From Halopedia, the Halo wiki
HAZMAT (short for Hazardous Materials) teams are personnel tasked with cleaning up potentially dangerous spills and handling material that is not safe for other personnel to handle. They clean up things like broken and leaking Shaw-Fujikawa Translight Engines.
When they do not remove these problems directly, they may tag a space-borne hazard with HAZNAV (navigation hazard) satellites to warn other ships from straying too close to it.
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