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Dose limits For many years, the concept of maximum permissible dose ( MPD ) defined occupational exposure DLs. This approach is now considered obsolete, and MPD has been replaced with effective whole-body DLs (Connor, 2019b). The DLs recommended by the NCRP in 1993, which remain unchanged, are based on risk assessments and have been established to ensure that radiation workers have no greater risk than personnel who work in industries with no exposure to radiation (Choudhary, 2018; Bushong, 2013). The DLs are based on an LNT dose-response relationship that the expectation that unnecessary radiation exposure will be avoided and that the need for any dental or medical procedure which requires any form of ionizing radiation is justified (International Commission on Radiological Protection, 2019). Current DLs not only address whole-body exposures but also establish DLs for various organs. The DLs account only for occupational exposure and do not include exposures received as a patient. In fact, there are currently no DLs established for patient exposures received from diagnostic or therapeutic radiation sources. For nonoccupationally exposed individuals, the DLs have been established at one tenth of the DL for radiation workers. This category includes individuals in the general population, such as personnel who are not radiation workers but who work in areas adjacent to imaging facilities or persons accompanying patients who are in an adjacent waiting or reception area within the facility. It is important to note that most radiation workers do not receive the exposure established for nonoccupationally exposed individuals (Bushong, 2013). In addition, DLs have been established for educational programs in which student trainees younger than age 18 are engaged in x-ray imaging procedures. Exposure of students in this category must be monitored and must remain below the DL. It is general practice not to accept underage persons into radiologic technology programs unless their 18th birthday is to occur soon (Bushong, 2013). The reason for this practice is that minors may not appreciate or understand the responsibility and consequences of radiation misuse. The current recommendations for annual DLs are listed in Table 5. Radiographer protection Individuals operating dental radiographic machines should take steps to protect themselves from occupational radiation exposure. Excess radiation exposure to the dental radiographer can result from improper safety habits and exposure from the primary beam, secondary radiation, and leakage radiation. To avoid these sources of radiation, the radiographer should stand behind a protective barrier during radiographic exposures. The radiology operatory must be constructed from materials that shield surrounding areas from primary and secondary radiation. Many materials used in construction, such as drywall and sheetrock, provide the thickness necessary for protective shielding. For most dental x-ray installations, lead lining of the operatory walls is unnecessary. If no barrier is present, the clinician should stand at least 6 feet (2 meters) away and at a 90° to 135° angle to the primary beam. This practice is known as the distance and position rule (EPA, 2016b). Location of the exposure control panel and exposure switch outside of the dental operatory facilitates operator protection. A small amount of radiation may leak from the protective metal housing surrounding the tubehead. For this reason, a radiographer should never hold the tubehead in place during radiographic exposure. Drifting or “floating” tubehead suspension arms should be professionally repaired as soon as possible. In addition, radiographers should not hold an intraoral receptor in place in the patient’s mouth during exposure, nor hold or restrain the patient (EPA, 2016b). Although it may seem to be a reasonable option for the uncooperative, pediatric, or special needs patients, this practice is not recommended because it leads to unnecessary occupational exposure. In

Table 5: Recommended Annual Dose Limits

Effective Dose: Whole Body • Annual: 50 mSv. • Cumulative: 10 mSv × age.

Equivalent Annual Dose for Tissues and Organs • Lens of eye: 150 mSv. • Thyroid, skin, hands, feet: 500 mSv. • Total equivalent dose: 5 mSv. • Equivalent dose/month once pregnancy known: ≤0.5 mSv. • Lens of eye: 15 mSv. • Skin, hands, feet: 50 mSv. • Lens of eye: 15 mSv. • Skin, hands, feet: 50 mSv.