Article updated: 2024 Jan 28 / Author Leslie Griver, Robert Henry

Why You Can Fail a Drug Test From Second-Hand Marijuana Smoke

Yes, you can fail a drug test from second-hand marijuana smoke. Several scientific studies confirm that.

But this is true if you spend your time near weed smokers in a non-ventilated room. If you were exposed to second-hand smoke just once in a well-ventilated room – you don’t need to worry.

We’ll explain how much THC gets into your system with second-hand smoke, according to the science.

How much THC is required to fail a drug test

Drug tests detect THC metabolites in the urine, saliva, and hair. Each drug test has a “cutoff-level.” It is the maximum allowed concentration of THC metabolites in the sample.

If the concentration is above this level, you’ll fail the drug test.

Urine drug tests usually have a 50ng/ml, and hair drug tests have 1 pg/mg cutoff levels.

Can you reach that level from second-hand smoke?

How much THC in second-hand smoke

The United States National Institute of Drug Abuse (NIDA) reports that only 20% to 37% of the THC is inhaled during smoking. 23% to 30% is destroyed by heating. And 40% to 50% is lost in the sidestream.
So this sidestream THC is what you can inhale as a second-hand smoker.

The modern joint may contain between 60 to 150 mg of THC or more.

Let’s follow the worst scenario and imagine this situation:

Let’s calculate how much THC will be in your system after that.


The following calculations are approximate. It’s impossible to make precise calculations because the results depend on age, weight, body fat, etc.

But using these calculations, we can understand if it is ever possible to fail a drug test from second-hand smoke.

The excretion half-life of THC is approximately 31 hours for infrequent users. It means that half of THC will be excreted from your system in 31 hours.

According to a scientific study that will help our calculations, if there is 27mg of THC in a joint, the amount of THC-COOH in a smoker’s urine would peak at 180 ng/mL after 4 hours.

You inhaled 75mg of THC (50% of the original 150mg) as a second -hand smoker.

This will result in 500ng/mL in your urine after 4 hours.

After 31 hours, the concentration would be 250ng/ml.

After 62 hours, it would be 125ng/ml.

It will drop to 32ng/ml after 124 hours. This concentration is below the 50ng/ml cutoff of the urine drug test.

So it will take 5+ days for the concentration of THC-COOH in the urine to drop below the detectable level.

Again, this is the worst scenario. It will happen if you inhale ALL the sidestream smoke.

If you are in a well-ventilated room and someone smoked one joint near you, most probably you don’t need to worry.

But if you spend a lot of time in a room with several weed smokers, you’ll test positive.

Let’s check if there are any scientific studies about THC in second-hand smoke.

Scientific evidence

Five drug-free male volunteers were passively exposed to the sidestream smoke of 4 and 16 marijuana cigarettes (2.8% THC) for 1 hour each day for six consecutive days in two separate studies.

After that, the lab tested their urine for the THC metabolites. The studies show that all subjects absorbed significant amounts of THC. As a result, they had significant amounts of THC metabolites in the urine.

In another study, six cannabis users smoked cannabis cigarettes in a room. Six non-smokers were seated with smokers in an alternating manner.

There were three sessions of a study. Sessions 1 and 2 were conducted with no ventilation, and ventilation was employed in Session 3.

Maximum THC-COOH concentrations for non-smokers were 57.5 ng/mL.

THC-COOH concentrations generally increased with THC potency, but room ventilation substantially reduced exposure levels.

The results of this study demonstrate that second-hand weed smoke can produce positive urine tests.

This review of multiple marijuana studies confirms that exposure to second-hand marijuana smoke leads to THC-COOH metabolites in the urine. And people experience ‘high’ effects after such exposure.