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Thread: How much CYA is ok for your health?

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    How much CYA is ok for your health?

    Hi folks.
    I own a 380 gal spa and I'm using dichlor as primary sanitizer along with HS silver-ION cleaning.
    I cannot go with the "dichlor then bleach method" since Klorox is not available where I live. I found clorine but the concentration is only 3.75% and PH is 13. (I'm in Sweden)
    So, basically I'm stuck with dichlor.

    So the question is, how much CYA is too much? 80 ppm? 100?
    I have googled around but can't find a good answer. My water is only a month old and my CYA is already ~60ppm.
    I have no problem changing the water often, but it would be nice to know what CYA level is considered too much.

    And it is strange that the dealers/manufacturers doesn't teach this to their customers...

  2. Back To Top    #2
    susa's Avatar
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    Re: How much CYA is ok for your health?

    Hejsan, while these are US standards, here's the MSDS sheet

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    Re: How much CYA is ok for your health?

    Welcome to TFP!

    The CYA level itself would have to get to extraordinary levels to be directly unhealthy. The LD50 (lethal dose with 50% killed) for mice is around 3000 mg/kg while the NOAEL (no observed adverse affect limit) for rats is 150 mg/kg/day so a person would have to drink ridiculous amounts to be harmful. Dermal (skin) absorption of CYA is minimal as described in this link and this PDF file at 5 µg/kg/day.

    However, it is the effect that CYA has on the active chlorine (hypochlorous acid) level that is more of concern. As the CYA level rises, the active chlorine level falls assuming you are maintaining the same Free Chlorine (FC) level. Most bacteria are killed very quickly even at rather low active chlorine levels (see the table in this post). However, when I compiled a list of hot tub itch/rash/lung incidents on another forum, it appeared that it was more likely to occur from Dichlor-only use after two months with a small increased risk after one month. This roughly corresponded to around 100 ppm CYA per month though this varies depending on usage (these spas had people soak longer and some had 2 people). Usually by the time the CYA gets to around 300 ppm the water is dull or cloudy so is when people typically change their water though this may be due to the slower oxidation of bather waste (or lotions or oils) from the lower active chlorine level rather than from bacterial growth.

    With your CYA at 60 ppm after one month and assuming a 5 ppm loss of CYA over the month (chlorine slowly oxidizes CYA and this is noticeable in spas at a rate of around 5 ppm per month), this implies an average Dichlor use of 65/30/0.9 = 2.4 ppm FC per day which implies around 60*(2.4/7)*(380/350) = 22 minutes of one person soaking per day in a hot (104ºF) spa, assuming there is no ozonator. Does that sound about right?

    The standard Water Replacement Interval (WRI) for spas is defined as (1/3) x (Spa Size in U.S. Gallons) / (# of Bathers) where the soak time is probably around 20 minutes or so. This comes to 127 days (around 4 months) for your spa, assuming 20 minute soak time for one person per day. The CYA would get to around 250 ppm by that time and the water might start to look a little dull.

    Overall, I'd say you would reduce your risk by changing the water after around 2 months, maybe 3; I wouldn't wait for 4. If you have access to Cal-Hypo, you might consider using that part of the time though it does increase Calcium Hardness (CH) so you wouldn't use it for too long. Nevertheless, if you used it for one month at the 2.4 ppm FC rate then that would increase CH by around 50 ppm which is quite tolerable unless your CH is starting high. So you could start with 100 ppm CH and use Dichlor for one month and then Cal-Hypo for one month and then Dichlor again for one month. Note that with Cal-Hypo, you'd want to pre-dissolve it in a bucket of water first before adding it to the spa.

    Note that there are a lot of people who use Dichlor-only that don't run into problems, but the above approach will lessen the already low risk and would keep the water clearer for a longer period of time.

    Quote Originally Posted by Nilking
    And it is strange that the dealers/manufacturers doesn't teach this to their customers...
    Hardly anyone in the pool and spa industry talks about the chlorine/CYA relationship. Real chemists at manufacturers know about it since it's real science definitively determined since at least 1974, but it's not disclosed to distributors and dealers. Even the basic chemical facts that are indisputable and independent of concentration of product and of pool/spa size are not disclosed:

    For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
    For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
    For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by at least 7 ppm.
    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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    Re: How much CYA is ok for your health?

    Quote Originally Posted by chem geek
    Welcome to TFP!

    With your CYA at 60 ppm after one month and assuming a 5 ppm loss of CYA over the month (chlorine slowly oxidizes CYA and this is noticeable in spas at a rate of around 5 ppm per month), this implies an average Dichlor use of 65/30/0.9 = 2.4 ppm FC per day which implies around 60*(2.4/7)*(380/350) = 22 minutes of one person soaking per day in a hot (104ºF) spa, assuming there is no ozonator. Does that sound about right?

    Thanks for a great reply. Sounds right except I do have an ozonator.

    Quote Originally Posted by chem geek

    The standard Water Replacement Interval (WRI) for spas is defined as (1/3) x (Spa Size in U.S. Gallons) / (# of Bathers) where the soak time is probably around 20 minutes or so. This comes to 127 days (around 4 months) for your spa, assuming 20 minute soak time for one person per day. The CYA would get to around 250 ppm by that time and the water might start to look a little dull.

    Overall, I'd say you would reduce your risk by changing the water after around 2 months, maybe 3; I wouldn't wait for 4. If you have access to Cal-Hypo, you might consider using that part of the time though it does increase Calcium Hardness (CH) so you wouldn't use it for too long. Nevertheless, if you used it for one month at the 2.4 ppm FC rate then that would increase CH by around 50 ppm which is quite tolerable unless your CH is starting high. So you could start with 100 ppm CH and use Dichlor for one month and then Cal-Hypo for one month and then Dichlor again for one month. Note that with Cal-Hypo, you'd want to pre-dissolve it in a bucket of water first before adding it to the spa.
    Sounds ok. I'm probably going to exchange water at least every two months then. It only takes two hours so no biggie.
    I have changed water every month for awhile now..

    Thanks for the great answer.

    //Thomas

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    Re: How much CYA is ok for your health?

    Quote Originally Posted by susa
    Hejsan, while these are US standards, here's the MSDS sheet
    Thanks. And a nice touch with the "Hejsan"

    //Thomas

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    susa's Avatar
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    Re: How much CYA is ok for your health?

    Thomas, Vi kan alla prata svenska.


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    Re: How much CYA is ok for your health?

    Quote Originally Posted by Nilking
    Thanks for a great reply. Sounds right except I do have an ozonator.
    Thomas,

    Well, an ozonator changes some things in the analysis. If you are using the tub every day, then this roughly cuts down the chlorine demand in half which would mean you could be soaking for twice as long (44 minutes) or with twice as many people (2 people). If, on the other hand, you used the spa infrequently, say once a week, then an ozonator roughly doubles the chlorine demand (oxidizing chlorine to chlorate or reducing it to chloride) but that seems unlikely given the amount of chlorine you need to use. Fortunately, the ozonator doesn't change the analysis regarding the amount of CYA in the water since that's just dependent on the amount of Dichlor you add. So changing the water after a couple of months would still be fine.

    Richard
    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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    Re: How much CYA is ok for your health?

    välkomna Nilking, (that's aboutas far as I go)
    Have you tried commercial cleaning suppliers for chlorine bleach, That is where I generally locate the best price/strength chlorine. The cleaning suppliers usually have a high turnover too so it's fresher than most pool shops who sell dichlor tabs more frequently.

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