Since I've found different, though similar, sources of information and have been asked about this in multiple posts, I thought I would consolidate some information on the safety of using borates in a pool (or spa).
It started with information in this report where the most reasonable number for the highest No Observed Adverse Effect Level (NOAEL) was 8.8 mg/kg/day (note that borates are meaured in units of Boron so this is milligrams of Boron per kilogram of body weight). The main adverse effect was seen in male dogs (at 29 mg/kg/day) some of whom had smaller testicles at higher borates levels after long-term (daily) exposure to higher borates levels. So to be safe, dogs should be trained not to drink every day from a pool using borates (one liter of 50 ppm Borate water drunk by a 22 pound dog is 5 mg/kg/day).
I then ran into this EPA report (see also this EPA report) that also quotes the 8.8 mg/kg/day as an NOAEL limit, but they use a factor of 100 Margin of Exposure (MOE) to be extra safe for humans (i.e. assuming that humans might be more sensitive than dogs since no direct toxicity tests were done on humans). Pool doses above 360 ppm Borates [EDIT] actually it's 52-53 ppm [END-EDIT] exceeded the margin of exposure limit for 7-10 year old swimmers assuming they accidentally drink 2 ml per day [EDIT] actually 38 ml (around 2-1/2 tablespoons) per day at 52-53 ppm [END-EDIT]. At 50 ppm Borates, [EDIT] an infant [END-EDIT] would have to drink 14 ml (about 1 tablespoon) per day of pool water. With no margin of exposure, this would be around 1.5 quarts per day. [EDIT] This assumes an 8 kg (17.6 pound) infant such as a 6-month old boy or 8-month old girl; a 7-year old weighs more like 23 kg (around 50 pounds) so could tolerate about three times as much, 38 ml per day or around 2-1/2 tablespoons every day and remember that this is with a safety margin of 100; with no margin of exposure it's over a gallon per day. [END-EDIT]
So are borates safe? It's all relative. There was no significant dermal (skin absorption) risk so it really comes down to regular drinking of pool water (one day or two means little; the studies on [EDIT] first noticeable effects [END-EDIT] were long-term daily doses). Even then, the risk is low, but if either a dog or a child were to drink significant quantities of pool water every day, then it would be better to keep them away from the pool water. The Borates do not bio-accumulate; they are naturally found in the environment and in some foods so the body does deal with them up to a point. It's higher regular doses that overwhelm the body's ability to remove it that are a problem.
[EDIT] Also note that Boric Acid is often used in eye wash solutions at 1-2% (1750 - 3500 ppm) so should not be irritating to the eye in pools. Also, the video link in the next post may not work anymore so one can use this link instead to learn how borates kill insects but at low concentrations are not a problem for humans. [END-EDIT] [EDIT] The boron content of some foods is listed in this link. Note that the Tolerable Daily Intake (TDI) in these tables is around 0.4 mg/kg/day (which is also quoted here) so is higher than the 0.088 used by the EPA for pool exposure for humans. [END-EDIT] [EDIT] For toxicity, LD50 oral toxicity (where 50% of subjects are killed) is 3450 mg/kg for male rats and >631 mg/kg for beagle dogs. Note that plain table salt has oral toxicity of 3000 mg/kg for rats so is similar to concentrated borate products. [END-EDIT]
[EDIT] Europe does not sell boron products such as borax or boric acid for consumer use in concentrated form as with Borax for laundry or boric acid as an insecticide. The reason is that the European Commission (EC) looked at essentially the same data as the EPA, but have different labeling requirements where the chronic effects data already referred to in the dog and rat studies showed developmental/reproductive issues at the observed effect levels (the EC uses 9.6 mg/kg/day as the NOAEL so no observed adverse effects). This link gives more detail and this link shows the required labeling. Interestingly, some cosmetic products may contain boron chemicals at < 5.5% concentration and are not required to have "Toxic to reproduction, category 1B" labeling. This makes no sense since it basically says that you can use the product on your skin, but you can't use a concentrated product (near 100% boric acid) that when added to pools dilutes to 50 ppm (0.005%) so is 1100 times less concentrated. Yes, there should be handling warnings for concentrated product, but its use is not in concentrated form. Unfortunately, no manufacturer that I am aware of is offering borax or boric acid to consumers because they want to avoid labeling they believe would result in virtually zero sales. [END-EDIT]