Now that masks have become common, they also have become big business. They can be found in a great variety of shapes, patterns and materials, but all masks aren’t created equal. Many factors, including the composition and thickness of the material, and the density of the filter fibres, determine their effectiveness.
Then there is the role played by environmental factors, such as the velocity and pattern of the airflow, and the charge of the target particle. Capacity to intercept the particles, inertial impaction and the role of gravity all have an impact on large particles. Particles with bigger sizes, faster velocities, and more compact density exhibit higher inertia or force of movement, and this process makes them more easily captured. Bacteria, the original culprits that masks were designed to protect against, are giants compared with even large viruses like coronavirus. Bacteria are examples of larger particles that are blocked by surgical masks. These particles are not able to flow around the mask fibers. Also, larger particles when they stray are more likely than smaller particles to collide with the fibers and can adhere to them.
Very small particles, such as the coronavirus, behave quite differently. Unlike larger particles, such as bacteria, very small particles do not aggregate on impact. Instead, diffusion is the main way these particles travel. The higher the speed the less aggregation of viral particles. High electrostaticity is more likely to block more viral particles.
Non-natural fibres are more likely to have increased electrostacitity which works with the diffusion to attract smaller particles to the outside of some synthetic masks, and prevent small particles from infiltrating through the mouth or nose.
Human variation cannot be discounted. The frequency of respiration, time of inspiration and expiration, relative humidity of the breath, temperature, face shape and fit also affect mask effectiveness. For example, cup-shaped or fold-up masks have the potential to fit better onto the face with less leak area than pleated masks like surgical masks. These masks, however, need to be individually fitted and tested to ensure maximum effectiveness. Very small leaks, even ones of one percent of the total sample, can substantially reduce the overall filtration efficiency of a mask down to 50% or even less compared with the tested value of the material itself.
The effect of humidity, not only in the air we breathe in, but also in our expired air can have a variable effect. Humidity swells the fibres in some masks, thereby reducing pore size and thus diminishing the capacity for smaller particles to enter. Conversely, humidity can reduce efficiency by aggregating fibres and creating gaps that a virus can sneak through.
Then there is the issue of re-use. Eco-sound face masks that can be reused lose 20% of their efficiency after a few washes. The process of repeated washing and drying enlarges pore size, but more importantly decreases the number of microfibers within the pores that trap the noxious agents.
Even the best of masks is no more than a series of fibrous webs. They will never be 100 percent effective, which is why it is so important not only to mask, but to limit the duration of social interactions, maintain social distancing, and wash hands.
Of course, there is one collateral benefit to all of us wearing masks. If everyone puts on a mask in public places, it will help to remove the stigmatisation that has hitherto discouraged masking of asymptomatic and symptomatic patients in many places.