Which scenario classifies a gas emergency related to readings inside a manhole?

The scenario that classifies a gas emergency related to readings inside a manhole is characterized by gas readings of 20% LEL (Lower Explosive Limit) or greater. This level indicates a significant concentration of flammable gas in the environment, which poses a serious risk of ignition and explosion if a spark or flame is introduced.

In the context of manhole conditions, readings above this threshold necessitate immediate action because they surpass the safety margin. Firefighters and first responders are trained to recognize that any LEL readings at or above this level require urgent investigation, evacuation of the area, and appropriate mitigation measures to ensure safety.

Conversely, a reading below 10% LEL indicates that the concentration is low and typically does not represent an immediate threat. Gas present in a solid structure, while also a concern, does not provide specific information about concentration levels relevant to an emergency classification. Lastly, the absence of gas measurements leaves uncertainty regarding the situation's safety and does not classify as an emergency based on measurable gas presence.