Why is the most common hydraulic fluid so poisonous? 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.
Fume events are from two sources in the vast majority of cases:  Deicing fluid, and engine oil.
The Aviation Herald event linked was poisoning from (really severe) engine oil contamination (mainly the toxic TCP additive; it's the same stuff used as a lead scavenging agent for running 100LL fuel on low octane engines, and it's also the oil additive that Lycoming specifies for the H2AD piston engines for the cam spalling problem). Phosphate Ester hydraulic fluid had nothing to do with it.
The oil contamination comes from 3 sources:

A failed compressor section seal on the main engine at some forward bearing that is able to feed oil into the compressor upstream of the bleed port used for air conditioning.
A blown seal on an air cycle machine (the turbocharger-like cooling unit in an air conditioning pack) allowing ACM lube oil into the conditioned bleed flow.
An APU compressor seal failure (more or less the same as an engine oil seal failure) getting into the APU bleed and into the A/C packs.

Probably the majority of fume events are from oil traces, only enough to make smells, but which is really hard to remove, and deicing fluid (when airplanes get deiced and too much was sprayed into the engine and the engine didn't get enough of a purge run to clean the compressor out before turning on the bleeds).  Deicing fluid fume events happen a lot and are probably the most common cause of fume turn-backs.
Skydrol/Hyjet is mostly a hazard to the techs that work on the planes.  Anyone here who turns wrenches on transport airplanes will tell you that Phosphate Ester fluid is nasty, nasty stuff to get on your skin and probably the single most unpleasant aspect of the job.