How are surgical masks and respirator filters tested?
Respirator filters must meet stringent certification tests (42 CFR Part 84) established by NIOSH. The NIOSH tests use what are considered “worst case” parameters, including:
- A sodium chloride (for N-series filters) or a dioctyl phthalate oil (for R- and P-series filters) test aerosol with a mass median aerodynamic diameter particle of about 0.3 µm, which is in the MPPS-range for most filters
- Airflow rate of 85 L/min, which represents a moderately-high work rate
- Conditioning at 85% relative humidity and 38°C for 24 hours prior to testing
- An initial breathing resistance (resistance to airflow) not exceeding 35 mm water column* height pressure and initial exhalation resistance not exceeding 25 mm water column height pressure
- A charge-neutralized aerosol
- Aerosol loading conducted to a minimum of 200 mg, which represents a very high workplace exposure
- The filter efficiency cannot fall below the certification class level at any time during the NIOSH certification tests
* Millimeters (mm) of water column is a unit for pressure measurement of small pressure differences. It is defined as the pressure exerted by a column of water of 1 millimeter in height at defined conditions, for example 39°F (4°C) at standard gravity.
As a result of these stringent performance parameters, fiber diameters, porosity, and filter thicknesses of all particulate filters used in NIOSH-certified respirators, including N95s, are designed and engineered to provide very high levels of particle collection efficiencies at their MPPS.
Manufacturers of surgical masks, on the other hand, must demonstrate that their product is at least as good as a mask already on the market to obtain “clearance” for marketing. Manufacturers may choose from filter tests using a biological organism aerosol at an airflow of 28 L/min (bacterial filtration efficiency) or an aerosol of 0.1 µm latex spheres and a velocity ranging from 0.5 to 25 cm/sec (particulate filtration efficiency). It is important to note that the Food and Drug Administration specifies that the latex sphere aerosol must not be charge-neutralized.