Some skin and hair care ingredients get a bum rap, with critics raising the alarm—claiming certain compounds are absorbed through the skin and into the body where they contribute to disease. But how valid are the allegations? YouBeauty takes a look at some of the most talked-about culprits.PARABENSWhat are they?Parabens are compounds that are used in makeup, lotions and shampoo, as well as in food and medicine. They’re preservatives that help prevent bacterial and fungal growth. The most common ones you’ll see listed on cosmetic labels are methylparaben, ethylparaben, propylparaben and butylparaben.According to the Food and Drug Administration, the most common parabens are safe at concentrations up to 25 percent. That’s much higher than the levels found in many products, which typically have concentrations of less than one percent. (It’s worth noting that in 2010, the European Union lowered the legally allowable levels of butylparaben and propylparaben in cosmetic products to 0.19 percent.)MORE: Beauty Products That Are Safe and EffectiveShould I be worried?Once inside the body, parabens can mimic estrogen, the hormone that controls most female reproductive functions. Like all hormones, estrogen sends biological signals by attaching to specific receptors throughout the body. The parabens act as a hormone disruptor—attaching themselves to the same receptors and interfering with the body’s natural communication system.High levels of estrogen may stimulate the growth of some types of breast cancer, and the concern is that estrogen-like hormone disruptors may do the same. But so far, it appears that it takes a higher concentration of parabens to have the same effect as a smaller amount of naturally-produced estrogen.In 2004, a team led by Philippa Darbre, Ph.D., from the U.K.’s University of Reading, published the first study showing evidence of parabens in breast tumors. Since then, additional research has found evidence of parabens in breast milk and non-cancerous breast tissue. But there is no proven link that indicates parabens cause breast cancer, explains Darbre.Another unknown: how parabens affect the body when other hormone disruptors, such as BPA (bisphenol A), or estrogen found in birth control or hormone replacement therapy are taken into account. Individually, these compounds may be harmless, but their cumulative effect on the body is unknown.Bottom line:According to Darbre, even if parabens do contribute to breast cancer (which hasn’t been proven conclusively), it may be difficult to tease out that link amongst all the other potential environmental and genetic influences. Since we might never know for sure, says Darbre, she advises avoiding paraben-containing products. To suss them out, check for any ingredient with “paraben” as its root on the ingredient list. Those listed at the top of the list generally appear in highest concentrations, while those listed near the end have the lowest amounts. And for people who don’t want to quit them altogether, the good news is they don’t appear to bioaccumulate, meaning the body flushes them out relatively quickly.QUIZ: Are You Doing All You Can For Your Skin?PHTHALATESWhat are they? Phthalates are plasticizers that make plastics less brittle. Over a dozen different phthalates can be found in everyday items from plastic toys to food packaging, as a cosmetic ingredient to keep nail polish from chipping, hair products from being too stiff, or as a solvent in fragrances.Should I be worried?Phthalates and the molecules produced as they break down in the body have been found in human milk, urine, saliva, blood serum and amniotic fluid—although it’s not clear if they got there through skin exposure.And, just like parabens, certain phthalates are known hormone disruptors. But rather than mimicking estrogen, phthalates interfere with testosterone production—meaning the body produces far less of this important male hormone than what’s needed for normal development, explains Russ Hauser M.D., M.P.H, Sc.D., an environmental health scientist at Harvard University.There is limited research on whether phthalates have a direct risk for women, although a 2003 North Carolina State University study showed that di(2-ethylhexyl) phthalate (DEHP) disrupted estrogen production in female rats. For now, the biggest concern is whether a woman who is pregnant or nursing can pass phthalates onto a developing fetus or infant.Studies on rats have also shown that certain phthalates cause abnormalities in male development, including undescended testicles and a shortness of the distance between the anus and the genitals. However, the rats in these experiments were exposed to levels of phthalates far higher than what humans typically experience.The good news is that the phthalate most commonly found in cosmetics, diethyl phthalate (DEP) doesn’t appear to have a strong anti-testosterone effect in rats, which may mean it is also safe for humans. But riskier phthalates do show up in some cosmetic formulations. And, as with parabens, the cumulative phthalate load is a concern.“We know people are exposed to hundreds of chemicals,” says Hauser. Even if the level of one phthalate is low, he adds, the combination of that phthalate with other phthalates and other chemicals is unknown.Bottom line:Just like parabens, most phthalates are eliminated by the body within a day or so. To check for them in cosmetics, look for any chemical name that includes “phthalate” on the ingredients list. Also be aware that phthalates are common in fragrances, and the F.D.A. doesn’t require companies to list individual ingredients if a scent is proprietary.MORE: Is Your Perfume Toxic?“CHEMICAL” SUNSCREENSWhat are they?Active ingredients in sunscreens are broadly divided into two categories: physical and chemical. Chemical sunscreens, which are soaked up by skin, can include one of over two dozen compounds—many of which are based on benzophenone. On a label, they’re listed as avobenzone, ecamsule (aka mexoryl) and oxybenzone, among others. They work by absorbing UV light. Physical sunscreens, in contrast, include components such as zinc oxide and titanium dioxide that sit on skin’s surface and reflect ultraviolet (UV) light away.Should I be worried?Many chemical sunscreen ingredients are endocrine disruptors. In 2010, Margret Schlumpf, an environmental developmental toxicologist from the University of Zurich, tested the breast milk of 53 mothers for environmental chemicals and found evidence of eight chemical sunscreen ingredients known for their endocrinal activity. In other studies, the compounds have shown up in blood and in urine.To be considered a broad-spectrum formula, sunscreens must contain both UVA- and UVB-blockers. Physical sunscreens are often considered the “safer” of the two, although there is growing concern over the increasingly small size of the particles used (see “Nanoparticles” below). With no clear evidence linking chemical sunscreens to human development or health problems, the overall impact remains uncertain.Bottom line: While the jury is still out on the long-term effects of these chemical ingredients, the link between sun damage and skin cancer is well-established—making sunscreen a pivotal tool to protect oneself from harmful UV rays.NANOPARTICLESWhat are they?Nanoparticles are tiny materials that are thousands of times’ smaller than the width of a human hair. Measured in nanometers, they can be synthetic or naturally-occurring, and are often used in cosmetic products.Since the term “nanoparticles” refers to their size rather than physical or chemical characteristics, it can be difficult to define what a particular nanoparticle does. Nano-sized inorganic compounds, such as zinc oxide and titanium dioxide are the compounds most often used in sunscreens.Should I be worried?Due to their smaller size, nano-sized zinc oxide and titanium dioxide effectively absorb and reflect light. This makes them ideal sunscreen ingredients. Before the advent of nanoparticles, these same ingredients gave sunscreens a thick, white appearance, making them notoriously hard to rub in and cosmetically-inelegant. By shrinking the ingredient, a lighter, easier-wearing product was created.It isn’t clear if nanoparticles cause any damage once absorbed; especially considering both already exist in our bodies. Zinc is an essential building block for good health while titanium is commonly used in medical and dental implants. “We just don’t know yet if there’s any long-term toxic consequence,” says Lisa DeLouise, an associate professor of dermatology and biomedical engineering and the lead author of a 2008 University of Rochester study.The University of Rochester Medical Center showed these particles penetrating mice’s skin and accumulating in their hair follicles. A 2010 paper from Macquarie University in Australia showed a similar absorption of zinc oxide that appeared in urine and blood samples.Bottom line:To see if your sunscreen contains nanoparticles of titanium dioxide or zinc oxide, check the label for either ingredient. Almost all sunscreens that contain either compound contain nanomaterials, particularly if the sunscreen goes on clear instead of white. But, as already mentioned, it’s important not to skip the sunscreen, as these compounds appear to be the safest ones on the market.MORE: Don’t Skip the Sunscreen—Here’s WhyALUMINUMWhat is it?Aluminium salts, including aluminum chlorhydrate and aluminum chloride, are active ingredients in many anti-perspirants that temporarily clog sweat ducts to keep sweat from escaping. Aluminum is not a normal component of biological tissue, so any traces of it in the human body originate from the outside.Should I be worried?Experiments by Philippa Darbre, the same researcher who works with parabens, suggest aluminum salts can penetrate the skin, showing up in breast tissue, breast milk, fluid from breast cysts and blood (though the studies couldn’t identify the source). And the salts may be able to penetrate shaved skin even more readily than unshaven skin, because razors make tiny abrasions in the skin— even when they don’t cause an obvious cut.According to Darbre, aluminum may act as an endocrine disruptor by mimicking estrogen in the body, which in theory, could contribute to some types of breast cancer.Bottom line:There is no clear link between the presence of aluminum in the body and breast cancer or any other illness. For every study that shows a potential relationship between aluminum and disease, there is another that shows none.Keep in mind: While aluminum (and even parabens, phthalates, nanoparticles and chemical sunscreens) may not be hazardous on its own, consider the combined effect. Your paraben-containing shampoo may not be a big deal, but add in that birth control pill, the tofu you eat (that contains plant-based estrogens) and the BPA-containing bottle you still drink out of, not to mention canned foods and who knows what else, its effects are still murky.  “What we really need is a better overview of all the chemicals in cosmetics,” says Darbre. “To draw accurate conclusions, we’ll need to look at the bigger picture, not just one individual chemical.”