Weed is most often consumed in a joint, pipe, or bong. Those methods involve smoke inhalation and have inherent health risks. Combustion of any plant material releases toxic compounds and irritants, and marijuana is no exception. In fact, smoking an unfiltered joint deposits more tar in the lungs than an equal amount of tobacco.
Vaporizers are able to circumvent these risks by gently heating marijuana to a thermal sweet spot that releases active compounds without creating smoke. This is achieved by hitting temperatures above the boiling point of cannabinoids like THC, but below the combustion point of the plant. The result is an aromatic vapor resembling steam that theoretically contains the compounds you want without the toxic by-products normally found in smoke.
The vaporizers that people are most familiar with heat up dry pieces of plant. There's also a subtype of devices called vape pens that heat up concentrated cannabis oil or wax rather than dry leaf (Juju Joints, on page 22, fall into this category). Cannabis oils are extremely potent, containing up to 80 percent THC compared to a measly 10 to 20 percent THC in marijuana buds, and because of this, vaporizers that utilize oils deliver higher doses of THC than vaporizers that utilize dry plant. Different manufacturers of oils have different extraction processes and might use additives, so marijuana oils might also contain chemicals that pollute the vapor more than dry leaf vaporizers do, but no studies have been done yet.
Although vaping releases as much THC as smoking (or more, if using oil), many users find the high to be different, usually finding it to be subtler. The reason is chemistry. Your experience is influenced by not just the one active ingredient of THC but the entire chemical input to your brain. For example, consider alcohol. The active ingredient is always ethanol, but wine can make you sleepy, vodka can make you loud, and tequila can make you slutty. In the same way, the combination of THC and other compounds will differ between smoke and vapor, and the effects will vary by the chemical cocktail consumed.
One example is cannabidiol (CBD), the medically relevant cousin of THC. Although not psychoactive like THC, it still binds neurons and can modulate the effects produced by THC. Because CBD has a higher boiling point than THC, it's present in smoke but not in vapor produced at lower temperatures. Vapor also lacks smoke toxins that cause drowsiness, like benzene and carbon monoxide. And there's usually less vapor itself, so the feeling isn't the same as deeply inhaling smoke into your lungs. In other words, although vaping produces plenty of THC to get you high, the chemical and physical experience is different.
So is vaping safer than smoking? The short answer is that we don't know all the risks, because cannabis research is desperately lacking. However, a handful of studies exist, and their limited results do support vaping over smoking. One study found that frequent smokers report improved respiratory health after switching to vapor, suggesting it causes less airway irritation. Chemical analysis showed that vapor contains fewer compounds overall, and the majority of toxins found in smoke were vastly decreased or absent (at least in studies using the Volcano vaporizer).
Still, it's important to note that vapor composition varies depending on many factors. First, the quality of weed matters—crap in, crap out. Temperature is important, too, because more toxins are released at higher temperatures. Cheap devices can heat inaccurately and may use plastic components in the vapor path. Heating plastic can release volatile toxins, but it's unclear which toxins (if any) are released at vaping temperatures. Devices using glass parts carry less risk of introducing contaminants into the vapor, but also carry larger price tags.
So, to answer the question—yes, vaping is safer, but it's not necessarily safe. More research is needed to understand the risks associated with the vapors produced by different devices.