"Casualty" can also be translated in this context as "el accidentado", "la persona que tuvo el accidente". According to the context provided, the text is about the measures that should me taken if someone accidentally inhales a specific toxic substance. The first step would be to place that person where s/he is no longer exposed to the (probably) gas.

Aislar a las víctimas afectadas

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Note added at 29 mins (2021-03-18 16:46:38 GMT)
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The first aid procedures outlined here will help the person attending a casualty to prevent serious damage to health and perhaps help to save life.
It is important to recognise that the casualty may not necessarily be suffering from poisoning, unless the circumstances clearly indicate that this is the case. The symptoms may be unrelated to the incident or may result from the physical effects of shock or an explosion. The first aid procedures detailed here would be of benefit whatever the cause and in any event, if the casualty has any symptoms he or she should be seen by qualified medical personnel. It is better to overestimate the risk to the casualty than to ignore symptoms.