A build-up of carbon dioxide in the face mask of a person will cause adverse effects. Feeling unwell and uncomfortable as well as experiencing the feeling of being suffocated are results of too high concentrations of carbon dioxide. To this end, protective masks may not cause a build up of carbon dioxide in the dead space of the mask.

For this reason, a test has been included in NEN-EN 149 to measure the carbon dioxide concentration in the mask dead space. The maximum allowed concentration of carbon dioxide which is present in the dead space of the masks is the same for all mask classifications.

The carbon dioxide test measures the CO2 content of the inhalated air, based on the NEN-EN 149 7.12. This test consist of a human breathing analog (Respironics CA-3200) which has the capacity of 2 liters per stroke 25 times per minute, two CO2 concentration sensors (GSS SprintIR-WF-20) and a CO2 supply. The machine produces an exhalation of 2 liter to which 5% CO2 is added, verified and measured using by a CO2 sensor. This air is exhaled through the half face mask fitted around a dummy head to produce a representative trapped volume. Subsequently, the machine produces an inhalation of 2 liter, of which the CO2 is also measured as well. The CO2 concentration in this inhalation stroke may not exceed an average of 1,0 % (by volume) when wearing a half face mask. The cycle of inhalation and exhalation is continued until the inhaled CO2 concentration reaches a steady-state value at 25 times per minute.