OEL, PEL, MAC, and TLV are the same definition with different wording. OEL = Occupational Exposure Limit. PEL = Permissible Exposure Limit. MAC = Maximum Admissible Concentration. TLV = Threshold Limit Value.

They all refer to airborne concentrations of substances and represent conditions under which it is believed that nearly workers may be repeated exposed day after day without adverse health effects. For example, the OEL value for Calcium Carbonate dust is 5 mg/m3. If the contaminants in your work area is 60 mg/m3 and the total exposure time is 7 hours. Then the 8 hour time-weighted average concentration = 60 X 7 /8 = 52.5 mg/m3. Divide TWA concentration by OEL = 52.5 /5 =10.5 x OEL. Then you should select a mask which can provide 10.5 X OEL protection level at least. For EN149:2001 standards, FFP1 mask can provide 4 or 5 x OEL, FFP2 mask can provide 12 x OEL and FFP3 mask can provide 50 x OEL. Therefore, you should select a FFP2 mask.
For U.S. NIOSH N95 approved masks, they can provide 10 X OEL protection.

If you are interested in knowing the OEL value for various air contamination, you may visit OSHA (Occupational Safety and Health Association)
web-site : www.osha.gov and look for document no.1910.1000 for TABLE Z1, Z2 or Z3, or just click : TABLE Z-1 Limits for Air Contaminants. As for the detailed OEL calculation method, please also visit OSHA web-site and look for document no. 1910.1000, or
just click : Air contaminants.