The usual hydraulic fluid, Skydrol, is poisonous. If it makes it to the air conditioning system, it causes fume events that may have adverse health effects up to and including at least one fatality. And it is not easy to completely prevent the possibility of it creeping into the air ducts as bleed air is also used for pressurizing the hydraulic reservoirs (at least on some aircraft).
So why was this organophosphate-based fluid selected over anything safer? Are there no safer alternatives?
Update: I should have linked the Qantas A332 at Sydney on Dec 15th 2019, hydraulic leak prompts evacuation accident, which actually inspired the question. That one is clearly hydraulic-related (while in most other fume events we never know for sure), and there is a comment down the page:
By geezer on Monday, Dec 16th 2019 14:48Z:
Nothing to do with the apu other than the duct by the packs. Hydraulic reservoirs are (gas) pressurised by either an engine bleed or from the pneumatic duct in the pack bay. Hence a tee piece. If one system loses pressurisation, say overnight, then with a couple of leaky check valves one reservoir can slowly fill another back through the hydraulic service manifold, as all three systems are serviced from one inlet using a three-way valve. Eventually one tank is overfull to the point where the fluid now travels back down the air pressurisation pipe (a small bore pipe). Another check valve stops it flowing back to the engine so it ends up dribbling into the pneumatic duct in the pack bay. We know this because we have found skydrol in these small bore pressurisation lines. I hope you get all that. I think I need a little lie down now.
It should be noted that this is not verified and may not be the cause, but A330 has APU intake on the bottom, so the APU could have also ingested the hydraulic fluid dripping from the bottom of the fuselage into the inlet.