'Effects' in the context of a HazMat incident relates to:

In the context of a Hazardous Materials (HazMat) incident, 'effects' primarily refers to the consequences and impacts that the hazardous materials can have on humans, the environment, and property. Reducing size and harmful effects pertains to the actions taken to mitigate the immediate hazards posed by the materials involved, such as containment, cleanup, and remediation efforts. This option emphasizes the importance of controlling and minimizing the damage caused by a HazMat incident, which is a central focus during response and recovery operations.

Considering the other options, while preventing future occurrences is crucial for ongoing safety, it does not directly address the immediate effects of a current incident. Changing the timing of events may involve coordination and logistics, but it does not directly relate to managing the consequences of the hazardous material itself. Monitoring environmental impact is important for understanding broader implications but does not specifically target the size and harmful effects that need to be dealt with in the course of a response. Thus, the focus on reducing size and harmful effects is the most directly relevant to managing the immediate risks associated with HazMat incidents.