Thanks for the query.The cause could be multifactorial and needs to be assessed. It could be genetic, environmental, constitutional or some nutritional deficiencies. While the treatment options are limited, an attempt to prevent further graying can definitely be made. Hope it helps.
Next Steps
PPD is not a problem if you are not reacting to it, There are certain group of people who do not tolerate the PPD component of hair dye and develop allergic reactions, They then need a PPD free hair dye. You can continue either.