These two concepts are often confused:

External exposure occurs when all or part of the body is exposed to a penetrating radiation field from an external source. During exposure this radiation can be absorbed by the body or it can pass completely through, similar to a chest x-ray. Note that exposure to a radiation field does not cause an individual to become radioactive; the radiation exposure ceases as soon as the individual leaves the radiation field.

All ionizing radiation sources produce an external radiation field.  However, some fields are so small they pose no external radiation risk at all. Examples include these low and moderate energy beta radiation emitters:

• H-3
• C-14
• Ni-63
• P-33
• S-35

Other sources of ionizing radiation produce much higher energy external radiation fields, and care must be taken to shield the source and to monitor exposure while working near these sources. Examples include:

• Am-241/Be neutron sources
• P-32 beta sources
• Cs-137 gamma sources
• Co-60 gamma sources
• X-ray machines (only when the machine is energized)

Internal
The other type of radiation injury involves contamination with radioactive material. Contamination means that radioactive material in the form of gases, liquids, or solids are released into the environment and contaminate people externally (such as on the skin), internally (such as by ingestion), or both.

Contamination by radioactive material can lead to incorporation of radioactive material into the body. This can be the result of uptake of radioactive material by body cells, tissues, and target organs such as bone, liver, thyroid, or kidney. In general, radioactive materials are distributed throughout the body based upon their chemical properties.  For example, radioiodine, such as 125I, is concentrated in the thyroid gland of the body, just as non-radioactive iodine does.

All radioisotopes are potentially hazardous if inhaled or ingested. This includes low energy isotopes such as 3H and 14C. Frequent monitoring for contamination is necessary when working with any unsealed isotopes, and periodic leak tests are conducted for sealed sources (usually every 6 months).

X-ray machines contain no radioactive material, and thus pose no threat of contamination even when energized. When energized, an x-ray machine is a source of external radiation exposure.