Technically, black henna is not henna because it is not a natural plant derivative; instead, it often contains coal tar, acetone, lighter fluid, turpentine and PPD (p-Phenylenediamine). The ingredient causes allergic reactions such as dermatitis, eye irritation and tearing, but may cause more severe reactions, including asthma, renal failure and high levels of toxicity. And once you are sensitized to PPD, it becomes a lifelong issue, meaning that you may have future allergic reactions to everything from perfumes to sunscreen to printer ink.
Essentially, the FDA says henna is okay in your hair because it will not come into contact with your scalp or other parts of your skin for long periods of time, if at all.
And there are studies that show cause for concern when it comes to lawsonia inermis many claim that temporary henna tattoos cause dermatitis and other ailments. However, these studies only cited henna applied to skin as being an issue, not hair dye.