This question originally appeared on Quora.
Answer by Matt Harbowy, Data Wrangler, former Ph.D. student, Chemistry, Cornell University, 1991-1993
They're not, except when they are. See also, "Is Sodium Lauryl Sulphate Harmful?")
That's not intended as a flip answer, because there are so many confounding factors. When people mention an entire class of compounds, they are typically using shorthand to describe specific chemicals that have a history of how they were made. Carrageenan, for example, is just a highly purified form of compound found in algae ("Irish Moss") and might be more cost effective to make rather than grow and harvest. Either way, there's a legacy to both: Natural carrageenan might have been grown in waters contaminated by heavy metals or radioactive substances, and synthetic or bioengineered carrageenan might have residues of the process by which it was made. In both cases, there are typically strict controls on the use of food and cosmetic additives to help prevent random acts of bioterrorism.
If you do a search on carrageenan, you'll find thousands of sites devoted to the horrible digestive problems "caused" by carrageenan, and you'd probably think that it is made in a vat by a company like Monsanto rather than harvested from the rocks of the Irish coast. (for example: "Is Carrageenan Safe? -- Ask Dr. Weil".) Agar is another "non nutritive" carbohydrate derived from seaweed and digested by intestinal bacteria, but is consumed harmlessly and is a popular, natural, vegan ingredient. (I don't know, but there might be Agar websites devoted to how vegan gelatin will kill us all.) So how does one know, simply by applying a technical synonym or class name of a compound, which ingredients are safe and which ingredient is going to cause me to die a slow, horrible death?
You don't!
What is perfectly OK for someone to eat may be deadly for another person. Aspartame and other similar chemicals, both natural and man-made, are deadly to someone who has the disease PKU (Phenylketonuria). Most people are perfectly capable of making their own Tyrosine, an amino acid that is used by our bodies to make proteins and is found in abundance in cheese. But for people with PKU, they lack an enzyme that makes tyrosine from phenylalanine, and phenylalanine accumulates as phenylpyruvate instead, causing the person to suffer increasing mental retardation while they might be innocently consuming foods high in phenylalanine that their parents, siblings, and friends can eat without problems.
The things that cause us to slide into disease tend to get highlighted, but even things you need, like glucose and oxygen, can cause your body to go off the rails, and everybody is different. Some things, like roasted potatoes or grilled peppers, are perfectly fine to eat even though laden with poisons and carcinogens because most people are capable of digesting and excreting the poisons those foods bring. You eat them in moderation, so you lower your chances of dying of anthracene-induced cancer or suffering painful kidney stones, or even nightshade poisoning. That which makes life enjoyable might also be killing us, but being an ascetic is not the solution, since very few average ascetics live long and happy lives beyond what is average for everyone else.
So how do I make rational choices?
Best thing is experience. When choosing a cosmetic, is it produced by a business with a long reputation of testing for safety? Do you have many friends and family members who use it without irritation or problems? Get the advice of others that you trust, and who don't have a reputation for gossiping about the latest deadly chemical. You'd be surprised, but listening to your own body and what it is telling you is a highly underrated skill. I am horrified when I hear stories of women using cosmetics or other goods because they want to conform to a particular beauty craze or fashion trend and wind up sick or disfigured 10, 20, even 50 years later. Every day as a young woman, my grandmother wore "shoes that hurt" because "they looked good," and wound up crippled as an old lady due to bad feet. And just because something is "natural" or "earth friendly" doesn't mean it isn't a dangerous fad with unintended consequences. Trusting that something is "kosher" or "vegan" does not mean "safe for everybody at any speed."
At one time, soaps were made from animal fat and lye. Likewise, you can use some vegetable-based oils and aggressive alkalis to make vegan soaps. People who were born before 1950 or so remember that folk wisdom was that the burning, reddening of the skin meant you were doing a good job with cleaning, but in reality, they were likely being burned by traces of the harsh chemicals used in their manufacture, and abrading skin and creating open sore wounds with the grit in e.g. pumice soaps. You don't have to be Howard Hughes level OCD to use perfectly safe, occasional items in a way that is seriously detrimental to your health. Listen to what your body says, and if you develop reddening, rashes, or other unexplained phenomena, stop using it and see if it goes away. Science works - do an experiment. Slowly introduce it back - do your symptoms come back? At what level of use? Don't overdo things, and don't follow fads and fashions blindly.
Finally, learn something every day. Having an open mind and a questioning, skeptical attitude (in appropriate doses) lets you see when you are right, and helps guide you when you are wrong. Question yourself harder than anyone else, because the easiest person to fool is yourself.
So, let's talk about the chemical classes discussed.
Parabens:
Wow, looks nasty. But then again, lets look at cinnamic acid, ubiquitous in nature, in almost all plants, and eponymously named for cinnamon:
So how do you decide between these? Well, some people can lie comfortably on grass, and some people are allergic. Is it the gallates, tannins, or cinnamates? Probably not. But, you have to listen to your own body and trust that almost nobody intends to harm you deliberately. If you are feeling unsure, write a nice letter or email to the manufacturer - something might have gone horribly wrong, and the only way the people selling you the cosmetic can know there is a problem is if enough people allow them to correlate incidents with particular batches or lots.
A quick story: the chemical above is a nasty pesticide called 2,4-D. It's not something you would leave out next to the pet dish - it's a poison, but at low levels it is a selective poison that is useful for no-till weed control. In the 1950s, the US Government asked Monsanto to make this and 2,4,5-T and called it agent orange. When made properly and used sparingly, it is powerful. However, if you don't watch the temperature that you make these compounds, and let a little "get burnt," you form the deadly dioxin 2,3,7,8-TCDD
Reputable companies don't want to poison you, and listen very carefully to their call in centers to determine if there are safety issues. Companies that respond quickly and aggressively are your best ally. There's a long chain of supply on almost everything you eat, drink, and wear, and the number of ways that it can go "horribly wrong" is infinite. Following the advice above might have made the problems with agent orange so much less damaging, even above and beyond the advice "don't wage chemical war."
Sulfates.
Boy, this is a hard one to talk about. Sulfate of what? Sulfate is part of the sulfur-cycle of microorganisms, including ones that turn carbon dioxide into sugar and oxygen. It's also the salt of sulfuric acid, a nasty oxidizing acid. But just like sodium chloride, the salt of two deadly poisons, is much less of a poison and unlikely to kill you at levels that either sodium and chlorine would, sulfate salts are common throughout the world.
Things like the mineral gypsum (calcium sulfate), and other Sulfate minerals
But sulfate is also a component, a side chain, of a number of organic salts that are common detergents and cleaning agents - if you buy cheap shampoo or dish cleaning detergent, you may have seen Sodium dodecyl sulfate (aka sodium lauryl sulfate) or Sodium laureth sulfate. But to hear certain sites (e.g. Sodium Lauryl Sulfate: Facts Versus Fairytales) refer to these compounds, you would think they are deadly poisons: and they are! at very, very high levels. Rub some natural soap in your eyes, or lemon, and you know intuitively that anything at the right level and in the wrong application can be problematic. There's no reason to overdo these things, but mindlessly calling them unsafe at any speed seems overly harsh. Use them sparingly, and if you have problems, discontinue using them. But the overwhelming majority of people should have no problems with these chemicals.
Even uranium has a sulfate: Uranium sulfate. Don't write off an entire family of compounds just because of one bad actor.
Think about things like methanol and ethanol. Methanol is a dangerous poison, causing blindness. Ethanol is ... also a poison, but the kids seem to like drinking it, and many live to tell the tale. There are both methyl and ethyl parabens, and likewise, there are many different compounds where a single carbon is the difference between safe and poison. Ethylene glycol is a sweet tasting antifreeze that is deadly, causing liver failure. This poison was added to cough syrup, "Elixir sulfanilamide" for children in the 1920's because they wanted to make it sweet and palatable, not realizing that it will kill them. The existence of the FDA is a direct result of elixir sulfanilamide, leading (among other horror stories) to the 1928 Federal Food, Drug, and Cosmetic Act. And yet, one more carbon and you have Propylene glycol, which is probably as safe as water, slightly sweet, and used ubiquitously. Dr Mercola and his kin will denigrate things the FDA calls safe, but the FDA was created to protect you from the very kind of dangers he is claiming. That's not to say people don't make mistakes. They do - and it's tragic when a mistake results in lives lost. But to ignore all the hard work that goes into keeping people safe against the "free market" of the 1920's is sadly often forgotten to time.
Finally, phthalates.
I really don't know how to feel about this. I don't think it's as simple as "BPA bottles are the devil." Just about any chemical, including plutonium, can be detected in the body, but even zero levels of plutonium don't mean that you aren't going to die of cancer. People have been dying of cancer for millenia.
IF I had to guess, I suspect that there's a trace contaminant like dioxin that is super poisonous, hard to detect, and present in only some batches of phthalates, because they look a lot like (the highly dangerous, chlorinated pesticides mentioned above) other things that form dioxins (NB: dioxins also form in nature. See: Dioxin Controversy - Industry Responses - dioxin is natural). But almost universally, what is talked about is the chemicals' estrogenic effect, on the idea that anything with two alcohol (OH) side groups a similar distance apart on either side will act "just like estrogen"
Genistein
Estrogen (estradiol)
People have been going on and on about phytoestrogens like Genistein for decades, being a natural component of a number of beans including soy. I've heard that its estrogenic effects are responsible for why there's such a low incidence of breast cancer in Japan, and I've heard that it's responsible for early onset of puberty in girls due to GMO soy. Both claims are likely bullshit, or the difference in one out of millions of people.
There's probably something to the whole phthalate controversy, which is why chemical suppliers and bottle manufactures have been sharply decreasing their use in baby toys and bottle nipples. But I doubt we know why, because it is often easier (and less costly) to irrationally overreact than understand and act appropriately.
Again, very few people set out with the intent of poisoning babies, and it's often a tragedy when it happens accidentally. The twentieth century has seen a dramatic increase in lifespan, largely correlated with and attributed to the move from an agrarian to a manufacturing economy. This is well documented by farmers such as Paul Conkin (http://www.amazon.com/Revolution...) and the tremendous lifesaving work of e.g. Norman Borlaug and his green revolution. We understand chemicals better than we did 100 years ago, and we will understand chemicals better 100 years hence. Go slow, be wise, and always in moderation. Replacing nearly unbreakable phthalate bottles with breakable glass or dentable, leaching metal certainly is no complete solution.