All life on earth has evolved in a continuous exposure to natural background radiation (Fig. 3-1; see Table 3-1). Background radiation from space and various terrestrial sources yields an average annual effective dose of about 3.1 mSv in the United States.

Humans have contributed many additional sources of radiation to the environment (Fig. 3-2). The largest of these sources is medical imaging, with much smaller contributions from consumer pro­ducts and other minor sources.

Approximately 400 million x-ray examinations are performed annually in the United States; about one quarter of these are dental. More recent estimates suggest that medical exposure in developed countries has grown rapidly in recent decades, particularly computed tomography (CT) of the chest and abdomen and increased use of cardiac nuclear medicine studies. The average doses from medical exposures are comparable to natural background exposure. CT (see Chapter 14) contributes more than half of medical exposure. In contrast to background radiation exposures, which affect everyone relatively uniformly, the distribution of medical exposures is highly skewed with older and sicker individuals receiving most medical exposures. Dental x-ray examinations, although made relatively frequently, are responsible for only 0.26% of the total exposure from medical imaging.

Recognition of the harmful effects of radiation and the risks involved with its use led the International Commission on Radiological Protection (ICRP) to establish guidelines for limitations on the amount of radiation received by both occupationally exposed individuals and the public (Table 3-2). These limits pertain to planned exposure situations, not to background radiation and not to emergency situations. Since their establishment in the 1930s, these dose limits have been revised downward several times reflecting increased knowledge concerning the harmful effects of radiation and the increased ability to use radiation more efficiently. Occupationally exposed individuals include dentists and their staffs. Individuals in the reception area or who are walking in a corridor outside a dental office are members of the public. Dose limits for members of the public—individuals not exposed occupationally—are generally set at 10% of occupationally exposed individuals. The current occupational exposure limits have been established to ensure that no individuals will have deterministic effects and that the probability for stochastic effects is as low as reasonably and economically feasible.

Dentists and their staff are occupationally exposed workers and are allowed to receive an average of 20 mSv of whole-body radiation exposure per year. Although this level of exposure is considered to present only a minimal risk, every effort should be made to keep the radiation dose to all individuals as low as practical. The dental profession does well in this area. The average dose for individuals occupationally exposed in the operation of dental x-ray equipment is 0.2 mSv—1% of the allowable exposure.

There are no limits on the exposure a patient can receive from diagnostic examinations, interventional procedures, or radiation therapy; this is because these exposures are made intentionally and for the direct benefit of the recipient. Individual circumstances make the setting of limits inappropriate. However, increasing concern for minimizing patient exposure has led multiple institutions, including the National Council on Radiation Protection and Measurement (NCRP) to issue diagnostic reference levels (DRL) for medical and dental diagnostic imaging. DRL exposure values represent the acceptable upper limits for patient exposure (75th percentile of general practice), whereas achievable doses represent the median dose (50th percentile) in general practice. The NCRP recommends a DRL of 1.6 mGy entrance skin dose for intraoral periapical and bitewing radiography. The NCRP further recommends an achievable dose of 1.2 mGy for intraoral radiography. These standards require that most users of D-speed film convert to E/F-speed film or a digital system. Multiple means to minimize unnecessary patient and operator exposure are described next.

Patient dose from dental radiography is usually reported as effective dose, a measure of the amount of radiation received by various radiosensitive organs during the radiographic examination. Table 3-3 shows typical effective doses from common dental intraoral, extraoral, and medical examinations. The equivalent exposure in terms of days of natural background radiation is shown. Dental exposures are a small fraction of the annual average background exposure.