Radiation Limits
By: Radiological71
16 January 2008

There are two main distinctions in the biological effect of radiation.
The usual terms are "stochastic" (random) effects, and "non-stochastic" (deterministic or acute effects). An acute effect is associated with killing cells. Surface skin burns from contact with radioisotopes ("beta burns") are an example. A stochastic effect is usually associated with a modification (mutation) of a cell. The mutation may or may not be passed on, may or may not manifest itself, and is less predictable.

Acute Effects

The cataract formation low dose limit for a single exposure of radiation appears to be about 2 Sv, and about 4 or 5 if the exposures occur in multiple fractions.However, neutron irradiation has a much greater relative biological effectiveness than gamma rays, so the neutron induced cataract formation is about 10 times lower compared with x-rays or gamma rays. Radiation induced cataracts usually form at he back of the lens, whereas age related cataracts tend to progress from the front of the lens backwards Other examples of deterministic effects are death LD_50/60 4.5 Sv whole body dose, and sterility in men (3 Sv gonad dose).

When a single large dose is confined to the skin, early transient reddening (erythema) occurs at 2 Sv, and appears in a few hours. Main erythema, permanent epilation and dry desquamation occur at the 6 Sv to 10 Sv range and appear at around 10 days to 4 weeks. Moist desquamation, ulceration and necrosis are typical in the 15 Sv to 20 Sv range and manifest themselves around 4 to 10 weeks. More information on this can be found on the CDC website. Examples of these effectects are seen in some difficult radiology proceedures.

Picture following is graphic in nature.

Figure 1: The result of ulceration after 5 hours of fluoroscopic x-rays

Stochastic Effects

The USA whole body annual dose limit for a radiation worker is 50 mSv or 5 rem [10 CFR 20.1201]. At a rate of 5% per Sievert, this corresponds to a 25% chance of cancer sometime during a persons life, if the 5% rule is accuate. (100 years x 0.05 Sv/yr x 5%). A 10 mSv (1 rem) exposure every year would correspond to a 5% chance of the radiation producing cancer.The ICRP dose limit for radiation workers is 0.02 Sv (20 mSv). For a member of the general public, the dose limit is 5 mSv or 0.5rem.[10CFR 20.1301] The NRC requires that any additional radiation from a unit site visit or facility produces no more than 1m Sv (0.1 rem) per year. Nasa limits for astronauts are much larger than a usual radiation worker, with 1 to 4 Sv for an entire career. Internally ingested radioisotopes are a different subject. An injested isotope can be selectively absorbed in a particular organ. For example ¹³¹I in the thyroid or ⁸⁹Sr in the bones. The "whole body dose equivalent" in such a case would be small, but the effect would be much larger than the simple 5% per Sv rule.

Dose Rate

References