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Alen · air purifier

Alen BreatheSmart FLEX replacement filter

Use part B4-Pure. Replace every 6-12 months.

The filter that fits

Maker lists this model

Alen 45i & FLEX Filter, Essential (Pure)

BreatheSmart 45i & FLEX HEPA Filter, Essential (Pure)

Part number
B4-Purealso sold as FL40-Pure
How many
1 per unit
How often
Replace every 6-12 months
Buy 45i & FLEX Filter, Essential (Pure) on AmazonManufacturer page

Opens an Amazon search for Alen 45i & FLEX Filter, Essential (Pure) B4-Pure. Check the listing says B4-Pure.

See the sources

Look-alike models that take a different filter

The BreatheSmart 75i, T500, A350 and FIT50 each take their own filter families (B7, TF60, BF15A, FF50); 45i and FLEX filters do not fit them.

Other genuine options from Alen

Variants and kits the maker sells for this model. Same fit, different media or packaging.

  • BreatheSmart 45i & FLEX HEPA Filter, Advanced (Fresh), formerly B4 Fresh
    F10017 · Find on Amazon
  • BreatheSmart 45i & FLEX HEPA Filter, Ultimate (Odor), formerly B4 Odor
    F10014 · Find on Amazon

Sources

High confidence: the manufacturer names this exact model on a live page. Each line is the page we read and the day we read it.

  • Which filter fits alen.com Maker's own page checked Oct 1, 2026
    BreatheSmart 45i & FLEX HEPA Filter ... Q: Will this filter also fit in Breathsmart Flex unit? A: Yes. The BreatheSmart 45i and BreatheSmart FLEX share the same filters.
  • How many web.archive.org Maker's archived page checked Oct 1, 2026
    Page for the BreatheSmart FLEX (per its URL): Remove the used filter by pulling out on the pull strings ... insert it into the air cleaner with the pull strings facing outward (archive.org copy of 2025-08-13; live page returns 403)
  • How often alen.com Maker's own page checked Oct 1, 2026
    Purifier includes filter rated for: 6-12 months
  • What's in it alen.com Maker's own page checked Oct 1, 2026
    All of our filters come with a Prefilter attached. On the BreatheSmart 45i filters, the prefilter is the black-mesh attachment with Velcro on the front of the filter.
  • Other genuine options alen.com Maker's own page checked Oct 1, 2026
    (Formerly B4 Fresh) ... (Formerly B4 Odor)
Show all 9 sources

Which filter fits

  1. alen.com Maker's own page checked Oct 1, 2026
    BreatheSmart 45i & FLEX HEPA Filter ... Q: Will this filter also fit in Breathsmart Flex unit? A: Yes. The BreatheSmart 45i and BreatheSmart FLEX share the same filters.
  2. alen.com Maker's own page checked Oct 1, 2026
    Our entry-level filter. Features a medical-grade HEPA filter that captures 99.9% of airborne dust, viruses, and allergens. (Formerly B4 Pure)
  3. alen.com Maker's own page checked Oct 1, 2026
    BreatheSmart 45i & FLEX HEPA Filter variants: Essential (Pure) F10016; Advanced (Fresh) F10017; Ultimate (Odor) F10014
  4. web.archive.org Maker's archived page checked Oct 1, 2026
    4 Filter options: Pure, Fresh, Odor, and VOC/Smoke | BreatheSmart FLEX & 45i filters are compatible (archive.org copy of support.alen.com, 2025-09-05; live page returns 403)
  5. web.archive.org Maker's archived page checked Oct 1, 2026
    Breathesmart FLEX HEPA Air Filter: B4-Pure / FL40-Pure (archive.org copy of 2023-06-03)

How many

  1. web.archive.org Maker's archived page checked Oct 1, 2026
    Page for the BreatheSmart FLEX (per its URL): Remove the used filter by pulling out on the pull strings ... insert it into the air cleaner with the pull strings facing outward (archive.org copy of 2025-08-13; live page returns 403)

How often

  1. alen.com Maker's own page checked Oct 1, 2026
    Purifier includes filter rated for: 6-12 months

What's in it

  1. alen.com Maker's own page checked Oct 1, 2026
    All of our filters come with a Prefilter attached. On the BreatheSmart 45i filters, the prefilter is the black-mesh attachment with Velcro on the front of the filter.

Other genuine options

  1. alen.com Maker's own page checked Oct 1, 2026
    (Formerly B4 Fresh) ... (Formerly B4 Odor)