I've written about the concept of the reserve of chlorine and the speed at which chlorine gets released and the concept of why that speed doesn't matter in terms of the rate of killing pathogens in
this post,
this post and
this post.
When I wrote "Free Chlorine (FC) does not measure active chlorine, but rather the chlorine reserve or reservoir that is mostly inactive", the "chlorine reserve or reservoir" includes both the chlorine bound to CYA as well as that which is unbound, namely HOCl and OCl
- which is why I said such reserve was mostly inactive since the chlorine bound to CYA has very little reactivity.
If you really want to know how much HOCl there is in the water, then you can use my spreadsheet to calculate it using primarily the FC, CYA and pH though other parameters also have a minor effect (there is also a temperature dependence, but I have that turned off at line 225 "Use Temp. Dependent Cl-CYA" in the spreadsheet since it came from Wojtowicz and not from a normal peer-reviewed journal). Roughly speaking, the HOCl level is proportional to the FC/CYA ratio and is very roughly half that ratio near a pH of 7.5 (Jason referred to a better formula I have for it, but even that is an approximation and only for a pH of 7.5 -- the spreadsheet does the accurate calculations).
Your pool does not become unhealthy when the chlorine gets below the minimum FC for your CYA level. It just becomes more likely to develop algae. Most bacteria and viruses are very easy to kill or inactivate and require very little chlorine to do so (some papers showing kill times are linked to in the "Chlorine / CYA Relationship" section in
this post and there is also
this CDC chart). The primary reason we have the minimum FC be higher is to prevent algae growth and to have faster bacteria and virus kill/inactivation rates to prevent person-to-person transmission of disease. It's not like all of a sudden the pool becomes unsanitary -- that pretty much won't happen unless the chlorine level is close to zero.
At the minimum FC/CYA ratios recommended on this forum for non-SWG pools, namely an FC that is around 7.5% of the CYA level, this is equivalent in HOCl concentration at a pH of 7.5 to a pool with an FC of 0.062 ppm with no CYA. Most heterotrophic bacteria have CT values of 0.04 for a 99% kill which means such a kill is done in 0.04/0.062 = 0.65 minutes or under 40 seconds. It takes at least 15 minutes for bacteria to double in population so this level of chlorine would kill bacteria faster than they can reproduce for any with a 50% CT of 0.93 which roughly corresponds to those listed with a 2-log 99% kill CT of around 6, a 99.9% CT of 9, a 99.99% CT of 12, etc. Pretty much everything in the CDC table is handled except for the bacteria
Burkholderia pseudomallei,
Vibrio cholerae (rugose not smooth strain),
Yersinia enterocolitica. Though the 99.99% inactivation time for
Poliovirus would be around 100 minutes, viruses do not reproduce outside their host. Similarly, the protozoan oocysts of
Entamoeba histolytica and
Giardia lamblia would take around 4 hours to inactivate 99% to 99.9%. The protozoan oocysts
Toxoplasma gondii and
Cryptosporidium parvum are essentially not inactivated by normal chlorine levels in any reasonable amount of time.
Richard