How did MOM come out with the trigger level of PSI 400, above which respirators with higher PF should be used?

In deriving this estimate, we had considered the particulate level and the duration which workers may be exposed to haze at work, as well as practical factors. When prolonged outdoor work is carried out at PSI levels above 400, an N95 mask with a protection factor of 10 may not provide the workers with sufficient respiratory protection. A respirator with higher PF will be able to provide sufficient respiratory protection and reduce exposure.