For dual protection (ear muffs and plugs are used simultaneously) use the following:
Estimated Exposure (dBA) = TWA (dBC) – (NRRh + 5)
Estimated Exposure (dBA) = TWA (dBA) – [(NRRh- 7) + 5]
Example:TWA=110 dBA, plug NRR=29, and muff NRR=25 dB Estimated Exposure = 110 – [(29 – 7) + 5] = 83 dBA