Filtration & Pressure Drop (Breathability).
When it comes to looking for a mask what most people first think of is filtration, and how well this mask will work.
The other important consideration is breathability, which is measured by pressure drop. A low pressure drop means the mask will be more breathable.
A breathable mask is easier to wear for longer periods of time, and reduces the need to touch and adjust the mask while it is being worn. To offer the most protection possible masks need to be worn properly. A mask with a higher pressure drop can be more difficult to wear for any period of time as it is more difficult to breathe through.
A great breakdown can be found here, discussing research and finding out of the University of British Columbia.
(2020) Properties of materials considered for improvised masks, Aerosol Science and Technology,