I'm creating a universe where humanity has reached the stars. They've colonized a few star systems and, obviously, FTL drives are a common thing.
The part of the story I'm currently working on features a ship trying not to be captured by "space police". As a desperate move, they end up jumping into a nebula, but not before being damaged by the policemen. They can't jump away until repairs are complete, but they also can't stay very long inside the nebula - and the reason for this is what I want to know.
The conflict in this scene must be time-pressing, like "either we get out of here in x hours/days or we're screwed". The nebula is the bad guy here.
Maybe it's composition is toxic and, as soon as the ship's force field is down, the crew is doomed - or something like that. However, as soon as I took a better look at nebulae, I found out that a nebula's density is really low - something between 100 to 10.000 particles/cm³. According to my research, this amount is practically irrelevant. But is it enough to be dangerous?
The Actual Question
The ship is stranded in a nebula. The force field is damaged and will fade... soon.
Once the shields fail, what kind of danger is the crew in and how long do they have to get out?
Stuff you might want to know for a better answer
Composition: I don't really know what kinds of elements compose a nebula, so the ideal answer should enlighten me a bit about it and propose something that could be harmful to the crew. Remember that the whole point of this question is making nebulae the bad guys.
Threat: You've estabilished what it's made of, now tell me why is it bad. Is it a radioactive element? Corrosive to the ship's hull? Bad for human lungs? Give me something that is dangerous to the level of true concern. It has to be dreadful, but there must be enough time for the crew to try and figure out a way to solve this issue. Note that you can also choose ignore the force field's protection, if necessary.
Time Sensitivity: Like I said earlier, this scene has a time sensitivity to it, but it's all a matter of perspective. The ideal answer should be something capable of killing the poor castaways in a few hours after initial exposure ('cause the ship's shields integrity would be the countdown, the thing between them and death). But if you can find a way to do it in a few days, that's okay as well. I'm setting the maximum DEADline to a week.
The VIP: There's a genius inside this ship. In the narrative, she's an engineer that will help fix the ship's engine, but I will give you freedom to make her a doctor or a biologist or... something you need - if that helps keeping the crew alive for the time needed. Example: The shields are useless and everyone is already exposed in the moment they pop in the middle of the nebula. Knowing the effects of that particular element to the human body, Dr. Tina Awesome warns the crew to get into the vacuum suits to prevent further exposure - which will buy them a few days to make the needed repairs.