How can I separate drugs from blood

Got stoned from passive smoking?

“But I didn't smoke weed at all,” some are said to have complained when the urine sample tested positive for THC at the police check. Can this be? Can a significant amount of THC be detected if the person has only passively inhaled cannabis smoke? A German research group wanted to know exactly and, in cooperation with a Dutch coffee shop, carried out a study under realistic conditions.

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Anyone who sits behind the wheel in Germany stoned not only increases the risk of being involved in an accident, but also of losing their driver's license. However, the Federal Constitutional Court ruled in 2005 in a ruling that only a THC concentration of over 1.0 nanograms per milliliter (ng / ml) in the blood can actually be assumed to result in impaired driving ability. Because as a result of technical progress, the detection time for THC has increased significantly. Cannabis use that was a long time ago could still lead to a positive finding, even though one has not currently smoked weed.

Research under extreme conditions

Jörg Röhrich and his research team at the University of Mainz and the University of Jena name this aspect as the starting point for their specialist publication in the Journal of Analytic Toxicology. Previous studies could have shown that even passively inhaled cannabis smoke can generate significant THC concentrations in the blood. However, these experiments were carried out under sometimes extreme conditions, for example by squeezing five volunteers into a small car and enriching the little air in it with six burned joints. The results were up to 6.3 ng / ml THC in the blood of the passively smoking inmates. The driver's license would be gone without having pulled the joint.

Since Röhrich and his team found scenarios like the one described too unrealistic, they went a different way to find out whether passive smoking actually leads to critical THC levels. They put eight people - four men and four women - who had never smoked pot, in a coffee shop in Maastricht, the Netherlands. They had to hold out for three hours while the shop was buzzing. Up to 25 people were smoking weed at the same time in the 84 square meter room. To be on the safe side, an additional 8 grams of cannabis was burned in an ashtray - to compensate for periods with few visits. The THC concentration in blood and urine was then measured at several times.

None hit threshold

In contrast to the results of previous studies, Röhrich and his research team could not find any seriously elevated THC levels. According to the authors, none of the test subjects had reached the threshold value of 25 ng / ml THC in the urine. It should also be mentioned that none of the abstinent men and women felt stoned.

Röhrich and his team therefore come to the conclusion that it is very likely that none of the test subjects would have tested positive for cannabis during a routine police check. A blood analysis would also have shown values ​​below 1 ng / ml THC. Passive smoking cannot be used as an excuse if the test strip works.

Withdrawal of driving license for passive smoking

However, since everything is only a question of intensity, one should not rule out the possibility that passive smoking weed leads to critical urine values. In one case from 2004, the chill-out area of ​​a techno event was a man's undoing. He stayed there for two hours before he got behind the wheel. Although he did not smoke weed himself, others in the room did so even more. When the police arrested the man, he was found to have a THC concentration of 5 ng / ml in his serum, whereupon his driver's license was immediately revoked.

The complaint of the accused against the withdrawal of the driving license was finally rejected by the administrative court of Baden Württemberg. Justification: It is generally accepted that the consumption of cannabis affects the ability to drive. The applicant was well aware of the considerable inhalation intake of cannabis in the chill-out area. If a driver sits behind the wheel despite this knowledge, he is to be treated like an active drug user. He can only regain his driving license if he can prove through a positive medical-psychological report (MPU or "idiot test") or a year of abstinence from drugs that he can distinguish between consuming and driving a motor vehicle.


Science has shown that passive smoking can lead to measurable THC concentrations in the blood. Although it is unlikely that these will exceed the critical threshold of 1 ng / ml THC in the blood, it cannot be ruled out, as the extreme examples of research or the case law show. In case of doubt, it is better to stop the car.

Röhrich, J., Schimmel, I., Zörntlein, S., Becker, J. Drobnik, S., Kaufmann, T., Kuntz, V. & Urban, R. (2010). Concentrations of? 9 -Tetrahydrocannabinol and 11-Nor-9-Carboxytetrahydrocannabinol in Blood and Urine After Passive Exposure to Cannabis Smoke in a Coffee Shop. Journal of Analytical Toxicology, 34, 196-203. Summary