# Why can't you use HEPA material to make masks? **URL:** \ **Category:** The Quarantine Zone\ **Tags:** covid-quarantine\ **Created:** [April 5, 2020, 9:52pm UTC](https://boards.straightdope.com/t/why-cant-you-use-hepa-material-to-make-masks/851151 "2020-04-05T21:52:20Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Damuri\_Ajashi](https://avatars.discourse-cdn.com/v4/letter/d/898d66/32.png) [@Damuri\_Ajashi](https://boards.straightdope.com/u/Damuri_Ajashi)\ **Post date:** [April 5, 2020, 9:52pm UTC](https://boards.straightdope.com/t/why-cant-you-use-hepa-material-to-make-masks/851151/1 "2020-04-05T21:52:20Z")
I’ve been making masks using HEPA material. Everything I have read leads me to believe that HEPA is a higher standard than N95. It clogs up from moisture but if you can figure out how to inhale through the filter and exhale through a valve, it works pretty well. I am using a cheap snorkel purge valve to try and achieve this and it seems to work pretty well. The HEPA filter material doesn’t come close to clogging up during the course of a day and I can replace the HEPA insert every morning. ---
**Author:** ![PastTense](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pasttense/32/14550_2.png) [@PastTense](https://boards.straightdope.com/u/PastTense)\ **Post date:** [April 6, 2020, 3:49am UTC](https://boards.straightdope.com/t/why-cant-you-use-hepa-material-to-make-masks/851151/2 "2020-04-06T03:49:21Z")
There is a New York Times article: **What’s the Best Material for a Mask?** > [@](#): > > In recent tests, HEPA furnace filters scored well, as did vacuum cleaner bags, layers of 600-count pillowcases and fabric similar to flannel pajamas… > > The best medical mask — called the N95 respirator — filters out at least 95 percent of particles as small as 0.3 microns. By comparison, a typical surgical mask — made using a rectangular piece of pleated fabric with elastic ear loops — has a filtration efficiency ranging from 60 to 80 percent. > > Dr. Wang’s group tested two types of air filters. An allergy-reduction HVAC filter worked the best, capturing 89 percent of particles with one layer and 94 percent with two layers. A furnace filter captured 75 percent with two layers, but required six layers to achieve 95 percent… > > The problem with air filters is that they potentially could shed small fibers that would be risky to inhale. So if you want to use a filter, you need to sandwich the filter between two layers of cotton fabric. > **[What’s the Best Material for a Mask?](https://www.nytimes.com/article/coronavirus-homemade-mask-material-DIY-face-mask-ppe.html)** > > Scientists are testing everyday items to find the best protection from coronavirus. Pillow cases, flannel pajamas and origami vacuum bags are all candidates. ---
**Author:** ![Folly](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/folly/32/3219_2.png) [@Folly](https://boards.straightdope.com/u/Folly)\ **Post date:** [April 6, 2020, 3:55am UTC](https://boards.straightdope.com/t/why-cant-you-use-hepa-material-to-make-masks/851151/3 "2020-04-06T03:55:33Z")
If a mask is more to protect others from yourself than vice versa, then it seems to defeat the purpose to exhale through a valve… ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [April 6, 2020, 3:58am UTC](https://boards.straightdope.com/t/why-cant-you-use-hepa-material-to-make-masks/851151/4 "2020-04-06T03:58:57Z")
Since there’s no way that a face mask is making a gas-tight seal at the edges, it’s probably self-defeating to use a high-pressure-drop material, which will face un-filtered air around the mask. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [April 6, 2020, 4:16am UTC](https://boards.straightdope.com/t/why-cant-you-use-hepa-material-to-make-masks/851151/5 "2020-04-06T04:16:24Z")
HEPA filters are designed to work in HVAC systems that generate a measurable constant pressure differential. Air is drawn through because of that pressure differential. When you breath, you have a very low and unsteady pressure differential so a mask with a good seal is going to require a lot of effort to draw air through the material. HEPA filters can also produce fibers that are generally no worse than what you get in the ambient environment but if you are breathing them in constantly may irritate the respiratory passages, actually make you more vulnerable. I don’t know what the actual effectiveness of HEPA materials are in filtering viruses but clean rooms and biocontainment facilities actually use ultra-low particulate air (ULPA) rather than HEPA standard filters, and then decontaminate the exhaust with high heat or UV-C irradiation. It would seem that standard HEPA filters are just not adequate for that purpose, although they are obviously better than plain woven textiles like cotton or polyester blends. The filter material in N95 and normal surgical masks is a non-woven textile produced by special melt-blown process. This material actually has particularly electrostatic properties that attract small particulates like bacteria and virions out of the air. Because there only a few companies that make machines that produce this material, and getting them to work reliably is a very finicky process engineering problem, the ability to rapidly increase material production is limited. Stranger ---
**Author:** ![ratatoskK](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ratatoskk/32/2987_2.png) [@ratatoskK](https://boards.straightdope.com/u/ratatoskK)\ **Post date:** [April 6, 2020, 2:35pm UTC](https://boards.straightdope.com/t/why-cant-you-use-hepa-material-to-make-masks/851151/6 "2020-04-06T14:35:36Z")
There’s some DIY info [here](https://www.instructables.com/id/DIY-Cloth-Face-Mask/). Scroll down to “Step 8: Filters”. ---
**Author:** ![Damuri\_Ajashi](https://avatars.discourse-cdn.com/v4/letter/d/898d66/32.png) [@Damuri\_Ajashi](https://boards.straightdope.com/u/Damuri_Ajashi)\ **Post date:** [April 6, 2020, 4:41pm UTC](https://boards.straightdope.com/t/why-cant-you-use-hepa-material-to-make-masks/851151/7 "2020-04-06T16:41:46Z")
> [@Folly](#): > > If a mask is more to protect others from yourself than vice versa, then it seems to defeat the purpose to exhale through a valve… The valve is inside the fabric layer.