They must be replaced when they are soiled, damaged, or there is a noticeable increase in breathing resistance. This all may seem a little confusing. However, once you know the hazard and apply its characteristics to the equipment's specifications, you will begin to see the logic. Selecting proper protection will become a simple task. Until then, use all available resources.
Manufacturers are extremely helpful.
Consult those listed in last month's article. They will be happy to assist you.