What type of decontamination is performed to reduce contamination to natural background levels?

The correct answer is thorough decontamination. This type of decontamination aims to significantly reduce contamination levels to those comparable to natural background levels. It involves comprehensive cleaning and decontamination processes that not only address visible contamination but also work to eliminate residual hazardous materials. This method is typically necessary in environments where there has been substantial contamination, ensuring that the area is safe for re-entry or in preparation for further use.

Intermediate, limited, and surface decontamination methods focus on specific areas or types of contaminants without necessarily achieving the level of cleanliness that thorough decontamination targets. Intermediate decontamination may involve a higher level of cleaning than limited decontamination but does not guarantee that the decontamination will reach natural background levels. Limited decontamination is usually a focused effort on designated areas or aspects of contamination. Surface decontamination, on the other hand, primarily deals with cleaning visible surfaces but may not remove all contaminants sufficiently to achieve the intended safety standards.