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

The 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 5mg/m3. If the contaminants in your work area is 60mg/m3 and the total exposure time is 7 hours, Then the 8 hour time-weight average concentration=60x7/8=52.5mg/m3. Divide TWA concentration by OEL=52.5/5=10.5x 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.NOISH 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. 1910.100 for TABLE Z1, Z2 or Z3 or just click: TABLE Z-1 Limits for Air Contaminants.