Notice
Sources and dates
How every sheet is sourced, what "as at" means here, and how corrections are made.
Mask advice changed quickly between 2020 and today, and much of it was written for an emergency. A sheet that quotes a rule without saying when it was written can mislead. This page explains how the editors pick sources, how dates are shown and what the register at the foot of the page records.
How sources are chosen
The editors prefer public-health agencies (WHO, ECDC, national health services, CDC and NIOSH), regulators and standards bodies, systematic reviews and peer-reviewed papers, and museum or archive pages for history. A source is used only if the editors opened the page and it supports the sentence it is attached to. If a page cannot be opened, or does not say what was expected, the claim is dropped or described in general terms. Where one country's agency is cited, the sheet names it and does not present it as universal.
What "as at" means
Every rule, threshold and recommendation is dated to the page it came from, using the publication or last-updated date shown on that page. A line that starts "As at" tells you that the statement was true of that source on that day, not that it is in force today. Where a page shows no date, the sheet says the page is undated and gives the day the editors opened it. Regulations and public-health advice can change, so check the issuing body for the present position.
Withdrawn and archived guidance
Some pandemic-era guidance has since been withdrawn from its original address. Where a sheet relies on such a page, it uses an archived copy, says so, and gives the date of the copy. Archived guidance is shown for what it said at the time and is never presented as current advice.
Corrections
If you find an error or a source that has been superseded, write through the contact form. A factual correction is made on the sheet, and the sheet's update date changes. Nothing is quietly deleted: a substantive change is noted on the sheet itself.
Guidance dates by sheet
| Sheet | Title | Guidance dates |
|---|---|---|
| 01 | How masks and respirators work | NIOSH 2009 and 25 Mar 2025; WHO 9 Jul 2020; FDA 21 Oct 2024; review 4 May 2021 |
| 02 | Types of masks and respirators | NIOSH 25 Mar 2025; FDA 21 Oct 2024; review 4 May 2021; HSE 7 Aug 2024 |
| 03 | The standards behind masks | Review 4 May 2021; NIOSH 25 Mar 2025 and 21 Jan 2025; HSE 7 Aug 2024; CEN 22 Nov 2022 |
| 04 | Fit and seal check | NIOSH, 2 Nov 2017; OSHA 1910.134 (amended 2019); HSE, 8 Apr 2025; CDC, 12 Mar 2025 |
| 05 | Putting on and taking off | WHO, 12 Oct 2023 and 1 Dec 2020; CDC, 10 and 12 Mar 2025; NIOSH guide undated |
| 06 | Counterfeit respirators | NIOSH 3 Feb 2025 and 21 Jan 2025; HSE 7 Aug 2024; MHRA 1 Jul 2023 |
| 07 | Caring for reusable masks | WHO, 1 Dec 2020; CDC, 10 and 12 Mar 2025 and 11 May 2023; NIOSH 2018 |
| 08 | How to make a cloth mask | CDC archived copy, reviewed 4 Apr 2020; WHO interim guidance, 1 Dec 2020 |
| 09 | What the evidence says | Cochrane, 30 Jan 2023; Annals, 29 Nov 2022; WHO, Dec 2021; CDC, 18 Aug 2025 |
| 10 | Masks and children | WHO/UNICEF, 7 Mar 2022; CDC archive, 11 May 2023; CDC, 18 Aug 2025; NIOSH, 25 Mar 2025 |
| 11 | Glasses and hearing aids | AAO, 22 Jan 2021; case report, 8 Aug 2021; Henry Ford, 5 Dec 2020; CDC, 12 Mar 2025 |
| 12 | Headaches and skin irritation | Headache, May 2020; JCAD, 2021; AAD, 7 Aug 2024; NIOSH, 25 Mar 2025 |
| 13 | Who should still wear one | ECDC, 27 Mar 2024; CDC, 18 Aug 2025; CDC wildfire, 24 Aug 2026; UKHSA, 16 Jun 2022 |
| 14 | A short history | Medical History, Apr 2022; BiblioAsia, 7 Jul 2020; NIOSH, Jun 1995; CDC, 3 Apr 2020 |
The short version
Sources are opened, not assumed. Every rule carries the date of the page it came from, and old guidance is labelled as old.