It's one of those things that help your cosmetics not to go wrong too soon, aka a preservative. Its strong point is being effective against yeasts and molds, and as a nice bonus seems to be non-comedogenic as well.
It is safe in concentrations of less than 0.1% but is acutely toxic when inhaled, so it's not the proper preservative choice for aerosol formulas like hairsprays. Used at 0.1%, Iodopropynyl Butylcarbamate has an extremely low rate of skin-irritation when applied directly for 24 hours (around 0.1% of 4,883 participants) and after 48 hours that figure was 0.5%, so it counts as mild and safe unless your skin is super-duper sensitive.
Show me some proof
- Lanigan, Rebecca S. "Final report on the safety assessment of iodopropynyl butylcarbamate (IPBC)." International Journal of Toxicology 17.5_suppl (1998): 1-37.