fmviz

This is a key question.

We note that the focus of this data is on the materials themselves. As such, these curves do not 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.

Konda et al. (2020), 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 ballistically (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.

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.

Overall, any mask is still better than no mask as (1) the remaining filtration efficiency (~15% for the N95-compliant masks in the Konda et al. (2020) study) will filter some of the smaller particles and (2) the mask should significantly reduce the fraction of larger droplets released into the air.

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.

A pertinent example is the knit cottons, which have variable quality despite being a similar material to the consumer.

In the plot below, dashed lines correspond to lines of constant quality.

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, Q, use the natural number as a base (this varies in the literature and should be checked between studies); pressures drop, Δp, in kPa; and the filtration efficiency, η, as a fraction; in the following expression:

Q = −ln(1 − η)/Δp

For more details on how quality is defined and what it means, we refer the reader to Rogak et al. (2021) and the references therein.

Simply put, these are codes we assign to each case, composed of characters identifying a material, its structure, number of layers used during measurement, and 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.

For those more interested:

1. The first letter or pair of letters generally denotes the material structure, using W for woven materials, K for knit materials, CP for cut pile materials (e.g., corduroy), and nW for non-woven materials. Composite masks, such as the N95-compliant materials tested, have different codes (e.g., ASTM 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.

2. 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.

3. Next, some codes append x* to denote if multiple layers were used during the test. For example K4x2 is four layers of the K4 material (Double-knit jersey, yellow). If the x* component is missing, a single layer of the material was used.

4. Finally, some codes append a space followed by a code denoting if a sanitization treatment was applied to the material. Examples include, HS for heat treatment, IPA for isopropyl alcohol treatments, WD for laundering, and SW 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 x* (e.g., WDx10 denotes materials laundered ten times).

For example, a code denoting a knit material that has been laundered multiple times is K4x4 WDx4. A list of the codes is included in the table at the bottom of this page.

We phrase things in terms of the aerodynamic diameter, which is more relevant for the impaction and interception mechanisms.

Estimates for multilayered masks are only approximate and are computed using the product of the particle penetration (one minus the filtration efficiency as a fraction) for individual layers.

In short, yes. It is well-established that the filtration efficiency consistently decreases as the particles get smaller (to the left) for this size range. The most penetrating particle size (MPPS), that is, the minimum in these kinds of curves, typically occurs in the 50-500 nm range and is the focus of several other similar studies (see references in Rogak et al. (2021)). Here we focus on the larger particles, which remain relevant for bioaerosol applications.

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 Rogak et al. (2021) for a more detailed discussion.

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.

Another example is the dried baby wipes, which would work fine as an insert but may be suboptimal in constructing the outer layers.

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).

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.

These give a sense of the significance of the pressure drop, using the N95-compliant mask as the standard (or moderate) 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.

Generally, for improvised masks, low pressure drops are preferred, as the masks do not fit as well as N95-compliant masks.