What must be done before unloading a HAZMAT car with a vapor hazard?

Before unloading a HAZMAT car that presents a vapor hazard, it is crucial to ventilate the area. This step is vital for ensuring the safety of personnel and reducing the risk of harmful vapor accumulation. Ventilation helps to disperse potentially toxic or flammable vapors, thereby creating a safer environment for the unloading process. Effective ventilation diminishes the chances of explosive atmospheres and exposure to dangerous chemicals, which could pose significant health and safety risks.

Considering the other options, sealing the car may be beneficial in certain situations, but it does not address the immediate need for airflow to reduce vapor concentrations during unloading. Waiting for the temperature to stabilize might be relevant in other contexts, but it does not directly mitigate the risk associated with the presence of vapors. Checking the placard for evacuation procedures is important for overall safety, but it does not directly contribute to the immediate action required for safe unloading in the presence of vapor hazards. Thus, ventilating the area is the most effective first step to ensure safety before proceeding.