow, can underperform.
- ENERGY STAR’s specification notes that CADR measures the air cleaner as a complete system. That is why CADR plus independent verification tells you more than a filter label alone. ENERGY STAR
Practical rule: treat “HEPA” as the minimum for particle filtering. Then compare tested CADR, not filter names.
A note on ozone and ionizers
The EPA advises avoiding air cleaners that intentionally produce ozone, a lung irritant, and notes that some ionizers, electrostatic precipitators, certain UV designs and plasma devices can emit ozone. California has required CARB certification for portable indoor air cleaners since 2010, with a 50 ppb ozone limit for electronic models. CARB also states that certification does not measure effectiveness. Checking a model against the CARB list is a useful safety step. epaca
Do You Need an Activated Carbon Filter?
If odors, smoke smell or VOCs are on your list, yes, and the amount of carbon matters.
- HEPA filters capture particles. They are not designed to remove gases.
- The EPA says to choose a unit with an activated carbon filter or another filter designed for gases, and notes that carbon can be effective when a large amount of material is used. Thicker is generally better. epa
- Because there is no standard performance rating for gas removal, you have little to compare. Look for a stated carbon weight or volume. A thin carbon-coated sheet is not the same as a deep bed of granules or a honeycomb block.
- Carbon fills up over time, so it needs replacement like any filter.
Be realistic about VOCs. The EPA says source control and ventilation come first. A purifier with carbon can be a supplement, not the primary fix for a strong chemical source. epa
Choose an Air Purifier Based on Your Main Problem
| Your main concern | What to prioritize | Caveats |
|---|---|---|
| Dust | Dust CADR; washable pre-filter; sized for room | Regular vacuuming and cleaning still do most of the work |
| Pollen | Pollen CADR; run during high-pollen seasons; keep windows closed while running | Studies show possible improvements in some allergy symptoms, but not always major ones |
| Pet dander | Strong CADR across all three numbers; easy-to-clean pre-filter for hair | Grooming and cleaning the source matter; add carbon for pet odors |
| Smoke (wildfire, cooking) | High smoke CADR; consider CADR ≈ room area for wildfire smoke per AHAM; a good carbon layer for smell | Close windows during smoke events; run at higher speed as needed |
| Odors | Substantial activated carbon; find and remove the source | HEPA alone won’t handle odors |
| VOC concerns | Carbon or another gas filter, ideally with a stated amount | No standard gas rating; source control and ventilation first |
| Bedroom | Low noise at the speed you’ll run; dim display or light-off option; adequate CADR | Oversizing slightly lets you run quieter |
| Large living room | High CADR (often 250+ CFM in larger rooms); check ACH; consider two units for open plans | Square-foot claims at 1 ACH can mislead |
Room-by-room quick guide
- Bedroom (100–200 sq ft): Noise is the deciding factor. Choose a unit whose lowest speeds are quiet and still clean adequately. Aim a bit above the EPA minimum so sleep mode still moves enough air.
- Living room (250–500 sq ft): Prioritize CADR and ACH. If it connects to a kitchen, size to the combined space.
- Home office (100–200 sq ft): You sit close to the unit, so noise and draft matter. The EPA says to redirect airflow if it causes an uncomfortable draft. epa
- Kitchen and open plan: Cooking creates particles and odors. Look for higher CADR plus carbon, and don’t ignore ventilation such as a range hood.
How Quiet Should an Air Purifier Be?
A single decibel figure is almost meaningless. Noise depends on fan speed, and the number on the box is usually the quietest setting.
- Sleep or lowest speed: whisper-quiet on paper, but airflow is lowest. Cleaning here is much slower than the CADR rating suggests.
- Low: suits background operation in a bedroom or office.
- Medium: noticeable, but fine for daytime in a living space.
- High: where CADR is measured. Expect a clear fan sound.
Manufacturer ranges show the spread. Levoit’s spec table lists 22–52 dB and Coway lists 22–54 dB(A). That is a huge gap between the lowest and highest settings. Even one product page can be inconsistent. Levoit’s page also advertises noise “as low as 24 dB” in its summary. LevoitCoway
For reference, the EPA says about 50 dB is roughly the operating noise of a modern refrigerator, and that if noise might stop you from running the unit, you should look for a low published noise rating. epa
Why this matters: a purifier that is too loud gets turned off. A slightly larger unit running at a quiet speed often beats a smaller unit that must run on high to keep up.
Air Quality Sensors and PM2.5 Displays
Many purifiers include a sensor. Levoit’s page, for example, describes a laser dust sensor with a PM2.5 display. Levoit
What they do
- Give a rough, real-time indication of airborne particles
- Drive auto mode, ramping the fan up when particles rise
- Show whether something you did (cooking, vacuuming) caused a spike
What they don’t prove
- Consumer sensors are not laboratory-grade instruments.
- Readings vary between brands and between a purifier’s sensor and a separate monitor.
- A “green” display doesn’t prove the air is healthy. It reflects what that sensor can detect, mostly particles.
- Most don’t detect gases or odors unless a specific sensor is stated.
Use the display as a trend indicator, not a measurement you’d rely on for health decisions.
Smart Features: Which Ones Matter?
| Feature | Verdict | Why |
|---|---|---|
| Auto mode | Useful | Adjusts fan speed to conditions, saving noise and energy |
| Sleep mode | Useful (bedrooms) | Dims lights and lowers fan noise |
| Timer | Useful | Lets you run the unit for set periods |
| Filter-life indicator | Useful | Helps you replace filters on time |
| PM2.5 display | Nice to have | Helpful for awareness, not precise |
| Wi-Fi and app | Optional | Handy for remote control and filter tracking, but not needed for cleaning |
| Alexa/Google voice control | Optional | Convenience only |
Wi-Fi adds cost but no extra CADR. For reference, Levoit’s app and voice control features and Coway’s Smart Mode, timer and filter alerts illustrate the typical feature set. LevoitCoway
Filter Replacement Cost
Filters are the real long-term cost. The EPA says all filters need regular replacement, because a dirty, overloaded filter won’t work well. Before you buy, check: epa
- Replacement interval (months of use)
- Filter price for the exact model
- Availability from the manufacturer and major retailers
- What’s included (some units use one combined filter, others separate HEPA and carbon layers)
Beware off-brand filters. AHAM warns that off-brand or unverified filters may not meet the original’s standards and can reduce airflow or filtration. AHAM Verifide
Worked cost example (official prices, Sept 21, 2026)
The prices below come from official manufacturer pages. Levoit lists its 3-stage replacement filter at $49.99, and Coway lists its Max2 filter at $89.00 with a stated life of one year. Coway’s own product page also describes filter life as “6–12 months”. The replacement schedules below are assumptions for illustration. Use the interval in your manual. Levoit Core® 400S-P Smart Air Purifier for Large Rooms +2
| Levoit Core 400S-P | Coway Airmega 250 | |
|---|---|---|
| Replacement filter price | $49.99 | $89.00 |
| Yearly cost, replaced every 12 months | ~$50 | $89 |
| Yearly cost, replaced every 6 months | ~$100 | ~$178 |
| 3-year cost with unit ($189.99 / $399 at time of check), 12-month filters | ~$290 | ~$577 |
| 3-year cost with unit, 6-month filters | ~$440 | ~$844 |
Calculation assumes the included filter covers the first interval and the listed unit price. This is an illustration of ownership cost, not a product ranking.
For a closer look at Coway’s real-world filter and features, see our Coway Airmega 250 review.
Rule of thumb: if two purifiers have similar CADR and noise, the one with cheaper, longer-lasting filters usually costs less over three years.
Energy Use
Wattage tells you how much power the purifier draws. Your real cost depends on fan speed and runtime.
Energy (kWh) = watts × hours ÷ 1,000
Manufacturer ratings: Levoit lists 38 W rated power and under 2 W standby, and Coway lists 64 W. LevoitCoway
Worst-case illustration, running at rated power 24/7 (LifeCareBalance calculation, using an assumed $0.17 per kWh; check your own utility bill):
- 38 W → about 333 kWh a year → roughly $57
- 64 W → about 561 kWh a year → roughly $95
Most homes will use less, since auto or lower speeds draw less power.
ENERGY STAR. ENERGY STAR says certified room air cleaners are more than 50% more energy efficient than standard models, saving about 211 kWh a year. It also notes that larger models use more energy, so size sensibly. The EPA stresses that the ENERGY STAR label refers to energy efficiency and says nothing about how well a unit cleans air. Don’t treat it as a performance certification. ENERGY STARepa
Where Should You Put an Air Purifier?
The EPA suggests placing the unit in the room where you spend the most time, keeping airflow unobstructed and staying away from curtains and anything that might block it. epa
- Bedroom: place it a few feet from the bed, not blowing directly at you. Keep intake and exhaust clear.
- Living room: central or along a wall with open space around it.
- Home office: off to the side, away from your direct line of airflow.
- Floor vs tabletop: Many units are designed for the floor, while tabletop and wall-mounted designs also exist. Follow the manual for optimal placement. ENERGY STAR
- Keep intake and exhaust unobstructed: no furniture pushed against the vents, and no curtains draped nearby. Check your manual’s clearance guidance.
- Open-plan spaces: Consider the entire space, and consider multiple units if one can’t cover it. epa
Common Air Purifier Buying Mistakes
- Trusting the headline square footage without checking ACH.
- Ignoring CADR and buying on looks or filter names.
- Buying too small because the room “isn’t that big” (ceiling height counts).
- Focusing on pollen or dust CADR when smoke CADR is the better sizing number for fine particles.
- Assuming HEPA removes odors or gases.
- Buying carbon “in name only.” Check for a stated amount.
- Judging noise by the lowest dB figure and never checking higher speeds.
- Skipping replacement-filter prices until the first filter dies.
- Using off-brand filters without checking they meet the original’s performance.
- Choosing ozone-producing or unverified “electronic” devices.
- Paying for Wi-Fi and app features you won’t use.
- Blocking airflow with furniture, curtains or walls.
- Expecting a cure. Purifiers may help reduce airborne particles, but they don’t replace source control, ventilation or medical care.
- Never replacing the filter. A clogged filter cleans poorly.
Beginner Air Purifier Buying Checklist
- Room size measured (length × width × ceiling height)
- CADR checked (smoke CADR vs your room, adjusted for ceilings)
- ACH/coverage assumption checked
- Filter type checked (HEPA-grade, with a stated efficiency or tested CADR)
- Activated carbon considered (with a stated amount if odors/VOCs matter)
- Noise checked at the speeds you’ll actually use
- Filter replacement cost and interval checked
- Energy use checked
- Sensor and features evaluated (useful vs optional)
- Placement considered (clear airflow, right room)
FAQ
What is the most important thing to look for in an air purifier?
CADR sized to your room. It tells you how much clean air the unit delivers. Then check filters, noise and ongoing filter cost.
How do I know what size air purifier I need?
Measure your room’s square footage and ceiling height, then match the smoke CADR using the EPA’s sizing chart. The EPA’s chart suggests about 65 CFM per 100 sq ft, assuming 8-foot ceilings. Go higher for tall ceilings, open plans or smoke. epa
What CADR should I look for?
About 65 CFM for a 100 sq ft room, 130 for 200 sq ft, 195 for 300 sq ft and 260 for 400 sq ft is the EPA’s approximate minimum. AHAM suggests roughly matching room area for wildfire smoke.
Is higher CADR always better?
Higher CADR cleans faster, but there are trade-offs. Bigger units can cost more, use more energy and be louder at high speeds. A modestly oversized unit running quietly is often the sweet spot.
Is True HEPA better than HEPA-type?
Not automatically as a label. “True HEPA” generally signals a filter meeting the HEPA benchmark, while “HEPA-type” doesn’t promise it. In real life, tested CADR and independent verification tell you more than either phrase.
Do I need activated carbon?
Only if you’re dealing with odors, smoke smell or VOCs. HEPA handles particles. For gases, look for a substantial amount of carbon.
How quiet should an air purifier be for a bedroom?
Choose a unit that’s quiet at the speed you’ll run overnight. Many products list about 22–24 dB at their lowest setting, but that doesn’t reflect noise at higher speeds. For scale, the EPA says about 50 dB is comparable to a modern refrigerator. epa
How often should an air purifier filter be replaced?
Follow the manufacturer’s schedule, which is commonly somewhere between 6 and 12 months for combined filters. The EPA notes that all filters need regular replacement, and dirty ones work poorly. epa
Are air purifier square-footage claims accurate?
They can be accurate for the assumption they use, but the assumption is often 1 ACH, which is slow cleaning. Check the ACH note or calculate coverage from the CADR.
Should an air purifier run all day?
The EPA says longer run times and higher fan speeds increase the amount of air filtered. Many people run a unit continuously on auto or low and turn it up during dusty or smoky events. Balance filter wear, noise and energy cost. epa
Where should I place an air purifier?
In the room you use most, with clear space around the intake and exhaust, away from curtains and walls, and not blowing directly at you.
Is an expensive air purifier worth it?
Not necessarily. Price often reflects features, build and brand, which don’t always raise CADR. Compare CADR, noise, filter cost and warranty. A rough value check is price divided by smoke CADR, though that ignores filters and noise.
Final Thoughts: Your Air Purifier Buying Guide in One Minute
If you remember one thing from this air purifier buying guide, remember this: match a verified smoke CADR to your room (adjusted for ceiling height), read the ACH behind any coverage claim, and price the replacement filters before you buy. Everything else, from sensors to apps to filter branding, is secondary.
Use the checklist above, compare two or three models on the same numbers, and buy the one that fits your room, your noise tolerance and your long-term budget. Purifiers may help reduce airborne particles, but pair them with source control and ventilation for the best results.