Hi Joy,
There are ways to color gray hair without damage to the hair. First of all, never retint hair that is already tinted. Only apply color to the hair that has not been tinted before - the new-growth. Sometimes you can refresh the color by diluting it and applying it to the previously tinted hair for no more than ten minutes.
This article explains how to retouch new-growth.
Retouch If you are worried about damage to your health from hair color, use a lighter color on your hair. The lighter colors contain less dye stock - less of the ingredients that are thought to be harmful.