Posting radiation areas is one of the most visible elements of a radiation safety program. Signs on doors, ropes around work areas, and labels on containers tell workers and the public where radiation hazards exist. The NRC prescribes specific posting requirements in 10 CFR 20.1902 based on dose rate and contamination level thresholds defined in 10 CFR 20.1003. Getting the posting right is not complicated, but getting it wrong, either by under-posting (a compliance violation) or by unnecessary over-posting (which desensitizes workers to signs they see everywhere), undermines the entire safety communication system.
This guide covers the area classifications, the specific posting requirements for each, the access controls that accompany posting, exemptions that may apply, and practical guidance for implementing a posting program that is both compliant and effective.
Area Classifications and Dose Rate Thresholds
10 CFR 20.1003 defines the following area classifications based on dose rate. The measurement point is 30 cm from the radiation source or from any surface that the radiation penetrates, unless otherwise specified:
- Radiation Area: An area, accessible to individuals, in which radiation levels could result in an individual receiving a dose equivalent in excess of 5 mrem (0.05 mSv) in 1 hour at 30 cm from the radiation source or from any surface that the radiation penetrates.
- High Radiation Area: An area, accessible to individuals, in which radiation levels could result in an individual receiving a dose equivalent in excess of 100 mrem (1 mSv) in 1 hour at 30 cm from the radiation source or from any surface that the radiation penetrates.
- Very High Radiation Area: An area, accessible to individuals, in which radiation levels could result in an individual receiving an absorbed dose in excess of 500 rads (5 Gy) in 1 hour at 1 meter from a radiation source or from any surface that the radiation penetrates.
Note the unit change for Very High Radiation Areas: the threshold is in rads (absorbed dose), not rem (dose equivalent), and the measurement distance is 1 meter, not 30 cm. For gamma radiation with a quality factor of 1, rads and rem are numerically equal, so 500 rads equals 500 rem for gamma sources.
The "accessible to individuals" qualifier means that an area inside a locked, shielded vault is not classified based on the dose rate inside the vault if no one can access the vault interior while the source is exposed. The area outside the vault is classified based on the dose rate at accessible locations.
Area thresholds at a glance: Radiation Area: >5 mrem/hr at 30 cm. High Radiation Area: >100 mrem/hr at 30 cm. Very High Radiation Area: >500 rad/hr at 1 m. The dose rate that defines the classification is not the dose rate at the sign; it is the dose rate at 30 cm from the source or surface.
Radiation Area Posting Guide
Determine radiation area posting requirements from dose rate at 30 cm. Identifies Radiation Area, High Radiation Area, and Very High Radiation Area classifications per 10 CFR 20.1003 and 20.1902.
Sign Requirements Under 10 CFR 20.1902
10 CFR 20.1902 specifies the signs required for each area classification. Each sign must bear the trefoil radiation symbol (magenta or black on yellow background) and the appropriate wording:
- Radiation Area: "CAUTION, RADIATION AREA" or equivalent conspicuous notice.
- High Radiation Area: "CAUTION, HIGH RADIATION AREA" or "DANGER, HIGH RADIATION AREA."
- Very High Radiation Area: "GRAVE DANGER, VERY HIGH RADIATION AREA."
- Airborne Radioactivity Area: "CAUTION, AIRBORNE RADIOACTIVITY AREA" or "DANGER, AIRBORNE RADIOACTIVITY AREA" (primarily relevant for unsealed source work).
Signs must also be posted for areas or rooms where licensed material is used or stored in quantities exceeding 10 times the applicable Appendix C values ("CAUTION, RADIOACTIVE MATERIAL" or "DANGER, RADIOACTIVE MATERIAL").
Containers of radioactive material must be labeled with the radiation symbol and the words "CAUTION, RADIOACTIVE MATERIAL" or "DANGER, RADIOACTIVE MATERIAL," along with sufficient information (isotope, activity, date, radiation level) to permit individuals handling the container to minimize exposure.
The sign must be visible and readable at the point of entry to the area. For rooms, post on or adjacent to the door. For roped-off areas in open spaces, attach signs to the rope or barrier at all access points and at intervals along the perimeter where people might approach. Signs that are faded, damaged, or too small to read from a reasonable distance do not meet the "conspicuous" requirement and should be replaced.
Using a "CAUTION, RADIATION AREA" sign where the dose rate actually qualifies as a High Radiation Area is under-posting, which is a compliance violation. Similarly, posting an area as High Radiation Area when the dose rate is below 100 mR/hr at 30 cm is over-posting, which erodes sign credibility. Survey and classify correctly.
Access Controls for High and Very High Radiation Areas
Posting signs is only part of the requirement. High Radiation Areas and Very High Radiation Areas also require physical access controls to prevent unauthorized or inadvertent entry.
High Radiation Areas (10 CFR 20.1601): Each entrance or access point must be equipped with one or more of the following: a device that energizes a visible or audible alarm when entered, a device that is locked (barricaded or otherwise controlled) to prevent unauthorized entry, or continuous direct or electronic surveillance capable of preventing unauthorized entry. Radiography operations under 10 CFR 34 typically use locked and barricaded boundaries (rope and padlocks) plus continuous direct surveillance (the radiographer observes the boundary during the exposure).
Very High Radiation Areas (10 CFR 20.1602): In addition to the above, the licensee must ensure that additional measures are implemented to prevent unauthorized entry and that an individual is not able to gain unauthorized access to the area. This typically means physical barriers (locked doors, interlocked doors that retract the source if opened) that cannot be bypassed without deliberate effort and authorization.
For fixed gauge installations (Cs-137 level gauges, for example), the gauge itself may create a localized Radiation Area at the beam window, but the dose rate typically drops below the Radiation Area threshold within a few inches to a foot of the gauge surface. The entire room is usually not a Radiation Area. Post the gauge with a "CAUTION, RADIOACTIVE MATERIAL" label on the device, and post a "CAUTION, RADIATION AREA" sign only if the area threshold is actually exceeded at accessible locations.
High Radiation Areas require locked/barricaded access, visible/audible alarms, or continuous surveillance. Very High Radiation Areas require additional measures that physically prevent unauthorized entry. Signs alone are not sufficient for either classification.
Exemptions from Posting Requirements
10 CFR 20.1903 lists several exemptions from posting requirements:
- Rooms or areas in hospitals where patients are undergoing diagnostic or therapeutic procedures, provided the patient's physician has authorized the procedure.
- Rooms or areas containing sealed sources where the dose rate at 30 cm from the sealed source container or housing does not exceed 5 mR/hr. Many properly shielded gauge installations fall below this threshold and do not require area posting (though the gauge container itself still needs a radioactive material label).
- Rooms or areas where radioactive materials are used or stored for periods of less than 8 hours, provided the materials are constantly attended during use and removed to a posted area when not in use.
The 8-hour exemption is often relevant for portable gauge users (moisture-density gauges used in construction) who bring the gauge to a job site for several hours and return it to a posted storage location at the end of the day. During field use, the gauge is constantly attended by the operator, and the 8-hour limit is not exceeded. The vehicle used for transport does not need Radiation Area posting if the gauge is in its approved shipping container and the dose rate at accessible surfaces is within DOT limits.
The RSO should document the basis for each exemption claimed and ensure that the conditions for the exemption are actually met in practice. An exemption that is claimed but not properly maintained (for example, material left unattended during the 8-hour period) becomes a violation.
If a shielded gauge installation produces less than 5 mR/hr at 30 cm, area posting is not required. But the gauge housing still needs a "CAUTION, RADIOACTIVE MATERIAL" label with isotope, activity, and date information. Survey the installed gauge to confirm the dose rate and document the exemption basis.
Radiation Area Posting Guide
Determine radiation area posting requirements from dose rate at 30 cm. Identifies Radiation Area, High Radiation Area, and Very High Radiation Area classifications per 10 CFR 20.1003 and 20.1902.
Container and Device Labeling
10 CFR 20.1904 requires that every container of licensed material bear a durable, clearly visible label with the radiation symbol, the words "CAUTION, RADIOACTIVE MATERIAL" (or "DANGER, RADIOACTIVE MATERIAL"), and sufficient information (isotope, quantity, date of measurement, radiation levels, name of responsible individual) to minimize exposure to individuals handling or using the container.
For sealed sources in shipping containers (Type A or Type B per DOT regulations), the manufacturer's labeling typically satisfies this requirement. For sources in storage (source pigs, gauge housings, camera storage boxes), verify that the label is present, legible, and current. Labels degrade from UV exposure, chemical exposure, and physical abrasion. Replace faded or damaged labels.
Some facilities add supplementary information to the label: the license number, emergency contact number, and the dose rate at 30 cm and 1 meter. This information is helpful for emergency responders and for anyone who encounters the container unexpectedly. For gauges mounted on process equipment, a sturdy metal tag attached to the gauge with wire or chain is more durable than an adhesive label.
Exemptions from container labeling under 10 CFR 20.1905 apply to containers holding material in quantities below the values in Appendix C Table 3, certain laboratory containers in immediate use, and containers that are being processed (opening, transferring) by an individual who is present. The exemptions do not eliminate the need for area posting if the area thresholds are exceeded.
Implementing an Effective Posting Program
An effective posting program is one where the signs accurately reflect the current radiation conditions and where workers trust the signs to provide meaningful safety information. Here are the practices that make it work:
Survey before posting. Do not post an area based on calculation or assumption alone. Survey with a calibrated instrument at 30 cm from the source or surface to determine the actual dose rate. Classify the area based on the measurement, not on what you think it should be. If the dose rate is 4 mR/hr at 30 cm, it is not a Radiation Area and should not be posted as one.
Update when conditions change. If a source is exchanged for one with different activity, re-survey and re-classify. If shielding is added or removed, re-survey. If a gauge is decommissioned and the source removed, take down the posting signs. Outdated signs on empty rooms or decommissioned equipment teach workers to ignore signs.
Use standardized signs. Purchase commercially printed signs that meet 10 CFR 20.1901 and ANSI N2.1 specifications. Hand-written signs on notebook paper do not project competence and may not meet the "conspicuous" requirement. Signs should be durable enough for their environment (outdoor signs should be weatherproof, industrial signs should resist oil and solvent splashes).
Train workers to read and respect signs. 10 CFR 19.12 requires that radiation workers receive instruction in the meaning of signs, labels, and posting requirements as part of their initial training. Refresh this training annually. Workers who understand why a sign is there (not just that it is required) are more likely to comply with the associated access controls.
Audit regularly. Walk the facility quarterly and verify that all posted areas are correctly classified, all signs are legible and properly placed, and no areas that should be posted have been missed. Document the audit results. This is a straightforward compliance activity that prevents findings during NRC or Agreement State inspections.
Conduct a quarterly posting audit: verify every sign matches the current dose rate, check that no new areas need posting, and confirm that decommissioned locations have had signs removed. A simple walkthrough log with dates and findings demonstrates program effectiveness to inspectors.
Common Posting Violations and How to Avoid Them
NRC inspection reports document recurring posting-related violations. The most common include:
Missing or inadequate container labels. Sources in storage without the required isotope, activity, date, and radiation level information on the label. The fix is to conduct an annual inventory of all container labels and replace any that are incomplete, illegible, or missing.
Under-posting High Radiation Areas. An area where the dose rate exceeds 100 mR/hr at 30 cm posted only as a Radiation Area, without the required access controls for a High Radiation Area. This typically occurs when a new, higher-activity source is installed without updating the posting. The fix is to re-survey and re-classify after every source change.
Missing access controls. A High Radiation Area posted with signs but without the required locking, alarming, or surveillance provisions of 10 CFR 20.1601. Signs alone do not satisfy the High Radiation Area requirements. The fix is to install locks, alarms, or establish continuous surveillance procedures.
Stale posting on decommissioned areas. "CAUTION, RADIATION AREA" signs remaining on rooms where sources were removed months or years ago. This is technically over-posting (not a violation by itself), but it erodes sign credibility and can lead to confusion. It also suggests a lack of program oversight that inspectors notice. Remove signs when the radiological condition no longer warrants them.
Failure to post temporary work areas. Radiography conducted in a general work area without establishing and posting the restricted area boundary. The boundary must be posted with appropriate signs and physically controlled (rope, barricade) before the source is exposed. This requirement is enforced under both 10 CFR 20 and 10 CFR 34.
Under-posting a High Radiation Area as a Radiation Area is a Severity Level III or IV violation that can result in a notice of violation and potentially a civil penalty. It is also a genuine safety hazard because workers may enter the area without the heightened awareness and access controls that High Radiation Area classification demands.