One common concern relating to drug testing is that exposure to secondhand cannabis smoke can cause a positive test result for THC. Many people believe that the recent rise in cannabis potency makes it more likely for false positives to occur.
There are many factors that can go into drug test results and potential intoxication from secondhand cannabis. Such variables include the amount of smoke in the surrounding area. As well as the amount of airflow in the area, or the lack of, such as hotboxing a car.
Perhaps the most important variable is the amount of smoke to which a non-smoking person would be subjected to.
Passive smoke exposure
Secondhand smoke, or “passive exposure” to marijuana produces THC levels that are much different than those produced under active exposure. When a weed smoker exhales, very low levels of THC are released back into the air.
This makes it extremely unlikely under normal circumstances for a non-smoker to inhale enough THC for a drug test to turn positive.
Extreme smoke exposure
In extreme cases, positive test results from passive exposure are possible but still unlikely. One study concluded that with an extreme lack of ventilation it is possible to test positive for THC from exposure to secondhand smoke immediately after exposure and “only under environmental circumstances where exposure is obvious.”
A second study placed both smokers and non-smokers together in a smaller environment: a vehicle. The results showed that when collected properly with a waiting period before collection, the risk of a false positive THC test was “virtually eliminated”.
The science of secondhand smoke
One study, conducted by researchers at the Johns Hopkins University School of Medicine, paired several regular pot smokers with nonsmokers and put them in a sealed compartment together for an hour, while one smoked a joint containing a relatively strong strain of cannabis.
The 12 nonsmoking participants were then tasked with peeing into a cup 13 times over the next 34 hours. Their urine was tested for 9-carboxy-THC, the marijuana metabolite commonly measured in standard drug tests.
The results give nonsmokers with weed-using friends reason to breathe easy. The scientists found that urine levels of this metabolite surpassed typically detectable levels (50 nanograms per milliliter) in only one experiment participant, and this happened during a brief window four to six hours after exposure.
Using a more sensitive test, however, which is not usually employed in the workplace, scientists could detect blood THC levels above the 20 nanograms per milliliter in several participants in the hours after exposure. But these concentrations dipped below this threshold for all participants within 24 hours, according to the study.
Positive tests are “likely to be rare” from secondhand smoke, the authors concluded, “limited to the hours immediately post-exposure, and occurring only under environmental circumstances where exposure is obvious.” Like, for example, sealing yourself in a car with several smokers for several hours and then peeing in a cup shortly thereafter.
When researchers ventilated the smoking chamber, thus making the smoke fumes less concentrated, the urine levels of THC’s metabolite did not come close to reaching the 50 nanograms per milliliter threshold for any participant.
Is there such a thing as a contact high?
Being near an abundant amount of weed smoke and in poorly ventilated areas (like a car with the windows rolled up or a small bedroom without a fan) may result in feeling a limited amount of the effects that the person smoking experiences.
But catching a whiff of marijuana fragrance through your apartment window or entering a room where people were smoking several hours ago is very unlikely (maybe even impossible) to affect you at all. That is of course unless you trick yourself into believing that you feel it.
Is THC active after cannabis smoke is exhaled?
By reviewing the 1999 British Journal of Anesthesia, we learn that the lungs absorb most of the THC when cannabis smoke is inhaled. Researchers have discovered that approximately 50 percent of THC and other cannabinoids present in a joint, for example, make up the smoke content and are inhaled.
“Experienced smokers, who inhale deeply and hold the smoke in the lungs … virtually all of the cannabinoids present in the mainstream smoke enter the bloodstream,” leaving very little THC in the surrounding air to be inhaled and absorbed by a passive inhaler.
When considering this, along with the 2015 Johns Hopkins study, you would need to be in an unventilated room for a long time to consume any THC-containing smoke from the air. More than likely the cannabinoids will have disappeared into the air before even reaching you.
Can you get secondhand exposure from vaping?
To date, no solid scientific research has been done on the impact of secondhand exposure to vaping weed. In regards to failing a drug test if exposed, the rule of thumb is similar to that of secondhand weed smoke; it is very unlikely to test THC positive.
In fact, vaping is a solution for cannabis users who may be concerned with health risks regarding smoking around non-smokers or adolescents. Vaping cannabis reduces the ingestion of smoke-related toxins and carcinogens such as carbon monoxide, tar, ammonia, and hydrogen cyanide that are typically produced when cannabis is ignited.
Limiting the exposure of others to secondhand smoke may be particularly salient to those seeking to use cannabis for medical conditions, where consuming a specific dose is required.
Vaping cannabis has been associated with fewer reports of airborne respiratory symptoms, but there have been no clinical trials that experimentally assigned vaping to a sample of cannabis smokers.
Very little is known about the health impact of secondhand cannabis smoke, but this will become more prudent as more and more US states regulate its use.