OBJECTIVE To verify the attenuation levels of two types of earmuffs under real operating conditions. METHODS A study with experimental design was carried out to obtain repeated measurements of the sound pressure levels inside and outside hearing protectors in a sample of workers of an oil and gas company working under normal process and exposure conditions. The results allowed determining differences between the attenuation levels established by the manufacturer, the adjusted attenuation levels under the NIOSH method, and the attenuation levels obtained experimentally. RESULTS The attenuation values established by the manufacturers are lower than those ones obtained under actual use conditions in all cases evaluated. Likewise, the attenuation values of the hearing protectors, once adjusted under the NIOSH method, reached values much closer to those obtained experimentally. CONCLUSIONS The variability between theoretical attenuation values and experimental values allows concluding that the attenuation levels obtained under controlled laboratory conditions do not take into account certain characteristics that, based on their use, affect the efficiency of the hearing protection device. This study encourages the implementation of comprehensive hearing protection programs that consider variables such as hearing protection effectiveness, under real use conditions, by applying fit tests or other adjustment factors like the one suggested by NIOSH. This would ensure an adequate selection that aims at achieving an effective control of this risk factor.