Hazard monitoring is the continuous measurement of conditions in an environment over time, so that developing hazardous states are found, recorded and routed to whoever can act on them.
Monitoring is a system, not an event. It includes establishing what normal looks like for a specific space, noticing change, keeping a record good enough to investigate afterwards, and getting alerts to a person who can do something. A detector alone does none of that.
Sensors sample continuously. Readings are evaluated, stored and compared against both fixed limits and the location's own history. Notable states raise alarms and create records. The distinction from detection is time: monitoring accumulates, detection decides.
A plant that logs air quality continuously can show that a shift pattern correlates with elevated VOCs. A plant with only an alarm knows nothing until the alarm fires.
Every reading is stored, not only the ones that breach a threshold, which is what makes trend and baseline analysis possible at all.
