Which one of the following is not a characteristic of a permit required confined space?

In the context of confined spaces, a permit-required confined space is defined by specific characteristics that make it potentially dangerous for individuals entering it. One such characteristic is the potential for a hazardous atmosphere, which includes situations where there is a deficiency of oxygen or the presence of toxic gases.

The characteristic of having an oxygen level at 21% does not align with the attributes of a permit-required confined space because an oxygen level of 21% is considered normal atmospheric oxygen content. For a space to necessitate a permit, the oxygen levels typically need to be below 19.5% or above 23.5%, indicating a potential risk to health when entering.

On the other hand, the other listed characteristics directly relate to scenarios where hazards can pose serious risks to entrants. For a space to be designated as a permit-required confined space, it must contain materials that could engulf entrants, have the potential for hazardous atmospheric conditions, or include any recognized serious health or safety hazards. Thus, option A stands out as not fitting the criteria required to classify a confined space as permit-required, making it the correct choice for this question.