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Thread: At what chlorine shock level is it unhealthy to swim ?

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    At what chlorine shock level is it unhealthy to swim ?

    At what chlorine shock level is it unhealthy to swim in the pool ? I have some mustard algea and CYA about 40 so according to this I need to raise free chlorine to 24
    chlorine-cya-chart-t2346.html#p18157
    Is that unhealthy if you jumped in, swam laps for 30 minutes wearing a mask to protect the eyes, then got out and took a shower?
    36 x 18 oval pool, inground, plaster pool.
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    Re: At what chlorine shock level is it unhealthy to swim ?

    Related question - If you went on vacation and just let the pool go, then came back and it was full of algea, would it be unhealthy to swim in it?
    36 x 18 oval pool, inground, plaster pool.
    27000 gallons
    1 hp pump. Sand filter
    Pool built in 1971.

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    Re: At what chlorine shock level is it unhealthy to swim ?

    I probably wouldn't swim in it at mustard level, although I'll leave that one for someone else to answer definitively.

    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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    Re: At what chlorine shock level is it unhealthy to swim ?

    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.
    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.
    36 x 18 oval pool, inground, plaster pool.
    27000 gallons
    1 hp pump. Sand filter
    Pool built in 1971.

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    Re: At what chlorine shock level is it unhealthy to swim ?

    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.
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    Re: At what chlorine shock level is it unhealthy to swim ?

    Assuming you lower the pH first before shocking to the high FC level,
    I remember reading here you should lower pH when shocking, but I can't find the info in pool school now. How low should you lower it? What is the reason for lowering it?

    By the way, thanks for all your help.
    36 x 18 oval pool, inground, plaster pool.
    27000 gallons
    1 hp pump. Sand filter
    Pool built in 1971.

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    Re: At what chlorine shock level is it unhealthy to swim ?

    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.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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