If a contamination risk exist and you need to provide first aid to a victim?

You need to make a determination about whether the physical environment is safe for the first aid team (yourself) versus the need to provide immediate aid. You also need to consider whether moving the victim out of the danger area is possible and warranted. There is no simple formulaic answer.

If the victim is the source of the contamination, then first aid must wait until adequate precautions are taken to protect yourself and the others in the first aid team.

Common practice requires the rescuer/first responder to take their own health into account before that of the victim. If you can't make the rescue safely -- don't. Work the problem until a safe rescue is possible.