In many departments, the hazmat truck itself's job is the investigation and mitigation of whatever hazardous agent is invovled in the incident. However, decontamination is typically it's own unit in a major CBRNE incident. Separate Hazmat and Decontamination Units would be a nice addition, with the requirement of people needing decontamination. Usually, a decon unit or tender will respond when there are a large amount of contaminated people,. If there is one or two patients, typically firefighters with Hazmat Operations or Technician certification can decontaminate people with the engine company. Contaminated patients typically are decontaminated prior to being triaged and treated medically.