The "h" in the chemical formula 194 ng.h/mL stands for hour.

This formula likely represents a concentration rate – the amount of a substance (measured in nanograms, ng) that changes over a certain time period (measured in hours, h) per volume (measured in milliliters, mL).

Example:

* 194 ng.h/mL could mean that 194 nanograms of a substance are released into a solution every hour per milliliter of solution.