The effects of secondhand and thirdhand tobacco smoke have been well studied in the past. By comparison, little is known about the effects of thirdhand cannabis smoke.
Now, a new study from researchers at the University of Toronto reports that ozone, an oxygen molecule present in the atmosphere, can react with the intoxicating THC compound on glass and cotton surfaces to produce new compounds, which the researchers have also characterized for the first time.
What is thirdhand smoke?
Smoking any substance, whether tobacco or cannabis, emits reactive chemical species into the air. For a period of time after smoking, these “secondhand smoke” particles will remain in the area, where they might be inhaled by non-smoking passers-by.
For tobacco, the US Centers for Disease Control and Prevention says that there is “no risk-free level of secondhand smoke exposure,” as the fumes are strongly associated with lung diseases, cancers, and strokes.
While secondhand cannabis smoke is less studied, there is early research that suggests secondhand cannabis fumes affect the cardiovascular system in a similar way to tobacco. Other studies indicate that extended secondhand smoke exposure could also result in “contact highs” and even failed drug tests.
Thirdhand smoke is where these airborne particles begin to settle and leave deposits on surfaces and clothing. And unlike secondhand smoke, thirdhand smoke can linger for a considerable amount of time after a person has stopped smoking. Touching contaminated surfaces can then lead to re-exposure to these chemicals.
Nicotine is a semi-volatile compound that can react with other chemicals on these surfaces, forming new compounds that may cause an additional risk. This is especially the case if those compounds are more volatile, and then can once again become airborne and contaminate the air. But little is known about THC’s behavior in the same conditions.
Ozone breaks down THC on glass and clothing
In this new study, the University of Toronto researchers examined how THC-coated surfaces might interact with ozone (O3), a particularly reactive form of oxygen that is produced naturally in the Earth’s upper-atmosphere.
The researchers first coated a sheet of glass and a piece of cotton cloth in a THC solution, to simulate windows and clothing. They then exposed the coated surfaces to concentrations of ozone equivalent to what would exist in indoor air.
Using a liquid chromatography–tandem mass spectrometry (LC-MS) technique, the researchers studied the THC layer under ozone exposure. They found that, over time, the concentration of THC present on both the glass and cotton cloth decreased, while the quantities of three THC oxidation products increased. These products were identified as an epoxide (THC + O), a dicarbonyl (THC +O2) and a secondary ozonide (THC + O3) product; the ratio of each was highly dependent on the humidity of the space.
In a second experiment, which used a smoking machine to deposit cannabis smoke onto cotton, the results were much the same; the three oxidation products were formed at roughly the same rate.
What does this mean in terms of health risk?
The researchers say that THC is less volatile than nicotine, as are the observed oxidation products. As a result, the THC deposits on surfaces from thirdhand smoke will have a less significant contribution to thirdhand cannabis smoke than the role that nicotine plays in thirdhand tobacco smoke. Compared to nicotine, THC and its products are much less likely to be re-emitted into the air and be inhaled.
Looking at the reaction kinetics of THC’s breakdown into the three oxidation products, the researchers say their results are also “consistent with a relatively short loss lifetime” for THC on both glass and cotton surfaces.
The researchers do caution that while THC and its oxidation products are less likely to cause problems by being emitted back into the atmosphere, exposure to thirdhand THC is still very possible in other circumstances; for example, if a person were to lick their fingers soon after touching a surface that had been contaminated by cannabis smoke.
With the health effects of thirdhand THC and its derivatives still unclear, further work is needed to evaluate the risk of such exposure.