Nonpyrogenic means the product has been tested for bacterial endotoxins and found within the pharmacopeial limit, which for bacteriostatic water is not more than 0.5 endotoxin units per mL. The word comes from pyrogen, the general term for substances that provoke a temperature response in a test system; in practice the pyrogen of concern in water products is endotoxin, the lipopolysaccharide of Gram-negative bacterial cell walls.
Why sterile and nonpyrogenic are two different claims
Sterility means no viable organisms. Endotoxin is a molecule, not an organism: it is left behind when Gram-negative bacteria die, it survives autoclaving, and it passes through sterilizing filters. Water can therefore be perfectly sterile and still fail the endotoxin test if bacteria grew in the water system at some point before sterilization. Manufacturers control endotoxin by keeping the water system free of Gram-negative growth in the first place, and they prove it by testing each lot. See sterile vs. nonpyrogenic.
How it is tested
The bacterial endotoxins test (USP <85>) uses Limulus amebocyte lysate, or a recombinant equivalent, which reacts to endotoxin at very low concentrations. The certificate of analysis usually reports the result as less than a threshold, for example <0.25 EU/mL, meaning no endotoxin was detected at the sensitivity of the method. See what is an endotoxin unit? and endotoxin testing: the LAL test.
What it means for handling
The claim applies to the contents of the sealed vial. Once a vial is open, anything introduced into it can carry endotoxin, and the preservative does nothing about that: benzyl alcohol acts on living bacteria, not on the molecules they leave behind. Clean technique at each withdrawal and discarding the vial 28 days after opening are what keep the contents in the state the label describes.
Related
Is bacteriostatic water sterile? · How to read a COA
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