The Reaction Control System (RCS) [1] uses hypergolic fuels [2] which are highly toxic. Such fuels are desirable in this application because they combust whenever the fuel and oxidizer are brought in contact. There's no need to an ignition system, which reduces the chance for failure.

[1] https://en.wikipedia.org/wiki/Reaction_control_system

[2] https://en.wikipedia.org/wiki/Hypergolic_propellant

Hydrazine is one nasty substance and that's whats being used for maneuvering in orbit[1].

[1]: https://en.wikipedia.org/wiki/Hydrazine#Occupational_hazards

As others have said, the hydrazine / nitrogen tetroxide propellant for the reaction control system is highly toxic. The retired space shuttle used the same type of propellants in its RCS, auxiliary power unit (APU) and orbital maneuvering system (OMS), so it was also met upon landing by a hazmat suited service crew.

This NASA safety video from the Apollo era is incredible.

https://www.youtube.com/watch?v=ND2TeNfcmKA

Most likely they're working with very hazardous fuels and oxidisers like hydrazine[0].

Edit: it seems that the orbiter was upgraded to use hydrazine and nitrogen tetroxide[1] from the somewhat less toxic hydrogen peroxide/JP-8 combination originally specced for its AR2-3 engine.

[0] https://en.wikipedia.org/wiki/Hydrazine

[1] https://www.airspacemag.com/space/spaceplane-x-37-180957777/

Hydrazine is hella toxic

Andromeda Strain?