1. How to calculate protection factor ?

You should aim at reducing the concentration inside the respirator to below any applicable occupational exposure limit (OEL). RPD uses a system called Assigned Protection Factor (APF), the factor by which the equipment reduces the contaminant concentration. An APF of 4, for example, means that the contaminant concentration will be reduced by a quarter. Choose the equipment that has an APF that exceeds the OEL, leaving a margin for equipment error and other factors.