What sign is used for labs with more than 5 millirem per hour exposure risk at one foot from the source?

The appropriate sign for labs where the exposure risk exceeds 5 millirem per hour at a distance of one foot from the radiation source is the "Caution - radiation area." This designation is crucial for ensuring safety in environments where radiation could pose a significant risk to individuals.

In radiation safety practices, this sign specifically indicates that the area has levels of radiation that could be harmful if proper precautions are not followed. The threshold of 5 millirem per hour serves as an important marker; levels above this indicate a need for heightened awareness and protective measures.

Other signs, while relevant in different contexts, do not convey the specific radiation exposure risks associated with the defined limits. For instance, signs indicating "Caution - radioactive materials" refer more broadly to the presence of radioactive substances without necessarily indicating the exposure levels. Similarly, signs that state "Caution - authorized personnel only" focus on restricting access without informing individuals about specific radiation hazards, and "Caution - hazardous materials" pertains to general chemical risks rather than radiation exposure specifically. Thus, "Caution - radiation area" is clearly the most appropriate choice for this scenario.