HOW DO HUMIDITY INDICATOR CARDS WORK? Humidity indicator cards are impregnated with a solution of cobalt chloride that has been carefully mixed to change color from blue through various shades of magenta - red, to a pink. ... enjoy a much longer and more reliable life if they are maintained at a very low relative humidity.
Are humidity indicator cards reusable?
The humidity card itself is reusable. When the humidity level is low, the dots will change from pink to blue again. With long use, the color of the dots may faint. Then, you can replace the card.