Which of the following is a key characteristic of permit required confined spaces?

A key characteristic of permit-required confined spaces is that they contain materials that can engulf an entrant. This can pose significant risks to individuals entering these spaces, as engulfment can lead to serious injuries or fatalities. The potential for engulfment means that specific precautions and procedures must be in place before entry, which is why a permit is required.

While it is true that trained professionals might enter these spaces and that some confined spaces may require more than one entrant for safety reasons, those are not defining characteristics of permit-required confined spaces. Additionally, many confined spaces are not always well-ventilated, which is another factor that necessitates safety measures and a permit system. The presence of materials that can engulf a person is a critical concern that influences the establishment of safety protocols for these environments.