The short answer is, the solvents in nail polish are very hostile to microbial survival. The NMC (Nail Manufacturers Council) was asked this question years ago by a regulatory agency, and in response, we contracted with an independent lab to perform a study. The study found that when nail polish was deliberately contaminated with microbes in a lab test, the microbes quickly died off.
For a more “real-world” test, we visited several salons and purchased back a large number of half-used nail polish bottles — which clearly had been used on numerous customers — and sent them to the lab for testing. In all cases, the used bottles from the salons had no detectable microorganisms.
As a result of these findings, regulatory agencies have continued to allow an exception to the “no double-dipping” rule for nail polish.
You can view the research project, titled Investigation of the Potential for Microbial Contamination in Nail Polish at www.probeauty.org/nmc.
— Paul Bryson is principal scientist, basic and long-term research, for OPI (www.opi.com).