4000bce - 399
400 - 1399
1400 - 1499
1500 - 1599
1600 - 1699
1700 - 1799
1800 - 1899
1900 - 1999
Their formal name is filtering facepieces, or FFPs. These masks are the respirators that will be part of the protective gear being made available to USPS employees. It is important to understand how they work and why they were chosen to protect the Postal Service's most vital asset - employees.
Particulate respirators are tested under "worst-case" conditions - that is, using very high air flows, very tiny particles that are neutralized to enhance penetration and very high dust concentration.
The respirators are certified to capture particles as small as 0.3 microns. That is much smaller than the period at the end of this sentence.
The anthrax bacterium in living tissue is approximately 3-4 microns in length. The free spores - that is, those that are airborne or otherwise outside of living tissue - range from 1-3 microns. Usually, the spores naturally tend to clump together to form even larger particles.
The masks selected for voluntary use by employees in areas not suspected to have an anthrax release are NIOSH certified. "NIOSH" stands for National Institute for Occupational Safety and Health. Certification is based on the amount of particles the respirator will filter out, and on how well the filter resists degradation from the type of particles being filtered out.
The postal respirators are designed to offer an additional measure of protection and security. They will be used in environments much cleaner and less challenging to the respirator than the test conditions.
The postal respirators aren't intended for use in suspected or known hazardous substance release areas, nor for cleanup or other work in a hazardous environment. A substantially higher level of protective equipment is required in such situations and can only be worn by specially trained personnel.
U.S. Government Website