Science & tech

The Physics and Chemistry of the Joint

Published on March 5, 2016 · Last updated July 28, 2020

When a joint gets passed around, it tends to bring out the armchair scientists.

Everyone has a theory. And most theories sound like they come straight from the mind of Ron Slater, Dazed and Confused’s stoner historian. There’s the temperature theorist, who’s convinced you’ve got to keep the joint hot. And the long-toke artist. And the many-short-hits believer.

Who’s right? Let’s look into the actual science of the joint, or as the peer-reviewed journals refer to it, the marijuana cigarette.

Why Does Cannabis Need to Be Burned?

First, a primer about why cannabis is burned and smoked in the first place. Eating a gram of cured flower straight out of the bag is a bad idea. It’ll taste like eating Kentucky bluegrass, and you won’t get the desired effect. The THC in the plant needs to undergo a process known as decarboxylation to become psychoactively available. Ed Rosenthal, one of the world’s leading experts on cannabis biology, explains the rest in this excerpt from one of his columns:

Marijuana produces THCA, an acid with the carboxylic group (COOH) attached. In its acid form, THC is not very active. It is only when the carboxyl group is removed that THC becomes psychoactive. When marijuana is smoked, the THC behind the hot spot is vaporized as the hot air from the burn is drawn through the joint or pipe bowl to the unburned material.

Decarboxylation guide: How to properly decarb weed

How Much THC Moves From Leaf to Bloodstream?

Budding cannabis flower on live plan in outdoor field

One of the earliest NIDA (National Institute on Drug Abuse) studies on cannabis cigarettes, conducted in 1982 by NIDA researcher Richard L. Hawks, estimated that 20 percent of the THC in a cannabis cigarette was delivered to the body when the smoker took a 5-second puff each minute. All the rest was lost to pyrolysis (burning) and sidestream smoke (the rising stuff from the smoldering end).

A later 1990 study by Mario Perez-Reyes, a psychiatric researcher at the University of North Carolina, put more specific figures to the path taken by THC. He estimated that 20 to 37 percent of the THC in a joint hits the consumer in mainstream smoke. Twenty-three to 30 percent is lost to pyrolytic destruction, and 40 to 50 percent goes up in sidestream smoke.

In these early studies, the scientific concern was all about THC. Other cannabinoids, like cannabidiol (CBD), and terpenes weren’t yet widely known. Also worth noting: All of these American studies were conducted using low-quality, low-potency (1.5 to 3 percent THC) cannabis supplied by NIDA.

Those estimates allow us to run some interesting numbers. If the average joint contains about 700 milligrams of cannabis flower—that’s the “scientific test joint” configuration—and today’s average THC level runs around 20 percent, that means 140 mg of THC are available in each joint. If 20 to 37 percent of that carries to the lungs, that’s a THC dose of 28 to 52 mg. Before you start comparing that to THC milligrams in edibles, though, consider that the body metabolizes and reacts to edibles differently than it does to inhaled smoke.

More Short Puffs, or Fewer Long Draws?

Woman puffs on cannabis joint

A 2008 study conducted by researchers at Leiden University, in the Netherlands, using much better cannabis (17.4 percent THC) supplied by Bedrocan, the company that grows pharmacy-grade cannabis for the Dutch Ministry of Health, specifically tested the toke question. Using joints with 700 mg of flower, volunteers tried a puff every which way. They took a two-second pull every 15 seconds, then every 30 seconds, then every 60. They tried a two-second pull, a three-second pull, and a four. Then the researchers drew blood from the subjects and measured their plasma THC levels. THC levels in the blood stair-stepped, as expected, in nearly every case. In other words, a longer toke drew more THC into the blood. A greater volume of inhaled smoke did the same.

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But here’s the interesting thing. The short, two-second puff every 30 seconds and every 60 seconds yielded about the same amount of THC, around 22 nanograms per milliliter. But the same puff every 15 seconds doubled the THC intake, to 44 ng/ml.

The conclusion: The average overall temperature of the joint remained higher when a toke was taken every 15 seconds. That kept the whole THC decarboxylation and delivery system up and running. When the joint was allowed to rest for 30 or 60 seconds, it cooled. It’s the difference between keeping a machine running and or shutting it down and starting it back up again.

Plus, as a number of these study authors noted, cannabis cigarettes don’t burn nearly as evenly or well as tobacco cigarettes. If you leave them untended for too long, they have a tendency to extinguish themselves.

Perez-Reyes observed a similar dynamic during his 1990 study. He asked study subjects to smoke joints extremely fast—a hit every six seconds—and then more slowly, taking a drag every 17 seconds. Which is still pretty fast. And his subjects got really high. The six-second-interval smokers registered peak THC blood plasma levels of 210 to 230 ng/ml. The 17-second-interval smokers hit 100 to 160 ng/ml. The legal limit for impaired driving in both Washington and Colorado is 5 ng/mL.

Delivery Efficiency: Joint vs. Vape vs. Bong

Pile of rolled cannabis joints on blue table

Nobody’s actually done an apples-to-apples-to-apples study on the question, or at least not one that’s been published in a peer-reviewed journal. But there is some information to be gleaned.

In 2007, Donald Abrams, a pioneering AIDS and medical cannabis researcher at the University of California at San Francisco, published a study of THC intake via the Volcano vaporizer. Abrams tested the vaporizer as a safer alternative to cannabis cigarettes. He was responding to a 1999 Institute of Medicine Report that found medical value in cannabis but hedged against recommending medical marijuana “because of the health risks associated with smoking.”

Abrams did find vaporization to be healthier. Compared to a smoked joint, the Volcano produced far less tar, carbon monoxide, and other combustion byproducts while delivering almost identical blood-THC levels. The vaporizer captured 54 percent of the THC in the leaf, as compared to the 20 to 37 percent available from a joint.

Bongs, by comparison, may deliver less THC per gram of flower. Perez-Reyes found that peak blood THC levels among his subjects using a water pipe were about 50 percent lower than the blood THC levels among the same subjects smoking the same amount of cannabis in a joint. That finding may lend credence to those who wonder if bong water is filtering out some of the cannabinoids that consumers desire.

Self-Titration Is a Real Thing

Woman holding cannabis joint and while smoke emits after she inhales

For UCSF’s Donald Abrams, the most surprising data from his 2007 study may have come in the area of titration—a factor involving concentrations of THC in the blood (more later). He asked his subject to consume three different potencies: 1.7 percent THC, 3.4 percent, and 6.8 percent. Under perfect conditions, the blood-THC levels of the subjects should have stair-stepped along with the increased potencies.

Surprise! They didn’t.

How long does weed stay in your system?

Smoking the 1.7 percent THC cannabis, his volunteers peaked at blood THC levels of 80 ng/mL. At double the leaf potency (3.4 percent THC), they peaked at 110 ng/mL. And at four times the potency (6.8 percent), they peaked at 120 ng/mL.

Even though the Volcano captured a higher percentage of THC compared to a joint, blood plasma THC levels in the subjects using those devices were comparable.

Here’s the really interesting part: Subjects in Abrams’ study didn’t know the THC content when they were consuming.

That suggests that the study subjects carried out some sort of self-titration, whether they were aware of it or not. Titration is a fancy word for dosing. Self-titration means smokers adapt their smoking behavior to obtain desired levels of THC from the particular delivery system, taking more puffs and/or inhaling more efficiently at lower, compared to higher, THC strengths.

“The phenomenon of self-titration of psychoactive drug intake from an inhaled delivery system is well documented for nicotine from cigarette smoking,” Abrams wrote, “but to our knowledge has not been previously reported for marijuana.”

Abrams’ study has interesting policy implications as well. One of the arguments used against recreational legalization is the fear that today’s higher-THC cannabis “is not the pot you knew in the 1970s.” That’s true. But it may also be true that consumers are simply inhaling less smoke or vapor than they did in the ’70s to achieve similar results.

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Bruce Barcott
Bruce Barcott
Leafly Senior Editor Bruce Barcott oversees news, investigations, and feature projects. He is a Guggenheim Fellow and author of Weed the People: The Future of Legal Marijuana in America.
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