Are e-cigarettes dangerous? The answer at this point is that science can't really say. That hasn't stopped some areas from taking preemptive measures to curtail health risks that might be posed by the increasing prevalence of these smoking devices.

On Tuesday, Los Angeles joined a growing list of cities that bans the use of e-cigarettes in public spaces, including parks, restaurants and workplaces, according to the Los Angeles Times. Over the past few years, e-cigarettes have grown in popularity, leading some public health officials to say that years of investment to educate the public on the dangers of tobacco will be undermined.

Although not explicitly marketed as such, e-cigarettes are generally considered a cleaner, healthier way of consuming tobacco products than their fire-induced alternatives. But if e-cigarettes are proven to pose a health hazard, this wouldn't be the first time that a product widely considered safe proves to be anything but.

SEE ALSO: 5 Must-Knows About E-Cigarettes

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Flame retardants protect against fire, but these synthetic chemicals can pose long-term health risks when they are ingested and travel through the bloodstream. Studies on these compounds, polybrominated diphenyl ethers (PBDEs), have linked them to health concerns, including various types of cancers, hormone disruption, brain development issues that affect memory and learning, and more.

In 2011, a study published in the American Chemical Society's (ACS) journal Environmental Science & Technology detected potentially toxic compounds associated with PBDEs in 80 percent of 101 baby products, as well as two other potentially carcinogenic chemicals. The products, which included mattresses, nursing pillows, strollers and more, used compounds banned in 172 countries and 12 U.S. states. Aside from baby products, flame retardants can also be found in household items such as furniture and electronics.

Such chemicals live not only in the products themselves, but also travel in household dust. A separate study published in 2012 in the same journal, and led by scientists at Silent Spring Institute, found 49 flame retardant chemicals in household dust.

What's worse is that some of these chemicals can actually make fire worse by spreading dangerous gasses in the air when exposed to flames, according to another study presented at an ACS conference in 2012.

SEE ALSO: Flame Retardants: Do We Need to Worry?

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Bisphenol A (BPA) is a chemical additive commonly found in resins and plastics, such as water bottles or food containers. It can also be found in household electronics, medical devices, dental fillings and sales receipts, just to name a few other applications. Over time, particularly with food containers that are reused, the BPA leaches out of the plastic and into the product itself.

Given that BPA is so widely available, particularly in food products, it should come as no surprise that just about everyone has traces of it in their bloodstreams. A study published 10 years ago by the Centers for Disease Control and Prevention (CDC) found BPA in 93 percent of urine samples from participants six years of age and older.

Although low levels of BPA exposure in humans have not been demonstrated to have negative consequences, in laboratory experiments, BPA has been linked with heart disease, various kinds of cancer, hormone disruption, behavioral disorders and more. A study published just last month, in fact, found that very low levels of BPA could affect organ development in primates.

SEE ALSO: Canned Foods Seal in BPA

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Now, even though we just finished explaining that BPA may pose health risks, it's worth noting that studies have shown the alternatives to the much maligned compound might not be any better.

In its latest issue, Mother Jones highlighted a 2011 study funded by the National Institutes of Health (NIH) that found that BPA-free products may even be worse than those containing BPA. Nearly all commercially available plastics sampled by the study's authors leached synthetic estrogen over time, even when exposed to conditions that shouldn't trigger the chemicals' release, such as heat from microwaves or dishwashers.

SEE ALSO: Are BPA-Free Plastics Just As Bad?

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Microbeads used in cosmetics and other products might be good for your skin, but they aren't doing the environment any favors.

After those tiny plastic beads, around 5 millimeters or smaller in diameter, make their way down your drain, they make their way into lakes and oceans. They are then ingested by fish and other marine life.

Personal care companies including Johnson & Johnson, Unilever and others have committed to replacing plastic microbeads with biodegradable alternatives in their product lines by 2015.

Consumer Product Safety Commission

Few people would ever consider window blinds a safety concern, but a sweeping recall issued jointly by the U.S. Consumer Product Safety Commission (CPSC) and the Window Covering Safety Council (WCSC) in 2009 shed light on the dangers.

Roman shades and certain roll-up blinds were responsible for five deaths and 16 near strangulations of children since 2006, and three deaths since 2001, respectively, as of the date of the joint release. A child could get his or her neck caught on the inner cord of Roman shades or loops of roll-up blinds. Tens of millions of blinds were recalled, and industry representatives offered free repair kits for anyone with either style window shades in their households.

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Prior to 2004, the anti-arthritis drug Vioxx had been used by some 84 million people worldwide, generating over $2 billion of revenue annual for drugmaker Merck.

Starting in 2000, reports surfaced that the drug could pose a danger to patients with heart problems during one study that explored whether Vioxx could prevent issues such as colon polyps, according to an article by National Public Radio (NPR). A study conducted by Merck in 2004 found that Vioxx increased risks of heart attack and strokes in patients susceptible to cardiovascular disease in those who had taken it for 18 months or more. Merck voluntarily recalled global stocks of the drug and three years later settled a class-action lawsuit to the tune of $4.85 billion for those adversely affected by the drug.

SEE ALSO: Deaths from Prescription Drugs Rising

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Thalidomide is a drug that has been used to treat a number of diseases and conditions, including cancer and leprosy. One of the earliest uses of the drug, however, alleviating symptoms of morning sickness in pregnant women, proved remarkably unsafe.

Pregnant women who first took the drug for this purpose in the late 1950s gave birth to children with malformed or missing limbs. Most did not survive. Today, NIH strictly warns pregnant women against using the drug, which can cause "severe, life-threatening birth defects."

Lori Cuthbert

Atrazine is a chemical most people haven't heard of, even though the compound is the most commonly used herbicide in the country. This weedkiller is used in fields growing corn, wheat, sugarcane and more. Aside from large-scale agricultural operations, atrazine is also used on residential lawns and golf courses, according to the Environmental Protection Agency (EPA).

The EPA has in fact been criticized for not recognizing the negative health effects of atrazine, particularly as it transfers from crops into the groundwater. Last month, the New Yorker published an extensive profile of the work of Tyrone Hayes, a University of California, Berkeley, biologist, a researcher who went from working for agribusiness manufacturing atrazine, Syngenta, to being one of the chemical's leading critics.

Over years of conducting experiments on amphibians, Hayes found that atrazine-tainted water led to abnormalities in the sexual development of frogs. Industry-funded researchers, predictably, were unable to duplicate Hayes's results, and a concerted public relations effort was launched to smear Hayes and discredit his findings, a campaign brought to light as a result of a class-action lawsuit that revealed internal emails, memos and more from Syngenta employees.

Other studies of atrazine, including a meta-analysis compiled by an international team of scientists in 2011, reached similar conclusions as Hayes using other creatures as well, including reptiles, mammals and fish.

SEE ALSO: Testes Hit by Herbicides

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It might be difficult to believe today, but for much of its history, tobacco was considered not only safe and desirable to consume, but also possessed health benefits.

When Christopher Columbus voyaged to the New World, he encountered tobacco and immediately extolled the supposed virtues of smoking the plant. Since Columbus's arrival through the late 15th century until the 20th century, Western medicine enlisted tobacco for pharmalogical purposes. The cigarette ad in this photo, for example, is from the late 19th century. These particular cigarettes are advertised to asthma sufferers, those over six years old anyway. In the 20th century, tobacco companies would feature doctors in their ads, providing an often explicit endorsement of cigarettes as a health product.

Although smoking and cancer were first linked in the 19th century, the scientific evidence to back up the connection wasn't produced until the 20th century. Tobacco companies did what they could to suppress evidence that smoking can lead to lung cancer and other respiratory illness, a fact they well knew beginning in the 1950s. Waves of lawsuits followed revelations of the industry's efforts, leading to the largest legal settlement in history.

However, despite cigarette warning labels, education and public health campaigns, tobacco use remains the number one preventable cause of death in the world today.

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Opium was the original cure-all. In fact, many painkillers available by prescription today are opiates. But when abused, the drug does far more harm than good.

The earliest usage of opium traces back to 3,400 B.C., when the opium poppy was cultivated in lower Mesopotamia. Spreading along the Silk Road, opium spread across the Mediterranean and into Asia. Hippocrates, known as the father of medicine, used opium regularly in his practice, citing its sleep-inducing qualities and other health benefits. Opium was a kind of cure-all starting in ancient times, and in fact, in some parts of the world, specifically Afghanistan, still is today.

In the 19th century, one of the earliest works documenting opium addiction, an autobiography by Thomas De Quincey entitled "Confessions of an Opium Eater," was published in an era when opium consumption for recreational and medicinal use was at all-time high. Decades later, China would go to war with the British empire over the opium trade.

The dangers of addiction to opium, as well as its derivatives, morphine and heroin, was widely recognized, and in the early 20th century, these drugs are either controlled or outlawed. Heroin has seen a resurgence in the 21st century, a trend highlighted by the recent death of actor Philip Seymour Hoffman.