There's the law and regulations and then there is reality. I'll give you a bit of both since the regs have not kept up with the latest science (and by latest, I mean as far back as 1974!).
First off, there are no national regulations or laws regarding chlorine amounts in pools or spas. Such regulations are at the state or county (i.e. associated with health departments) or sometimes the city level. However, the EPA does have a maximum chlorine level specified for drinking water as listed in their
Drinking Water Contaminants webpage where they list a maximum chlorine level of 4 ppm.
This post gives a few limits for some states and you will see that some exceed the 4 ppm EPA limit with the highest being 10 ppm in Florida. The EPA also regulates (via FIFRA) what can be written on pesticide product labels with regard to pathogen kill claims and dosages. So chlorine products will never say more than 4 ppm FC for pools.
One of the big problems is that current regulations look only at the Free Chlorine (FC) level by itself and not in conjunction with Cyanuric Acid (CYA). Separate limits are set for each. This is idiotic since the FC alone is irrelevant when it comes to reaction rates for disinfection, oxidation, corrosion, creation of disinfection by-products, etc. The FC level in isolation is only relevant from a capacity point-of-view so is relevant for drinking water, not swimming in it. Also, some people assume that the lower disinfection rates when CYA is present is always a problem rather than seeing that the higher active chlorine level without CYA is perhaps a bigger problem. Fortunately, it takes a very low active chlorine level to kill most pathogens and for those that are highly chlorine-resistant such as the protozoan oocyst
Cryptosporidium parvum, one can use supplemental systems such as ozone, UV, AOX or even overnight chlorine dioxide. To minimize the oxidation of skin, hair, swimsuits, corrosion of equipment, and rate of creation of disinfection by-products, I think that a reasonable target for commercial/public pools and spas is an FC that is around 20% of the CYA level so indoors that would be 4 ppm FC with 20 ppm CYA or 6 ppm FC with 30 ppm CYA while outdoors perhaps one could go lower if exposed to UV in sunlight so perhaps as low as an FC that is 10% of the CYA level so 5 ppm FC with up to 50 ppm CYA. For residential pools and spas, there are no regs, but if there were they should be looser and closer to the algae-growth prevention limits in the 5-7.5% FC of CYA range. The FC that is 20% of the CYA level (at 77ºF) is roughly equivalent in active chlorine concentration to 0.2 ppm FC with no CYA.
Are you asking about this due to the CDC MAHC's
Disinfection and Water Quality Code and
Disinfection and Water Quality Annex where they are proposing to ban CYA completely from indoor venues, spas and "increased risk venues" such as waterparks and whirlpool baths? You realize that public comment for this is now closed. I sent extensive comments about this to CDC MAHC. You can read more about this issue in the
Pool & Spa News "New Code Proposes CYA Restriction" article where you can also read some of my comments there (you'll also see that Bob Lowry has comments similar to some of my posts since I had been in contact with him previously, but he doesn't link to or refer to the sources). My take on the CDC MAHC proposals (as they currently stand) is that they are as biased in the direction of "more is better" for active chlorine and "CYA is bad so should be avoided" than the APSP-11 recommendations were in the direction of "CYA doesn't matter". The CDC MAHC is being driven by fear of Crypto outbreaks and pathogen control without regard to disinfection by-products or other side effects of high active chlorine levels while the APSP-11 was driven by chlorinated isocyanurate manufacturer desires for continued sale of their products. I'm oversimplifying, of course, since there were a lot of good science references written in APSP-11, but it was largely ignored when it came to setting FC and CYA levels. Ironically, the CDC MAHC also refers to peer-reviewed papers, but ignores whole categories they'd rather not deal with. Neither group looked at the raw fundamental and very basic science regarding the chlorine/CYA relationship and how CYA moderation of chlorine's strength can be seen as a good thing if done in moderation.
You can see from other CDC MAHC modules that my comments are largely being ignored in
1.0, 2.0, 3.0 Preface, User Guide and
6.1 Operator Training Module, but I am still hopeful that the
Disinfection and Water Quality Committee will take my comments more seriously.