Porosity is measured (in the U.S.) using a Gurley porosity meter.

A test specimen is placed in the Gurley porosity meter that measures the time (in seconds) required for 100 mL of air to pass through one square inch under the applied pressure of 1.22 kPa (approximately 0.177 psi). Typical results for porous substrates such as medical kraft papers and Tyvek®  "paper" or "material" can range from 20 to 500 seconds. It is important to know the porosity of a medical packaging substrate especially if the package is to be sterilized using ethylene oxide (EO). This process requires that the EO used to sterilize the contents of the package can be removed from the package using this porous substrate as the "vent". Sterilization typically employs successive cycles of pressure and vacuum to accomplish the flushing of the sterilant gases. Therefore, the package must be designed such that this can be accomplished without the package "ballooning", thereby stressing the seals to the point that the package integrity may be compromised.

Gurley porosity tester