At what chlorine shock level is it unhealthy to swim ?

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As for vacation, how long will you be gone? There are certainly ways you can set it up so that you don't come home to a mess.
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That was just a theoretical question. The thing I am curious about is health. If a pool is not cared for, and it get all overgrown with algae, is it unhealthy to swim in it? I am not going to do this with my pool. I am just curious.
 
Assuming you lower the pH first before shocking to the high FC level, then the active chlorine level will be roughly equivalent to a pool with 1.4 ppm FC with no CYA. This is roughly the level found in indoor commercial/public swimming pools. The chlorine (and chloramines) in such pools is usually noticeable and continued swimming in such pools usually results in swimsuits degrading, flakier skin and frizzier hair. Chlorine reacts with everything faster at this level so would produce disinfection by-products more quickly so you'd likely smell it on your skin more.

If someone did get into a pool at such high FC levels, then when they get out they should rinse off right away. This is because the high FC level will continue to react with your skin when you leave the pool and that will last much longer than a lower FC level and if you let that water evaporate, then that chlorine becomes more concentrated.

We don't recommend getting into a pool higher than normal shock level, but that's mostly to be conservative and not having enough experience with anyone using a pool at higher levels.

Swimming with algae is not unhealthy, but if the pool is cloudy so that you cannot see the bottom very well, then that is dangerous since you wouldn't be able to see someone who is drowning. Algae can also be slippery so that would be another hazard.
 
Adding a hypochlorite source of chlorine raises the pH. Though the pH will drop back down when the chlorine is used/consumed, it is high while the FC is high. Lowering the pH first lessens this effect. The active chlorine level is lowered at the higher pH, though not nearly as much as when there is no CYA in the water. Nevertheless, if you started at a pH of 7.5 and TA of 80 and CYA of 50 ppm and 4 ppm FC, then the following would occur:

adding 16 ppm FC to get to regular shock level of 20 ppm FC: pH rises to 8.47 and HOCl is 0.22 ppm
adding 26 ppm FC to get to yellow/mustard algae shock level of 30 ppm FC: pH rises to 8.74 and HOCl is 0.37 ppm

If you lowered the pH to 7.2 first, then you'd have:

adding 16 ppm FC to get to regular shock level of 20 ppm FC: pH rises to 7.84 and HOCl is 0.28 ppm (close to actual target of 0.3 ppm)
adding 26 ppm FC to get to yellow/mustard algae shock level of 30 ppm FC: pH rises to 8.38 and HOCl is 0.50 ppm

If you lowered the pH to 7.0 first, then you'd have:

adding 16 ppm FC to get to regular shock level of 20 ppm FC: pH rises to 7.33 and HOCl is 0.33 ppm
adding 26 ppm FC to get to yellow/mustard algae shock level of 30 ppm FC: pH rises to 7.79 and HOCl is 0.67 ppm (close to actual target of 0.7 ppm)

So generally speaking, lowering the pH to 7.2 before regular shocking is helpful while for yellow/mustard algae shock level one could start even lower (7.0 or 7.1). This mostly prevents the pH from getting so high as to risk scaling or lingering cloudiness or metal staining.

This information is not in the Pool School so as to keep things simpler and not require an extra step for shocking.
 
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