This story originally appeared on Rodale’s Organic Life in September 2017.

Where there’s smoke, there’s fire—and where there are clouds of vegetable glycerin vapor, there are probably essential oil vaporizer pens.

Often called a “personal diffuser” or “diffuser stick,” aromatherapy “vape” pens look like colorful hard plastic cigarettes and are filled with a mixture of essential oils, vegetable glycerin, and water. Inside is a heating element that’s activated by suction—when someone inhales, the mixture is heated and a white cloud of aromatherapy vapor emerges.

The vapor may look like smoke at first glance, but its composition is mostly water, and the ingredients are similar to those found in mood-setting hazes found at haunted houses, concerts, and sporting events.

Related: The 5 Most Versatile Essential Oils You Can Own—And 5 You Can Skip

Amazon reviews for diffuser sticks are overwhelmingly positive. Customers indicate that they can help combat stress and panic attacks, aid with smoking cessation, stave off menstrual cramps, and make for a great alternative to a smoke or coffee break when you just need a quick escape from the frenetic bustle of every day life. But are they safe?

The science behind essential oil vapes is murky

“The truth is, we just don’t know what’s happening to the essential oils when you heat them,” says Amy Kreydin, Certified Clinical Aromatherapy Practitioner and owner of The Barefoot Dragonfly, “Vape pens are too new for us to have that research, so we don’t know the full safety profile.”

We do know, however, that when you heat vegetable glycerin to more than 536 degrees Fahrenheit, a substance known as acrolein is produced, which is a known respiratory irritant and carcinogen.

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The FDA recently delayed regulations for e-cigarettes—but had regulatory procedures moved forward, vaporizer manufacturers would have been required to submit their products for testing. For now, until FDA testing is performed, there’s no way to be sure exactly what compounds are being inhaled once the mixture of glycerin, water, and oils is heated.

“We know the chemistry of essential oils changes when they’re heated,” Kreydin says. “The big question is, ‘What is the heat changing the oils into?’ Once we understand the chemistry changes taking place, we can look at the research and say, ‘Okay, this is no longer considered safe,’ or ‘Well, this didn’t change that much, so it’s probably not going to hurt you.’” (Here are 6 times you should never use essential oils.)

Looking for another option to relax? Watch the video to find out how to make a natural soothing calendula chamomile lotion at home:

preview for How To Make A Soothing Calendula-Chamomile Lotion

To inhale or not to inhale

Two of the biggest companies in the personal aromatherapy diffuser space, VitaStik and Monq, both offer 10 different “flavors” of sticks and boast the use of 100% organic essential oils and vaporizer blends. The companies differ, however, when it comes to the heating of the oils and the recommended use of their products.

Related: What You Need To Know About Using Essential Oils To Treat Your Acne

VitaStik, which infuses its blends with vitamins and supplements including B12, ginseng, and melatonin, recommends that consumers inhale and “breathe” the contents of the stick. (Some studies have indicated that B12 is more readily absorbed by the body when inhaled). The sticks cost around $15 each on Amazon. VitaStik devices heat up to between 110 and 200 degrees Fahrenheit—nowhere near the 536 degrees required to produce acrolein.

“There are a lot of products out there that aren’t good,” says Al Santos, founder and CEO of VitaStik. “Some of them heat up to 800 degrees and the flavors are made with propylene glycol, which can turn into formaldehyde when heated. God only knows what that’s doing to people. I’ve tested our formulas extensively and looked at them through a spectrometer, and I can tell you exactly what’s happening to them—you end up with exactly what you start with.”

Unlike VitaStik, Monq recommends that consumers don’t inhale, rather that they breathe in through the mouth without inhaling, “similar to sipping on a straw,” then exhale through the nose. Although Monq’s blends (from $20 per stick on Amazon) are safe to inhale, brand strategy manager Carlie Russell says the “mouth inhale” is recommended because it allows the oil blends to pass along the olfactory glands, giving a more enhanced aromatherapy experience than you would get if you inhaled into the lungs. Monq’s devices heat up to between 401 and 437 degrees Fahrenheit.

Related: 6 Best Essential Oils To Help You On Your Travels

In the absence of safety studies, it’s impossible to say exactly where aromatherapy pens fall on the spectrum of ‘safe for everyday use’ and ‘toxic,’ but at first glance, “essential oils seem a lot better than tobacco,” says Dr. Paul Christo, host of Aches and Gains radio show on SiriusXM and author of Aches and Gains: A Comprehensive Guide to Overcoming Pain.

“It’s unlikely that you’re putting organic oils into the vape and getting a toxic product out the other end, but it seems to me the lower the temperature, the better. The real question is: Is it helpful for people?”

Are essential oil vapes helping people?

Customers who reviewed Monq and VitaStik most frequently reported success using the diffusers to combat anxiety.

“If you’re saying, ‘I’m anxious and I feel like I could inhale one of these instead of taking a Xanax,’ sure, it seems reasonable that you’d try it,” Christo says, stressing that any tapering off of anxiety medication should be done under doctor’s supervision. “It is possible these could help people in terms of a calming effect, and help center them. But is it the placebo effect or is it the oils? And does it even matter, if it’s working?”

Related: 5 Best Essential Oils For Anxiety

If you’re looking for an aromatherapy diffuser stick, Santos says it’s important to choose one that’s completely natural: “You have to be careful—read all the ingredients on the package. Make sure it’s organic, free of all chemicals, and that the oils are made in the USA.”

Of course if Monq or VitaStik don’t sound like your cup of tea, there many other ways to relax with essential oils. Dr. Christo recommends an ultrasonic diffuser for the home, and for oils on the go, Kreydin recommends an essential oil inhaler, which is simply a hospital grade plastic container (that looks like a tube of lip balm) with a cotton wick inside that’s saturated with essential oils. (Here are 5 great essential oil diffusers you can buy.)

With these options, simply breathe in and relax. No smoke—or vapor—required.