How we evaluate compatible filters.

When we write that a filter is rated for a particular contaminant, that a part number cross-references a specific OEM model, or that a warranty position is protected by federal law, the claim is anchored to one of the following sources. Higher items in this list outrank lower items when they disagree.

1. AStandards-body documents. NSF International (info.nsf.org), AHAM, EN 1822 published text, ASHRAE 52.2, AAMI ST91, FDA Code of Federal Regulations. We cite these by standard number so you can verify directly at the issuer.
2. BManufacturer specification sheets. Downloaded from the manufacturer's own site, dated when retrieved. We do not rely on third-party reseller pages for spec claims because resellers have an incentive to round numbers favorably.
3. CGovernment and regulatory text. US Code (15 U.S.C. 2301-2312 Magnuson-Moss, 42 U.S.C. 300f Safe Drinking Water Act), FTC enforcement guidance, CMS Local Coverage Determinations.
4. DIndependent laboratory reports. Where a NSF-accredited lab or peer-reviewed paper has tested a specific filter or technology, we cite the report. These are rare for the consumer compatible-filter market.
5. ETrade associations. Water Quality Association, Asthma and Allergy Foundation of America, American Academy of Sleep Medicine guidance. Useful for consumer-facing thresholds (when does hard water become a problem, what GPG is "hard") but secondary to standards-body documents.

Our consolidated index of every standard cited across this site is at /standards. If a claim on our site is not linkable to one of the categories above, it is opinion, and we mark it as such.

The compatible-filter market is densely populated with phrases that sound like certifications but carry no verification. We do not treat the following as evidence that a product meets a standard:

• "NSF compliant." NSF compliance is a marketing claim by the manufacturer. It is not the same as NSF certification, which requires testing by an NSF-accredited laboratory and issues a verifiable certification number. Without a number, the phrase means the manufacturer believes the product would pass — not that it has.
• "Tested to NSF standards." Tested by whom? At what lab? Reported where? "Tested to" without a third-party report is functionally identical to "not tested."
• "HEPA-style" or "HEPA-type." True HEPA is defined by EN 1822 classes H13 and H14 and captures 99.97% or better at 0.3 microns. "HEPA-type" is an unregulated marketing term typically meaning 90-99% capture at 1-2 microns, a substantially weaker performance.
• "Removes 99% of contaminants." Which contaminants? At what challenge concentration? Under what flow rate? Without a tested contaminant list and challenge protocol, the number is unverifiable.
• Amazon product reviews. Reviews on a product's own listing are gameable, frequently paid for, and not a substitute for independent verification. We cross-reference review patterns only as a tiebreaker, never as primary evidence.
• Influencer endorsements. Affiliate or sponsored relationships between content creators and manufacturers are rarely disclosed in compatibility content. We do not weight a recommendation based on social-media presence.

Generative AI is now capable of writing fluently incorrect content at scale. The public web is filling up with templated AI buying guides that look authoritative and cite nothing. We have a position on this.

Every factual claim on this site has been cross-checked against a primary source listed in Section 01. The chain we follow: human draft → primary-source verification → optional AI grammar pass → human re-read → publish. When we have inserted experience content (a real photo, a measured dimension, actual unboxing notes), the page makes that explicit.

For every guide, the verification checklist is the same:

1. 01Every part number cross-references at least two independent sources (manufacturer site plus one major retailer listing).
2. 02Every certification claim links back to the standards body (NSF certification database, EN 1822 class definition, AAMI document, etc.).
3. 03Every legal claim links to the actual statute or regulation, not to a third-party summary.
4. 04Every replacement-interval or maintenance-schedule claim cites the manufacturer's published guidance, not a community rule of thumb.
5. 05If a guide includes a buying recommendation tier (Buy / Acceptable / Avoid), the tier criteria are explicit and the same across categories.

When a reader reports a factual error, we update within 48 hours and bump the "Last reviewed" date in the article's editorial footer. Corrections are silent — we want the current page to be accurate, not a transcript of past mistakes. The public Git history of this site shows every change if you want to see the diff.

The standards landscape changes. NSF revises certification protocols. New emerging contaminants get added to NSF/ANSI 401. The FDA reclassifies devices. CMS updates Medicare LCDs. Our update cadence:

Each guide's editorial footer shows the original publish date and the most recent review date. If you ever see a guide with a stale "Last reviewed" date older than 9 months on a topic where the standards have changed, email us and we will prioritize the review.

• ×Recommendations without certification verification. If a category has an applicable independent certification (NSF, AHAM, EN 1822) and a product does not hold it, we will not recommend it even at a lower tier.
• ×Paid product placement. No brand is paying for its position on this site. The directory is sorted by category coverage, not by margin.
• ×Templated AI-generated product pages at scale. The pre-2026 version of this site experimented with this pattern. It produced thousands of mediocre pages, ranked nowhere, helped no one, and earned a Google Helpful Content adjustment. We deleted that version and are rebuilding around hand-written editorial only.
• ×Sponsored reviews disguised as editorial. When manufacturers or distributors send sample products, we will disclose it explicitly inside the guide and add a permanent "Sample provided" note in the editorial footer.
• ×Affiliate links disguised as neutral references. Every Amazon link on this site is an affiliate link. The disclosure appears on every page that contains them. We use the standard rel="nofollow sponsored" attribute on every commercial outbound link.

Pick any factual claim on this site and trace it. The reference is either inline in the article, in the article's References & Standards section, or in the consolidated /standards page. If a claim is not traceable, that is an editorial error and we want to fix it. Email us at the address below.

The site's source code is in a public Git repository. Article changes, corrections, and version history are auditable per-commit. If you ever wonder whether a recommendation was added because of a manufacturer relationship or after an editorial review, the commit history will tell you.