Adjusting a commonly used respirator pressure drop equation for use with modern respirators

Millions of workers in the United States require the use of a respirator while at work. Occupational inhalation hazards have the potential to cause respiratory diseases. Wearing a FFR (filtering facepiece respirator) are commonly used respirators that can help protect workers against inhalation hazards. When choosing a respirator, an important parameter is pressure drop. Pressure drop relates to the collective effort of individual fibers in a respirator resisting airflow and affects comfort and breathability of a respirator.

There are many models to predict pressure drop. A commonly used pressure drop equation developed by Davies in 1952. Davies pressure drop model did not incorporate material used in modern respirators. The equation can be rearranged to back calculate an effective diameter rather than measuring the individual fibers themselves but may be larger than the actual fiber diameter. Given modern advancements in respirators this study has two specific aims: 1) To determine the key physical characteristics of filter media associated with modern respirators and 2) To evaluate the accuracy of a commonly used pressure drop model on modern respirator filter material.

This study used 10 FFRs models to determine average pressure drop, filter fiber diameter, how modern respirators lined up on a pressure drop model, and the accuracy of the effective fiber diameter calculations by rearranging Davies’, a model based on fiber diameter distribution, and an adjustment of Davies’ model.

Pressure drop values between the various FFRs varied indicating not all respirators are created equally. When placing the data in a pressure drop model, the respirators did not align with the expected curve from Davies model. The equation was adjusted aligned better with the curve of the adjusted equation. The adjusted equation provided a more accurate prediction of fiber diameter compared to the other to effective diameter equations.