Hydrochromic inks indicate moisture level as well as solvents. These inks are used in a variety of application such as test for covid, drugs, alcohol, DNA, and certain hormones. These tests are used by state Highway Patrol departments, clinics, and health departments, and, particularly with COVID-19, for mail-in tests in which those being tested never see a healthcare provider or officer. They are also used in commercial products like soap and toys.
The inks are available as irreversible and reversible. The inks change from no color to color when wet. The ink can expose images and texts when products get wet.