To avoid an exposure bias due to the use of hearing-protective devices at work, we calculated each participant’s level of noise reduction according to the noise reduction rating of the hearing-protective devices that he wore (29 dB for earplugs and 25 dB for earmuffs), the protection levels of the hearing-protective devices (an average of 28% for earplugs and an average of 62% for earmuffs), a comfort factor of 0.5 (because of the minimal percentage of comfort related to compliance with usage of hearing-protective devices) (36), and the percentage of working time that he used the hearing-protective devices (36, 37)。