pre-molded, formable, custom molded and semi-insert. Premolded earplugs do not require the plug to be formed before it is inserted into the ear. This prevents the plugs from becoming soiled before insertion. Formable earplugs are made of a variety of substances; however, all each substance shares the common feature of being able to be shaped by the user prior to insertion. One drawback of this is the obvious need for the user to have clean hands while shaping the earplug. They do have the advantage of forming to the users ear, while many premolded earplugs do not accomplish this very well. Custom molded ear plugs are unique for each person, since they are cast from each user's own ear canals. Therefore, they provide a personalized fit for each individual. Semi-inserts are generally a soft earplug on the end of band. The band aides in maintaining the earplug in position. They are often useful since they can be quickly removed and inserted. Earmuffs Earmuffs are another type of HPD. The main difference between earmuffs and earplugs, is that earmuffs are not inserted inside the ear canal. Instead the muffs create a seal around the outside of the ear to prevent noise from reaching the inner ear. Earmuffs are easy to wear and often provide a more consistent fit than an earplug. There are earmuffs available that use the principle of active noise control to help reduce noise exposures. However, the protection earmuffs offer may be mitigated by large sideburns or glasses as the seal of the earmuffs may be broken by these objects. Noise reduction ratings The United States Environmental Protection Agency (EPA) requires that all hearing protection devices be labeled with their associated noise reduction rating (NRR). The NRR provides the estimated attenuation of the hearing protection device. The NRR obtained in the lab is often higher than the attenuation provided in the field. To determine the amount of noise reduction afforded by a hearing protection device for the A weighted scale, OSHA recommends that 7 dB be subtracted from the NRR. This new NRR should be subtracted from the individuals time weighted average (TWA) noise exposure. It must then be determined if the attenuation is appropriate for the level of noise the individual is exposed to. There are several fit testing devices on the market that will measure the attenuation an individual receives when wearing their HPD. These systems