Search for Alen air purifier decibels and the product pages answer more directly than most. Every Alen spec table has a "Noise level" line with two labelled figures, one for the lowest speed and one for the highest. Alen also prints the airflow at every speed, which tells you how much air the quiet setting actually moves.
We read the US product page, the user manual and Alen's help center article on noise for every air purifier on Alen's US store. Five models are on sale, and all five carry a noise figure: the BreatheSmart 25i, 35i, 45i and 75i, and the T500 tower. This page lists all of them, explains what Alen's labels mean, and ranks each model only against others in the same room-size group.
Our picks are listed above, with full specifications for each.
Every Alen spec table gives two figures in the same form, for example "Lowest Speed - 23 dB | Highest Speed - 42 dB" on the BreatheSmart 35i. The labels matter. They tie each figure to a named fan speed, so the low end is always the slowest speed and the high end the fastest. Alen does not state a measuring distance or a test standard for any of them.
The highest speed is not the same step on every model. On the 25i and 35i it is Turbo, the fourth of four speeds. On the 45i the product page calls it Speed 4, and the manual calls the same button Turbo (T). On the 75i it is Turbo, the fifth of five speeds. On the T500 it is Level 3, which the manual calls "Maximum cleaning power".
The unit label changes too. The 25i, 35i and 45i spec lines say dB. The 75i and T500 spec lines say dBa. Alen's comparison table, which appears on each BreatheSmart page, gives all four BreatheSmart ranges in dBA, for example "20 dBA - 40dBA" for the 25i. Alen's help center says noise "is measured in decibels (dBa)". We take Alen at its word that all five are A-weighted, which is why we rank them against each other. If you want the background on why the A matters, our explainer on what dBA means covers it.
We group the models by Alen's own coverage at one air change an hour, the figure its spec tables print first. The small-room group is rated for up to 1,400 sq ft an hour. The large-room group is rated for 1,900 sq ft an hour or more.
Lowest speed, small-room group, quietest first
- BreatheSmart 25i: 20 dB; 45 CFM; 800 sq ft an hour.
- BreatheSmart 35i: 23 dB; 66 CFM; 1,400 sq ft an hour.
- T500: 42 dBa; 48 CFM; 500 sq ft an hour.
Highest speed, small-room group, quietest first
- BreatheSmart 25i: 40 dB on Turbo; 120 CFM.
- BreatheSmart 35i: 42 dB on Turbo; 190 CFM.
- T500: 56 dBa on Level 3; 130 CFM.
Lowest speed, large-room group, quietest first
- BreatheSmart 45i: 24 dB; 80 CFM; 1,900 sq ft an hour.
- BreatheSmart 75i: 25 dBa; 100 CFM; 2,800 sq ft an hour.
Highest speed, large-room group, quietest first
- BreatheSmart 45i: 50 dB on Speed 4; 255 CFM.
- BreatheSmart 75i: 51 dBa on Turbo; 375 CFM.
Two things stand out. In the large-room group, the gap between the 45i and the 75i is 1 dB at each end, which is too small to call a difference when Alen gives no distance. The 75i moves 20 CFM more on its lowest speed and 120 CFM more at the top. And the T500 sits apart from the BreatheSmart models: its lowest speed, 42 dBa, equals the highest speed of the 35i.
What the room sizes mean
Alen's headline coverage is large because it assumes few air changes. The 35i page says it "Purifies up to 1,400 sq ft every hour or 700 sq ft every thirty minutes", which is one and two air changes an hour. The usual sizing basis for a purifier, AHAM's, assumes 4.8 air changes an hour, which gives a much smaller room. ENERGY STAR, which uses that basis, lists the 25i for 150 sq ft, the 35i for 233 sq ft and the 75i_V2 for 574 sq ft. ENERGY STAR's search for Alen does not list the 45i or the T500, though Alen's spec tables say both are ENERGY STAR certified.
This is why sizing matters for noise. A purifier rated well above your room can stay on a low speed and still keep up. One rated near your room's size will need its higher, louder speeds. Our bedroom air purifier guide works through that sum for a typical bedroom.
What the gaps mean
Decibels are logarithmic. Roughly every 10 dB sounds about twice as loud. So the 25i's top figure, 40 dB, should sound about four times as loud as its 20 dB lowest speed, and the T500's 56 dBa at the top is a big step above any BreatheSmart model's top figure. Gaps of 1 to 3 dB between two Alen models are small, and with no distance stated they may be no gap at all. Our decibel chart sets published appliance figures beside everyday sounds such as a whisper and a fridge.
Alen's quiet features, by their real names
Pink noise. Alen lists "Sound type: Pink Noise | All speeds" for the 35i, 45i and 75i. The T500's sound type is "N/A", and the 25i's spec table does not list one. Alen describes pink noise as sound in which "The low tone sounds in pink noise are present in higher levels than the high tones." It is a claim about the character of the sound, not a lower decibel figure.
Sleep Mode. On the 25i, 35i, 45i and 75i, pressing Power once puts the unit into Sleep. The manuals say "It will continue to work, but the lights will turn off. To wake the unit from Sleep, press any button." Sleep Mode does not change the fan. A second press within 5 seconds turns the purifier off.
Light AUTO. The LIGHT button cycles through LO, HI and AUTO (shown as A on the 25i and 35i). The manuals say AUTO "adjusts control panel lights based on the ambient light level, dimming or turning off the lights", and the 25i, 35i and 75i manuals say it turns them off "when the room is dark". LO "dims the lights by 50%", per Alen's help center.
Auto mode. The BreatheSmart control panels say "Automatic Mode sets speed for optimal performance." Alen's spec tables describe the sensor as "Automatic speed adjustment and coordinated to 5 color rings". The manuals do not say Auto mode stays off Turbo, so it can reach the loudest speed when the air is bad.
Button chime. The 35i, 45i and 75i manuals say "A chime will indicate that the control has registered your touch." The 25i manual has no such line. It matters only if you adjust the purifier while someone sleeps.
Timer. The T500 has a "Twelve-hour timer" and no auto mode, sensor or light setting. Alen's app FAQ says the app has no Timer or Light control for the 45i.
The BreatheSmart pages also show a Quiet Mark Certified badge. What that says about Alen against other brands is Alen's claim, not ours, and we have not checked how it was reached.
Keeping an Alen quiet
Alen's manuals recommend a high speed for the first couple of hours in a new room, then a lower one. The 25i and 35i manuals say to run the purifier "continuously at the highest fan speed where the sound is acceptable, or set the unit on Auto mode." The 45i and 75i manuals suggest "speed two (2) or three (3)". For a bedroom, that means choosing the speed by ear, and sizing up so a low speed is enough. Clean or replace the filter on schedule. A clogged filter makes the fan work harder for the same air. Our nursery air purifier guide covers the 25i's lock, lights and cover in more detail for a baby's room.
Alen's help center article "How Loud Are Alen Air Purifiers?" gives figures that do not match three product pages. It lists the 75i at "Lowest Speed 25 dBa | Highest Speed 49 dBa" against 51 dBa on the product page, and the 45i at 23 and 49 dBa against 24 and 50 dB. The help center's 35i page lists "Lowest Speed 21 dBa | Highest Speed 42 dBa" against 23 dB on the product page. We rank on the product pages. On the help center figures, the 45i and 75i tie at the top.
Other claims conflict too. The 35i page's FAQ calls it "our quietest purifier", while the 25i is listed lower at both ends. Alen's explore page says "Alen runs as low as 25 dB", which is higher than the lowest-speed figure on three of its own models. The 25i's spec table gives CADR as 107 but airflow on Turbo as 120 CFM, and the T500 gives CADR as 145 against 130 CFM on its top speed. Alen does not explain any of these.
Alen's help center still lists figures for the BreatheSmart FLEX, Classic and FIT50, but their product pages on alen.com now redirect to the collection page, so we treat them as no longer sold by Alen and do not rank them.
How Alen compares
This page ranks Alen models only against each other. For how they sit among other brands, on each maker's own published figures, see our guide to the quietest air purifiers. Alen, like most makers, states no measuring distance or standard, so read our explainer on why decibel ratings rarely compare across brands before treating a small gap as real. For a brand that prints a range and names the setting for only some of its low figures, see our Levoit air purifier decibels page.
How we chose
We read Alen's US air purifier collection and its sitemap, and opened the product page, the user manual and the help center product page for every purifier on sale. We recorded each noise figure in Alen's words, with the speed it describes, and noted where a page, its comparison table and the help center disagree. We grouped models by Alen's own coverage at one air change an hour and ranked lowest-speed figures against lowest-speed figures, and highest against highest, only within a group. Prices are Alen's US list prices on October 4, 2026.
We did not measure any of these purifiers. Every figure here is Alen's own, from its product pages, manuals and help center, and Alen states no measuring distance or test standard for any of them. Owners' rooms, floors and filters will change what they hear.
