 The average adult uses nine personal care products each day with 126 unique chemical ingredients. Now, we used to think that anything applied to the skin would always just remain on the surface, and the only thing you had to worry about was like local skin irritation. But over recent decades, we started to recognize that some topically applied substances can penetrate into, or even through, human skin and end up circulating throughout our bodies. Take the toxic heavy metal lead, for example. To see if lead could be absorbed through the skin into the body, researchers applied lead to someone's left arm, and then they measured the lead in the sweat coming off their right arm over the next few days. There was a big spike proving nearly 30 years ago that lead can be absorbed through the skin and rapidly distributed throughout the body. This led to public health authorities to recommend parents avoid using cosmetics, at least on their children, that could be contaminated. Which cosmetics might those be? Lead has been found in a wide range of cosmetic products, because it's a natural constituent of many color pigments. The FDA has set an upper limit for lead at 20 parts per million, and though only some samples of henna exceeded that, because henna is used for temporary tattoos, these quantities of lead can remain on the skin for a long time and may not be safe. This study showed that lead may have no identifiable safe exposure level, with even the lowest levels shown to affect the brains of developing children. Thus, the use of henna, especially among children, may constitute a public health risk. So increasing awareness of henna's serious toxic implications may help end or at least reduce the use of such hazardous material, especially when children are involved. Now traditionally, henna was just the dried powdered leaves of some plant, but more recently other ingredients have been added to give it a stronger color, added ingredients such as lead, one of the most common and egregious additives in henna. But not as common as PPD, parafinaline diamine. The red paste traditionally used, known as red henna, rarely produces adverse effects, but to achieve a darker pigment, known as black henna, these additives may be used, including animal urine, but better pee than PPD, a cold tar derivative that can cause nasty skin reactions such as blistering and scarring. PPD has added to speed up the process from as long as 12 hours down to less than 2 hours. So while the use of black henna is maybe tempting, it has the potential for both short and long-term side effects. How common are these reactions? The best estimate is about 2.5%, so one in 40 kids that gets a black henna tattoo may have an allergic reaction. Unfortunately, this practice has become fashionable, thanks a lot, Spice Girls. There's no such thing as natural black henna, so perhaps it would be best to respect the more traditional practices lest a temporary tattoo turn into a permanent scar. The problem is that PPD can be found in products labeled as red henna, too, which is because it's red, doesn't mean it's not risky. Bad news for the $100 million industry, because henna of all colors is so often adulterated, under FDA guidelines henna shouldn't be applied to the skin at all.