Skin erythema doses: about 2 Gy for early erythema, 3 to 6 Gy for main erythema, depending on source

Skin reddens in waves, so there’s no single erythema dose. Published thresholds also differ. These are approximate single doses to the skin, in Gy, which is ARRT®‘s primary unit.

What are the thresholds?

Effect	Bushong, Table 33-4	ICRP draft, Table 2.2	CDC brochure	Onset
Early transient erythema	2 Gy	2 Gy	About 2 Gy	Hours
Temporary epilation	3 Gy	3 Gy	About 3 Gy (epilation)	About 2 to 3 weeks
Main erythema	6 Gy	6 Gy	About 3 Gy	About 10 days to 1.5 weeks
Permanent epilation	7 Gy	7 Gy	Not listed in Appendix A	About 3 weeks
Moist desquamation	15 Gy	18 Gy	About 15 Gy	About 4 to 6 weeks

Bushong also describes a mild first wave after 3 to 10 Gy, which is what our radiobiology chapter uses. So one book gives two early figures. If your program teaches one number, use it for class.

Why is the common “200 rem” or “600 rem” phrasing awkward?

Older pages use rem. Absorbed dose in the older system is the rad: 1 Gy = 100 rad. For x-rays the NRC quality factor is 1, so 2 Gy works out to 200 rem, and the numbers match. That’s why you see both. The ARRT outline says SI units are primary, so learn Gy first.

An earlier version of this page told you to pick 600 rem whenever a question said “skin erythema”. We found no source for that rule, and ARRT says it avoids content where resources conflict. Read the stem. If it says early or main, answer for that. Our judgment: if it gives only one matching option, take it.

What does “threshold” mean here?

Skin reactions are deterministic effects, also called tissue reactions, and the 2027 outline uses both terms.

• Below the threshold, the effect isn’t expected.
• Above it, severity rises with dose.
• The line isn’t sharp. The ICRP draft says these values sit near the dose that affects about 1% of people. Bushong says intermediate doses depend on the person, the dose rate and the field size. Bushong also puts the dose affecting 50% of people treated with superficial x-rays at about 5 Gy.

An earlier version said “there is no safe dose below the threshold” and “risk is 100% above it”. Both were wrong. Stochastic effects, such as cancer, are the ones modeled with no threshold.

Why does this matter at work?

High-dose fluoroscopy is the usual setting. Federal rules require fluoroscopes made on or after June 10, 2006 to display air kerma rate and cumulative air kerma. The 2027 outline lists cumulative air kerma and varying the beam angle under minimizing patient exposure. Early redness can also be gone by the time anyone looks, and main erythema shows up days later, so follow your facility’s dose documentation and follow-up process.

The Radiation Protection guide covers fluoroscopy limits, and the Radiation Physics practice set drills radiobiology. For staff protection, see lead apron attenuation.