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The CYA level itself would have to get to extraordinary levels to be directly unhealthy. The LD
50 (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.
Nilking said:
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.
