Measuring equipment

  • Particle generator: ATM226
  • Particle counter: PortaCount pro 8030

Validation target

Accurate filtration factor measurementwith the current setup.

Validation strategy

Using a set of face maskswith well known and documented filtering performance as a benchmark to calibrate the setup.

Relevant NEN standard

ClassificationMin. filtration 2% NaClMin. filtration paraffin oil
FFP180%80%
FFP296%96%
FFP399%99%
Table 1: Maximum penetration for FFPSource: NEN 149

As stated in table 1 the filtering capacity of a FFP2 mask should be such, that a maximum of 6% of NaCl particles can pass through the mask at a volumetric flow of 95 l/min. The PortaCount Pro 8030 gives a fit factor as output metric which can bedefined as:

Where FF is the fit factor and N_ambient is the number of particles present in the ambient air, before the mask, and N_mask is the number of particles presentbehindthe mask material. However, as the fit of the device is not assessed in this setup, FF can be defined as the filter factor.

A maximum penetration of 6% the ratio of particles in front of and behind the mask is 100/6 = 16.67. As such, a Filter Factor of 5 is necessary for a FFP1 classification with the conditions described in NEN 149, a FF of 16.67 for FFP2, and a FF of 100 for FFP3.

This standard prescribes a volumetric flow of 95 l/min through the total mask. Assuming a mask can be approached be a half sphere and a diameter of 11 cm, which is the average diameter of our FFP2 mask samples, the total area of an average mask is 0.18 m2. This means that the standard requires an average air velocityof about 0.1 m/s through the mask. However, this approximates a uniform flow of air throughout the mask. In reality, this would seem unlikely, as the flowin the area directly in front of the mouth and nose will be higher than at the side due to a higher pressure differential.

Figure 1: Fit factor of different mask samples with varying air velocities

To validate the setup, a number of masks, of which it is known they meet the NEN149 criterion for FFP2 or FFP3, is used, and the results compared to what would be expected. The values found are presented in figure 1 using 2% NaCl. Here, the lowest velocities, around 0.1 m/s, would correspond to the volumetric flow specified in NEN 149 most accurately. Here, we can see that the 3M 1862+ and 3M 9320+ fall nicely above the FFP2 threshold, but below the FFP3 line. This would indicate that both these masks would be placed in the FFP2 class, which is the case. With the 3M 8320, the FF at 0.1 m/s has been left out as it was too high to accurately measure, and would definitely be above the FFP2 and FFP3 line. Additionally, the Dräger Piccola FFP3 mask lies above the FFP3 line at the lowest air velocity, further indicating that setup measures correctly. The Medline surgical mask clearly showed inadequate performance for a FFP classification as anticipated, since it is not designed for this purpose.

Moreover, test data in figure 1 shows that air speed has a strong negative correlation to filtering capacity. This is to be expected, as a higher velocity results in greater momentum of the particles, and thus a greater chance of particles passing through the filter material. However, this indicates that the flow velocity in our setup will greatly affect whether or not a sample will meet the NEN 149 requirements. As such a well-founded reference value is necessary to both provide reliable data that approximates the NEN 149 values, but is also a realistic velocity. Therefore, we assume that the maximum air speed is 5 times higher than the average air speed, resulting in a maximum air speed of 0.5 m/s. This assumption is based on differencesfound in [1], in which air velocities during rest, moderate and heavy inhalation are determined, and in [2], which measured air velocities and the projected area during exhalation.So a 0.5 m/s air speed is the worst case scenario and will be used as a benchmark.

Conclusions

  • From the 95 l/min volumetric flow, we have deduced an average air velocity of 0.1 m/s through the mask.
  • The filter capacity is strongly dependent on the air velocity through the sample material.
  • FFP2, FFP3 and surgical masks are used as a calibration benchmark for the setup.
  • The measured fit factor is an accurate representation for the filter penetration.
  • We have determined that an air flow 0.5 m/s will provide a more reliable single velocity test result.

References

[1] Moya, J., Phillips, L., Schuda, L., Wood, P., Diaz, A., Lee, R., … & Chapman, K. (2011). Exposure factors handbook: 2011 edition.US Environmental Protection Agency.

[2] Anthony, T. R., & Anderson, K. R. (2013). Computational fluid dynamics investigation of human aspiration in low-velocity air: orientation effects on mouth-breathing simulations.Annals of occupational hygiene,57(6), 740-757.