Ionizing Radiation as a Veterinary Workplace Hazard

Ionizing radiation is high-energy radiation (particles or electromagnetic waves) that carries enough energy to detach electrons from atoms or molecules, creating ions. The main types are alpha radiation, beta radiation, gamma radiation, x-rays, and neutron radiation. Each particle type varies in terms of its physical characteristics, as well as shielding requirements to decrease occupational exposure (see the table Physical Properties of Ionizing Radiation).

Sources of ionizing radiation include radioactive materials and radiation-generating machines. Common sources of ionizing radiation in medicine include diagnostic imaging (x-rays, CT, fluoroscopy), radiation therapy, radiopharmaceuticals, and radiolabeled isotopes. Personnel can be exposed to radiopharmaceuticals by handling the drug, as well as by coming into contact with biological waste.

For information concerning radiolabeled isotopes, see the table Common Medical Radionuclides. In general, gamma radiation emitters (eg, technetium 99m [^99mTc], iodine 123 [¹²³I]) are used for imaging, beta radiation emitters (eg, yttrium 90 [⁹⁰Y], phosphorus 32 [³²P]) have therapeutic applications, mixed beta and gamma emitters (eg, iodine 131 [¹³¹I]) have mixed function, and positron emitters (eg, fluorine 18 [¹⁸F]) are used in PET imaging.

The rate of radioactive decay for each radioactive element is described by its half-life, the amount of time it takes for approximately half of the radioactive atoms to decay to a more stable form.

The emission of ionizing radiation (alpha, beta, gamma) can produce direct DNA strand breaks and promote free radical formation, resulting in oxidative damage and organ-specific uptake (eg, iodine accumulation in thyroid tissue). Adverse health effects associated with occupational exposure to ionizing radiation depend on the isotope, dose, and exposure route (external versus internal contamination).

Radiation dose depends on the duration of exposure, the amount of radiation generated from the radiation source, the distance from the radiation source, and the amount and type of shielding in place. Radiation dose is often categorized as either acute (minutes to days) or chronic (weeks or longer).

Veterinarians and their staff can be exposed to external radiation from a radiation source outside of the body (eg, x-rays from an x-ray machine) or through internal exposure after inhalation, ingestion, or dermal uptake of radioactive material (1).

Internal exposure to ionizing radiation is particularly hazardous. For example, radioactive material that emits alpha particles can be very harmful to living cells when the particles are inhaled, ingested, or absorbed into the bloodstream (eg, through a cut in the skin or an area of skin that is not intact). Children and fetuses are especially sensitive to radiation exposure, in part because of their higher cell division rates.

Health effects from ionizing radiation include deterministic (associated with increasing doses of exposure) and stochastic (associated with random exposure) effects. Deterministic effects occur after a threshold dose is reached, and the severity of the effect increases with the dose. Deterministic effects include birth defects at doses of ≥ 10–20 rads (0.1–0.2 gray) to the embryo/fetus, temporary or permanent sterility in men after doses of ≥ 15 rads (0.15 gray) to the testes, and lens opacities at ≥ 50 rads (0.5 gray) to the lens of eye. Radiation-induced erythema has a threshold dose of approximately 300 rads (3 grays) (2).

Additional signs associated with skin injury due to ionizing radiation can include itching, tingling, redness, and swelling of the skin. Exposure of nearly the whole body to ≥ 70 rads (0.7 gray) can result in acute radiation syndrome with associated bone marrow, GI, cardiovascular, and CNS effects (2). This form of radiation poisoning is not likely to arise from occupational exposure of veterinary workers.

The probability of cancer or other stochastic effects due to ionizing radiation increases with dose; however, the severity of the effect is dose independent. Stochastic health effects are assumed to lack a threshold dose below which they do not occur. This is the reason that no level of radiation is considered completely "safe" and why radiation doses should always be kept as low as possible for the intended purpose.

The primary control method for decreasing exposure to ionizing radiation in a veterinary clinic is to decrease the duration of exposure, increase the distance from the ionizing radiation source, and provide proper shielding (eg, lead, syringe shields, storage containers). Hands-free radiology is encouraged. Likewise, the use of proper shielding is critical when using portable x-ray units in a clinic or in the field (3). Additional controls include the use of designated work areas, warning signs, personal dosimeters, and contamination monitoring.

Animals treated with radiopharmaceuticals (eg, cats treated with ¹³¹I) should be isolated; and urine, feces, bedding, and other contamination sources should be carefully handled and disposed of. Control measures specific for each ionizing radiation source should be in place before their use (4).