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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>FMVIZ | Face mask filtration</title>
<meta name="author" content="Timothy Sipkens" />
<meta name="keywords" content="face mask, mask, filtration, Sipkens, Tim, Timothy, Rogak, N95, pressure drop, COVID, aerosol transmission, viz, graphic, aerosols, experiment, PPE" />
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<link href="https://pro.fontawesome.com/releases/v5.2.0/css/all.css" rel="stylesheet">
<!-- Load d3.js -->
<script src="https://d3js.org/d3.v4.js"></script>
<!-- Load jquery -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
<!-- Load styles -->
<link href="index.css" rel="stylesheet">
</head>
<body>
<header id="header">
<a href="#h1" id="logo">fmviz</a>
<nav>
<a class="nav-expand" onclick="openSideBar()"><i class="fas fa-bars" style="font-size:17pt"></i></a>
<ul class="nav-list" id="nav-list-a">
<li id="nav-fvp"><a href="#hfvp" class="nav-link">FILTRATION & PRESSURE DROP</a></li>
<li id="nav-diy"><a href="#hdiy" class="nav-link">DESIGN YOUR OWN MASK</a></li>
<li id="data-link-a"><a href="#htable" class="nav-link" onclick="highlightDataLink()">
<div style="display:inline-block;vertical-align:middle;">
<span><i class="fas fa-table" style="font-size:12pt"></i></span>
<span style="padding-left:4px;">LINK TO DATA</span>
</div>
</a></li>
<li><a href="https://github.com/tsipkens/fmviz" class="nav-link">
<img src="imgs/GitHub-Mark-64px.png" alt="GitHub" style="height:13pt; padding-right:5px;">
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</ul>
</nav>
</header>
<div class="side-nav" id="side-nav-list">
<ul class="nav-list">
<li><a class="nav-expand" onclick="closeSideBar()">
<i class="fas fa-times" style="font-size:17pt"></i>
</a></li>
<li><a href="#hfvp" onclick="closeSideBar()" class="nav-link">FILTRATION & PRESSURE DROP</a></li>
<li><a href="#hdiy" onclick="closeSideBar()" class="nav-link">DESIGN YOUR OWN MASK</a></li>
<li><a href="#htable" onclick="closeSideBar()" class="nav-link" onclick="highlightDataLink()">
<div style="display:inline-block;vertical-align:middle;">
<span><i class="fas fa-table" style="font-size:12pt"></i></span>
<span style="padding-left:4px;">LINK TO DATA</span>
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<div class="main" id="main-intro">
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<a class="anchor" id="h1"></a>
<div style="padding-top:60px;"></div>
<h1 style="margin-bottom:0px;padding-bottom:0px;">fmviz</h1>
<h3 style="margin-top:0px;padding-top:0px;padding-bottom:30px;text-align:center;">
<div style="margin:auto;width:100%;max-width:500px;">Visualizing face mask data.<br>How good are common materials?</div>
</h3>
<div style="text-align:center;padding-bottom:30px;">
<img src="imgs/header_img.svg" alt="Header image" style="width:40%;max-width:130px;">
</div>
<p>
The primary function of any mask is to filter particles out of the air that passes
through the mask, which is relevant to preventing the spread of the COVID-19
and other respiratory diseases. Filtration occurs as a result of several mechanisms, including
<b>impaction</b>, <b>interception</b>, <b>diffusion</b>,
and <b>electrostatic attraction</b>.
</p>
<div style="text-align:center;">
<img src="imgs/01_schem1.svg" alt="Impaction and interception" style="max-width:400px;width:100%;padding:13px;padding-bottom:0px;"></img>
<img src="imgs/01_schem2.svg" alt="Diffusion and electrostatic" style="max-width:400px;width:100%;padding:13px;padding-bottom:0px;"></img>
</div>
<p>
Impaction tends to dominate for larger particles,
while diffusion is most significant for smaller particles. Electrostatic charges
in the mask, as are typically present in N95-compliant masks, tend to enhance
particle filtration but also tend to hold up rather poorly when the mask
is sanitized. For example, washing most N95-compliant masks with soap and water
will significantly reduce their filtration properties.
</p>
<p>
This web app seeks to visualize the overall filtration measured for
a range of common materials, using data from
<a href="https://www.tandfonline.com/doi/full/10.1080/02786826.2020.1855321">Rogak et al. (2021)</a>.
In accordance with that paper, we consider relatively large particles in the typical <b>aerosol</b> range,
roughly from half to several micron. This compliments other studies that consider smaller particles.
Limited information is also provided in terms of sanitization treatments (e.g., laundering)
and their effect on material properties.
</p>
<p>
Before continuing, we make several general notes:
</p>
<a class="anchor" id="leakage-note"></a>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> <b>What about leakage around the mask?</b></button>
<div class="text-indent">
<p>
<b style="color:var(--a0)">This is a key question!</b>
</p>
<p>
We note that the focus of this data is on
the <b>materials</b> themselves. As such, these curves <b>do not</b> consider
leakage around the mask. This is another very important feature in
constructing an effective masks but requires different kinds of
tests that are wearer-specific. There are studies by other
aerosol researchers and health professionals considering this
aspect of mask construction.
</p>
<p>
<a href="https://pubs.acs.org/doi/10.1021/acsnano.0c03252">Konda et al. (2020)</a>,
for example, present the difference in filtration between candidate materials with and
without leakage. This research found a gap representing only 0.5-2% of the surface
area reduced the performance of an N95-compliant mask from 99% to less than 15% filtration.
It is worth noting that those meausurements correspond to where a majority of the
particles are near the lower end of the particle size range considered here.
The largest human-generated particles (that is, the particles you can see, which
are above the size range even considered here) will travel <b>ballistically</b>
(that is, more of less on straight trajectories), such that leakage should
trend downwards, even if some of the particles from the deflected spray
will still escape the masks in practice.
</p>
<p>
Regardless, the mask will function better if one reduces the impact of
gaps. We recommend maximizing the surface area
of air exchange to improve effective ventilation. A cup-shape or duck
bill design may be preferred. Other solutions to reduce gaps include
external braces, sponge nose pads, or reinforced nose wires.
</p>
<p>
Overall, any mask is still better than no mask as (1) the remaining filtration
efficiency (~15% for the N95-compliant masks in the
<a href="https://pubs.acs.org/doi/10.1021/acsnano.0c03252">Konda et al. (2020)</a>
study) will filter some of the smaller particles and (2) the mask should
significantly reduce the fraction of larger droplets released into the air.
</p>
</div>
</div>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> What flow rate was used?</button>
<div class="text-indent">
<p>
We used a flow rate of 1 LPM, yielding a face velocity of 4.9 cm/s,
below that used in the NIOSH N95 standard (8-9 cm/s).
</p>
</div>
</div>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> What type of particle diameter do we use? <b>Aerodynamic diameter</b></button>
<div class="text-indent">
<p>
There are multiple kinds of particle diameters that aerosol scientists use when
making these kinds of measurements, which may differ a bit from the physical diameter.
</p>
<p>
We phrase things in terms of the <b>aerodynamic diameter</b>, which is related to the
drag force on the particles. This is more relevant for the impaction and
interception mechanisms, which are dominant for the size range considered
here.
</p>
<p>
Note that the final plot below shows both the aerodynamic (top axis) and mobility (bottom axis)
diameters, for reference. This calculation assumes spherical salt particles with
a density of 2,160 kg/m<sup>3</sup>).
</p>
</div>
</div>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> Why are some of the materials identified by their physcial colour?</button>
<div class="text-indent">
<p>
We occasionally characterize the materials by their physical colour. This is
done as there is often limited information available about the specific material properties (e.g., thread count).
In this regard, the colour is simply used to denote different materials within
a similar material class.
Accordingly, this data should be interpretted as indicating the potential variability
of filtration and pressure drop that could be associated with a given
type of material.
</p>
<p>
A pertinent example is the knit cottons, which have variable quality
despite being a similar material to the consumer.
</p>
</div>
</div>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> What is up with the sample codes (e.g., W5, K2, nW3)?</button>
<div class="text-indent">
<p>
Simply put, these are codes we assign to each material, composed of characters
identifying a material, its structure, number of layers used during measurement,
and sanitization treatment applied (e.g., laundered), if any. While we provide these codes
for reference back to the associated paper, in most instances the reader can
ignore these codes.
</p>
<p>
For those more interested:
</p>
<p style="padding-left:10px;">
<b style="color:var(--a0)">1.</b>
The first letter or pair of letters generally denotes the material structure,
using <b>W</b> for woven materials, <b>K</b> for knit materials,
<b>CP</b> for cut pile materials (e.g., corduroy), and <b>nW</b> for non-woven
materials. Composite masks, such as the N95-compliant materials tested,
have different codes (e.g., <b>ASTM</b> for surgical masks that adhere to the
ASTM standard).
The different material structures can be added or removed from the plot using the
checkboxes in the controls above the first plot.
</p>
<p style="padding-left:10px;">
<b style="color:var(--a0)">2.</b>
The second component is a number used to identify the different
materials within a given material structure. As the number is only
used to identify the material, it does not contain any information
about the material makeup or structure.
</p>
<p style="padding-left:10px;">
<b style="color:var(--a0)">3.</b>
Next, some codes append <b>x*</b> to denote if multiple layers
were used during the test. For example <b>K4x2</b> is four layers of
the K4 material (Double-knit jersey, yellow). If the <b>x*</b>
component is missing, a single layer of the material was used.
</p>
<p style="padding-left:10px;">
<b style="color:var(--a0)">4.</b>
Finally, some codes append a space followed by a code denoting if
a sanitization treatment was applied to the material. Examples include,
<b>HS</b> for heat treatment, <b>IPA</b> for isopropyl alcohol treatments,
<b>WD</b> for laundering, and <b>SW</b> for washing with soap and water.
If no such code is appended, the material is in its original condition.
If santization was repeatedly applied, this latter part of the code
will also be appended with <b>x*</b> (e.g., <b>WDx10</b> denotes
materials laundered ten times).
</p>
<p>
For example, a code
denoting a knit material that has been laundered multiple times is
<b>K4x4 WDx4</b>. A list of the codes is included in the table at the bottom
of this page.
</p>
</div>
</div>
</div>
<div class="main" id="main-fvp">
<a class="anchor" id="hfvp"></a>
<h2>FILTRATION & PRESSURE DROP</h2>
<p>
This first graphic uses data from
<a href="https://www.tandfonline.com/doi/full/10.1080/02786826.2020.1855321">Rogak et al. (2021)</a>
to examine the filtration properties of face masks alongside a
second important property: <b>pressure drop</b>.
Filtration has rather intuitive consequences for face masks. If a face
mask has poor filtration properties, particles (perhaps containing virus) are allowed to
pass through the fabric and reach the wearer, reducing the utility
of the mask.
The consequences of a poor pressure drop are less direct.
Pressure drop denotes resistance. In this respect,
a low pressure drop is preferable for two reasons: (1) it makes
for a more breathable mask, which is less likely to irritate the wearer, and
(2) it results in less leakage, improving
the overall utility of the mask.
</p>
<p>
A good mask has a balance of both of these properties. The following plot considers
these properties for some common materials. Uncertainties in the data are generally larger for low
fitration efficiencies (negative filtration efficiencies occasionally reflect this; they
are artifacts of the measurements and are not physical).
Generally materials in the upper-left region of the plot (high filtration, low pressure drop)
are the best. N95 masks are located on the upper axis,
effectively filtering 100% of particles at these sizes.
</p>
<p>
Circles with thicker outlines indicate materials with some form of sanitization
treatment applied (e.g., laundered or heated).
</p>
<a class="anchor" id="hqual"></a>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> What do the dashed lines in the figure represent? The short answer: <b>quality</b>.</button>
<div class="text-indent">
<p>
In the plot below, dashed lines correspond to lines of constant <b>quality</b>.
</p>
<p>
A good (or high) quality denotes a hybrid of good filtration
while maintaining a low pressure drop. Infinite quality would correspond
to a material that filters all of the particles, with no pressure drop.
Naturally, real materials have trouble realizing this ideal. Masks with
the highest quality in this study include N95-compliant and surgical masks.
N95-compliant masks in some cases essentially blocked 100% of particles
(within the limits of the current measurement apparatus), such that the quality approached ∞.
Calculations of quality, <i>Q</i>, use the natural number as a base
(this varies in the literature and should be checked between studies);
pressures drop, Δ<i>p</i>, in kPa;
and the filtration efficiency, η, as a fraction;
in the following expression:
</p>
<p>
<i>Q</i> = −ln(1 − η)/Δ<i>p</i>
</p>
<p>
For more details on how quality is defined and what it means, we refer the reader
to <a href="https://www.tandfonline.com/doi/full/10.1080/02786826.2020.1855321">Rogak et al. (2021)</a> and the references
therein.
</p>
</div>
</div>
<a class="anchor" id="hastm"></a>
<div class="text-outside" id="tastm">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> What are the referenced <b>ASTM Level</b> lines?</button>
<div class="text-indent">
<p>
This classification refers to whether or not the mask meets the
breathability requirements of the
<a href="https://www.astm.org/Standards/F3502.htm">ASTM F3502</a> standard.
</p>
<p>
<b>Note</b> that the face velocity used for these measurements <b></b>differs</b>
from the standard, such that a correction
is applied (approximately halving the pressure drop requirements).
Part of that standard also restricts the filtration efficiency at
0.3 micron, which is not considered in the provided classification.
</p>
<p>
Also note that the N95 Δp refers to a measured pressure
drop, rather than to the NIOSH N95 standard.
</p>
</div>
</div>
<div class="iframe-container1">
<iframe name="graphic1" src="fvp.html" frameborder="0" scrolling="no" style="visibility:hidden;" onload="this.style.visibility = 'visible';">
</iframe>
</div>
<a class="anchor" id="hbw"></a>
<p>
The aforementioned <b>quality</b> (see the corresponding Q&A entry
<a href="#hqual">above</a>)
is a useful surrogate for determining
how well a material performs in optimizing both pressure drop and filtration.
The graphic below shows the range of quality observed for each
material structure, in the form of a box-whisker plot.
Here, the box corresponds to the 25th and 75th quantiles for a given structure,
while the whiskers correspond to the 5th and 95th quantiles.
In this plot, quality is capped at 1,000 and corresponds to the filtration at
an aerodynamic dimaeter of <b>2.76 μm</b>.
</p>
<div class="iframe-container1b">
<iframe name="graphic1b" src="whisker.html" frameborder="0" scrolling="no" style="visibility:hidden;" onload="this.style.visibility = 'visible';">
</iframe>
</div>
</div>
<div class="main" id="main-diy">
<a class="anchor" id="hdiy"></a>
<h2>DESIGN YOUR OWN MASK</h2>
<h3>Considering size-resolved, multilayer mask filtration.</h3>
<p>
Not all particle sizes are filtered in the same way. For example, small particles
tend to pass through masks more easily, resulting in lower filtration efficiencies.
This second graphic examines the size-resolved mask filtration of the data from
<a href="https://www.tandfonline.com/doi/full/10.1080/02786826.2020.1855321">Rogak et al. (2021)</a>.
Generally, curves higher on the plot are preferred; though, these materials
may result in unreasonable pressure drops that make the mask uncomfortable to wear
and lead to larger amounts of leakage.
</p>
<p>
In addition, this graphic <b>estimates</b> the filtration properties of
masks composed of multiple layers.
In this regard, solid, coloured lines indicate individual filter layers, while
the black, dashed line corresponds to an estimate combining all of the
selected feature layers.
</p>
<p>
Individual feature layers can be added,
removed, or changed using the dropdown boxes and
can include up to <b>three layers</b>, the number recommended by the WHO.
</p>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> How do we <b>estimate</b> the filtration efficiency of the combined layers?</button>
<div class="text-indent">
<p>
Estimates for multilayered masks are only <b>approximate</b> and are computed
using the product of the particle penetration (one minus the filtration efficiency
as a fraction) for individual layers.
</p>
<p>
It is worth noting that some recent work (e.g.,
<a href="https://pubs.acs.org/doi/10.1021/acsnano.0c05025">Zangmeister et al. (2020)</a> and
<a href="https://www.tandfonline.com/doi/full/10.1080/02786826.2020.1855321">Rogak et al. (2021)</a>)
suggests that the first layer may filter a disproportionate number of particles.
In these cases, the presented estimates are likely to be overly optimistic.
</p>
</div>
</div>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> Do all of the curves have a similar trend?</button>
<div class="text-indent">
<p>
In general, yes. It is well-established that the filtration
efficiency consistently decreases as the particles get smaller
for this size range.
The most penetrating particle size (MPPS), that is, the minimum
in these kinds of curves, is located to the left of the shown domain and
occurs when the balance of the diffusion and
impaction mechanisms are at a minimum. This typically occurs in the 50-500 nm range and
is the focus of several other similar studies (see references in
<a href="https://www.tandfonline.com/doi/full/10.1080/02786826.2020.1855321">Rogak et al. (2021)</a>). Here we
focus on the larger particles, which remain relevant for bioaerosol applications.
</p>
</div>
</div>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> Are all of these mask combinations realistic?</button>
<div class="text-indent">
<p>
Not necessarily! This graphic does not give an indication of the feasibility of the different layers,
as they do not have a standardized weight, pressure drop, thickness, or compatibility.
We again refer the interested reader to
<a href="https://www.tandfonline.com/doi/full/10.1080/02786826.2020.1855321">Rogak et al. (2021)</a>
for a more detailed discussion.
</p>
<p>
A pertinent example is gauze, which, when combined in multiple layers
(up to 48 layers if selected from the dropdowns, where each filtration
layer itself is composed of 16 layers of gauze),
yield face masks that are extremely bulky.
</p>
<p>
Another example is the dried baby wipes, which would work fine as an
insert but may be suboptimal in constructing the outer layers.
</p>
</div>
</div>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> What do the line colours in this graphic represent?</button>
<div class="text-indent">
<p>
The colours of the solid lines
correspond to the material classes for the individual filter layers,
with the same colours as those used in the previous graphic (for example,
yellow is for knitted materials).
</p>
</div>
</div>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> Why might one consider the poor filter layers?</button>
<div class="text-indent">
<p>
While some materials contribute very little to the filtration properties, they
may have other advantages. For example, some layers may absorb moisture.
However, it is important that these layer do not add a lot of pressure drop
to the mask, for reasons noted above.
</p>
</div>
</div>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> What do the qualifiers on the pressure drop (e.g., low) mean?</button>
<div class="text-indent">
<p>
These give a sense of the significance of the pressure drop,
using the N95-compliant mask as the standard (or <b>moderate</b>) pressure drop.
Pressure drops sufficiently below the measured N95-compliant pressure drop are
considered low, while pressure drops higher than the N95-compliant
masks are considered high.
The real significance of the pressure drop will depend on other factors,
such as how hard an individual is breathing. As such, these estimates
are only an initial guide to the user.
</p>
<p>
Generally, for improvised masks, <b>low</b> pressure drops are preferred,
as the masks do not fit as well as N95-compliant masks.
</p>
</div>
</div>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i>
</span> What is the referenced <b>ASTM Level</b> below? Short answer: <b>breathability</b>, not filtration.</button>
<div class="text-indent">
<p>
This classification refers to whether or not the mask meets the
breathability requirements of the
<a href="https://www.astm.org/Standards/F3502.htm">ASTM F3502</a> standard,
as per the <a href="#hastm" onclick="expandASTMNote()">note above</a>.
</p>
</div>
</div>
<div class="text-outside">
<button type="button" class="collapsible"><span class="plus"><i class="fas fa-chevron-up"></i></span> What does the share button
( <i class="far fa-share-alt" style="font-size:8pt;"></i> ) do?</button>
<div class="text-indent">
<p>
Clicking this button will copy a link to the clipboard for the current mask configuration.
One can then paste this link anywhere to reintialize the web app to this combination.
</p>
</div>
</div>
<a class="anchor" id="hviz2"></a>
<div class="iframe-container2">
<iframe name="viz2" id="viz2" src="curves.html" frameborder="0" scrolling="no" style="visibility:hidden;" onload="this.style.visibility = 'visible';">
</iframe>
</div>
</div>
<div class="main" id="main-data">
<a class="anchor" id="htable"></a>
<h2>SOURCE DATA</h2>
<p>
Source data in CSV format is available online at:
</p>
<div class="ref" style="display:flex;vertical-align:middle;">
<a id="data-link" href="https://github.com/tsipkens/fmviz/blob/main/data/fm.csv"><i class="fas fa-database" style="font-size:13pt"></i> FM.CSV</a>
</div>
<p>
This includes full details including the the quality and other properties for all of
the materials and santization treatments.
This package is released under a <a href="LICENSE">GNU GPLv3 license</a>.
Code supporting this web app is also openly available in the
associated <a href="https://github.com/tsipkens/fmviz">GitHub repository</a>.
</p>
</div>
<div class="footer" id="hfooter" stlye="bottom:0px;">
<div class="main" id="main-footer" style="margin-top:20px;padding-bottom:20px;font-size:10pt;">
<a class="anchor" id="hcite"></a>
<h5>CITATION AND CREDIT</h5>
<p>
<span class="footer-sup">1</span> This graphic was originally created by Timothy Sipkens
(<a href="https://github.com/tsipkens">@tsipkens</a>).
Maintenance is performed by members of the
<a href="http://www.aerosol.mech.ubc.ca/">Energy and Aerosol Laboratory</a>
at the Univeristy of British Columbia, including David Downey, Hamed Nikookar, and Thomas Bement.
</p>
<p>
<span class="footer-sup">2</span> This web app is associated
with the following paper:
</p>
<p class="ref">
<a href="https://www.tandfonline.com/doi/full/10.1080/02786826.2020.1855321">
S. N. Rogak, T. A. Sipkens, M. Guan, H. Nikookar, D. V. Figueroa & J. Wang (2021)
Properties of materials considered for improvised masks,
<i>Aerosol Science and Technology</i>, 55:4, 398-413,
doi: 10.1080/02786826.2020.1855321
</a>.
</p>
<p>
The original authors can be contacted at
<a href="mailto:[email protected]">[email protected]</a>.
When using this data, please cite the corresponding paper.
One can also consider citing this web app directly:
</p>
<p class="ref">
<a href="https://tsipkens.github.io/fmviz/">"FMVIZ: Face mask visualization." https://tsipkens.github.io/fmviz/.</a>
</p>
<p>
<span class="footer-sup">3</span> This work compliments active efforts
by many other aerosol scientists and health
professionals aimed at improving our understanding of
improvised mask materials and construction. We refer to many of these
studies in the associated paper. We would also like to acknowledge the
<a href="http://www.freemaskprojectvancouver.com/">Free Mask Project</a> for their
involvement through this project, and Eric Cytrynbaum and
Devon Ostrom for constructive feedback in regards to this web app.
We would also like to acknowledge multiple individuals at
<a href="https://makermask.org/">MakerMask.org</a> for providing us with
some of these samples and for many excellent discussions.
</p>
<p>
<span class="footer-sup">4</span> For best results, do not use Internet Explorer.
</p>
<p>
<span class="footer-sup">5</span> This app and its associated data are
released under a <a href="LICENSE">GNU GPLv3 license</a>,
with the associated conditions.
</p>
<p id="substar">
<span class="footer-sup">†</span> ASTM levels refer ONLY to
pressure drop and are corrected for face velocity, such that
actual levels differ from those reported in the standard. See
<a href="#hastm" onclick="expandASTMNote()">this note</a> for more information.
</p>
</div>
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