Phosporous Acid, the active ingredient in Exel & Agri-Fos, has been in use for at least 25 years & is also considered very safe.
http://www.epa.gov/pesticides/biopes...eet_076416.htm
http://www.epa.gov/pesticides/biopes...nvironmentrisk
http://www.epa.gov/oppbppd1/biopesti...eet_076002.htm
Test results indicate that the compound is practically nontoxic to birds and freshwater fish, and, at most, slightly toxic to aquatic invertebrates. Low toxicity, the proposed rate of application, and mitigating label language present minimal to nonexistent risk to wildlife.
...results from mammalian acute and subchronic toxicity studies indicate lack of toxicity, adding further weight to the lack of risk from exposure
Given the low toxicity of mono- and di- potassium salts of phosphorous acid as indicated by toxicity data, and a history of safe use in other parts of the world, a determination of reasonable certainty of no harm for the general population, as well as subgroups including infants and children, was made.
...a lack of reports of toxicity or allergic reactions from 12 years of use in other parts of the world...
This was issued in 1998, that means it's been safely used for a quarter of a century.
Investigating the tautomer (read phosphonic acid) reveals that it's been tested on a wide range of animals since at least the mid 1970's & was found to be non toxic even at massive sustained doses.
The doses used would scale to a 175lb male ingesting an ounce every day for 90 days.
http://www.epa.gov/IRIS/subst/0530.htm