The Colorado courts have made clear an employer can terminate an employee who tests positive for marijuana in violation of the employer’s illegal drug policy. But, what if the employee is a firefighter and she tested positive because she was exposed to high concentrations of marijuana smoke while responding to a large-scale fire at a marijuana grow operation? What about the paramedic who enters a house thick with marijuana smoke to treat a heart attack patient? How does an employer go about determining when an emergency responder’s positive test for marijuana may be the result of an on-duty exposure?

Testing Considerations

A number of laboratory tests can detect the presence of Tetrahydrocannabinol (“THC”), the controlled substance in marijuana.  Generally, the choice of test depends upon the desired goal – i.e., whether it is to serve as a screening tool, to confirm marijuana use in an individual subject, or to monitor the results of a patient’s treatment program. The fundamental trade-offs among the various techniques are cost, ease of administration and accuracy.

Urine and blood testing are the most common screening mechanisms, with urinalysis by far the more common of the two. Urinalysis specimens are generally screened first using a fairly inexpensive test called the enzyme multiplied immunoassay technique (“EMIT”). Studies focusing on the accuracy of the EMIT include a report concluding that the test is 87% accurate and a 1982 study conducted by the Department of Defense finding that the test is 88.9% accurate.Because EMIT has demonstrated an approximately 12% potential for error, it is generally regarded as suitable only for initial screening. Samples that test positive under EMIT are generally retested with a more accurate test, such as gas chromatograph mass spectrometry (“GC/MS”).

While GC/MS is considered a very reliable confirmatory technique, the high cost and complexity preclude its use in mass screenings. However, EMIT and GC/MS in combination is both cost effective and nearly 100% accurate in testing for THC. Moreover, a GC/MS test is reputed to be the only confirmation test that, when used with EMIT, is “fully defensible against legal challenge”.

Many variables can impact the potential for passive THC absorption

Studies have demonstrated that the amount of THC a person absorbs through second-hand exposure (i.e., passive inhalation) can be impacted by many different variables. In addition to factors unique to the individual, such as body composition and metabolism, external variables such as the duration of exposure, multiple exposures, quantity of smoke in the room, proximity to smokers, amount of ventilation and the THC levels of the marijuana inhaled can all influence an individual’s level of THC absorption, although experts disagree as to the absolute and relative levels of impact environmental variables may have. Further, the amount of time that has passed since the exposure has been shown to impact directly the level of THC excretion in the subject’s urine.

Ambient smoke concentration is recognized as one of the largest contributors to passive THC absorption

Some researchers have concluded that room air levels of THC are the most critical factor in boosting absorption levels during second-hand exposure. Room air levels are most directly impacted by the size of the space, the number of smokers and the level of room ventilation.

Most second-hand exposure (passive inhalation) studies have been conducted in small, closed rooms or in vehicles, in order to simulate high concentrations of ambient marijuana smoke. Many have utilized multiple test subjects with varying numbers of smokers, amount smoked, and ventilated or unventilated rooms. Often, air concentrations necessary to result in a positive urine specimen required a large number of smokers in an unventilated room. This created test conditions that were described as “extremely uncomfortable”, “obvious” and causing “universal discomfort”, such that in some studies subjects had to wear goggles. Therefore, any exposure at a level that may cause substantial THC absorption by an emergency responder should be very noticeable.

Marijuana potency also can impact absorption levels

The increased potency of marijuana has renewed concerns that second-hand exposure can produce positive test results. A 2015 study assessed the impact of increased marijuana potency and a lack of ventilation. The study included a first subject who passively inhaled marijuana smoke from 5.3% THC marijuana cigarettes in an unventilated room; a second subject who was under identical conditions with the exception of the marijuana potency, which was increased to 11.3% THC; and, a third subject who was under identical conditions to the second subject, except that the room was ventilated.

Notably, the mean maximum concentration of THC detected in the urine of those subjects who inhaled the more potent THC was over 2.5 times that of the mean maximum concentration observed in the subject exposed to the lower potency THC.  Both of these subjects were without ventilation. (Additionally, the mean maximum concentrations from the subject exposed to lower potency THC without ventilation were 1.5 times the mean maximum concentration from the subject exposed to the higher potency THC in a room with ventilation. This underscores the importance of room ventilation to passive THC absorption as compared to other factors.)

The study had many false positives (a positive urine screen for THC resulting from passive inhalation) at an EMIT threshold of 20 ng/mL. However, out of the eighteen subjects, only one false positive occurred at a 50 ng/mL threshold. This subject was exposed to the highest potency smoke in an unventilated room. The researchers explained that the marijuana used in the test was more potent than that used in earlier studies and was similar to the potency of marijuana available in the United States at the time of the study. However, recent news articles evaluating the Colorado marijuana industry have pointed out that marijuana sold in the state has an average THC level of 18.7%, with some strains surpassing 30%.  This represents, on average, a THC concentration of over 150% of the highest potency used in the past study. Because concentrations at the Colorado average potency level have not been tested, it is unclear what additional impact there might be on a passive inhaler’s THC absorption. However, at a minimum, the study suggests that increased THC could result from the additional increase in marijuana potency. This further underscores the need for emergency responders to log all potential exposure incidents in case later review is required.

Duration of exposure also can impact absorption

Duration of exposure to marijuana smoke also is recognized as a factor in the level of THC absorbed. However, duration impacts absorption in conjunction with air concentration and potency and is not by itself determinative. For instance, one study included subjects who tested positive for THC at 20 ng/ml after only thirty minutes of exposure in a very small unventilated space, while in another study no participants tested positive at 25 ng/mL after three hours of exposure in a relatively well-ventilated coffee shop.

As researchers have pointed out, duration of exposure can reasonably be assumed to impact levels of absorption in passive inhalers. However, it can be inferred from available literature that other factors have an equal or even more substantial effect. Therefore, it is important to recognize that false positives from passive exposure can result from a relatively short duration of exposure.

Excretion rates can be affected by similar factors as absorption

Large variations in excretion patterns have been noted following smoking of marijuana. How long THC is detected in the urine depends strongly on frequency of use. THC is stored in the adipose tissue and may accumulate faster than it can be removed in persistent users. In regular users (more than once a week), the THC builds up to a high level that can take weeks to decline below typical detection thresholds, while for occasional smokers (no more than once a week) it may take only a few days. A regular smoker (3 times per day) who stops using marijuana may still test positive at 50 ng/mL for over two months.

For occasional marijuana use (or single use), at the 50 ng/mL cutoff level, it would be unusual for urinary detection of THC to extend beyond 3-4 days following the smoking episode. One study concluded that passive exposure to 16 marijuana cigarettes could result in urinary THC at a level similar to actually smoking one. Therefore, the potential exists that after an extreme exposure incident an emergency responder could test positive on a drug screen for as long as 4 days. However, another study concluded that THC levels from passive inhalation would be excreted to undetectable levels in approximately 1 day. Notably, variables that impact levels of THC absorption (e.g. air concentration and potency) also affect the length of time before THC is no longer detectable in the urine.

One Approach to the Problem of Work-Related Second-Hand Exposure to Marijuana

The results of several studies demonstrate that extreme marijuana smoke exposure can produce positive urine tests at commonly utilized cutoff concentrations. This can be impacted by other factors as well, such as the amount of smoke, lack of ventilation, potency of the marijuana and duration of exposure. However, positive tests are likely to be rare, limited to the hours immediately after exposure, and occur only under environmental conditions where exposure is obvious.

A drug test protocol should balance the need for deterring drug use, the risk of detecting false positives when potential for second-hand exposure exists and the risk of spending excessive resources on more expensive, but more accurate, tests. A multi-tiered approach that begins with inexpensive initial screening followed by a more expensive confirmation test and then a case-by-case investigation into the potential impact of second-hand exposure or cross reactivity with other medications appears to be commonly employed, accurate and economical.

Initial Screening Levels

The chance of false positives due to environmental marijuana smoke exposure has led some researchers to recommend initial screening thresholds of 65 ng/mL to rule out second-hand exposure as a cause. Others have suggested cut off levels as high as 100 ng/mL. However, while false positives at a 20 ng/mL threshold are common, they are rarely observed at EMIT thresholds of 50 ng/mL or above. In those instances where an individual tests positive for THC above 50 ng/mL, confirmation testing and individual case review can be used. While some researchers recommend higher initial screening thresholds, doing so creates substantial risk of receiving false negatives and a failure to detect actual non-passive marijuana use.

Follow-up Testing

Because false positives are always a possibility, standard testing procedures generally include a confirmation test, most commonly using GC/MS. While not required, a confirmation test seems reasonable to include in an agency’s policy and is consistent with government guidelines. The standard threshold for confirmation on a GC/MS appears to be 15 ng/mL.

Case-by-Case Investigation

Further investigation after a failed confirmation test also is not a requirement. However, it can mitigate the risk of false positives due to medication cross reactivity or second-hand exposure. Additionally, because environmental marijuana smoke concentrations significant enough to produce results higher than 50 ng/ml only occur under circumstances in which exposure is obvious, the agency should require reporting, and maintain a log, of any such exposures to aid in the investigation process.

Based upon current scientific evidence, a reasonable testing policy might include:

  1. An initial screening using the relatively inexpensive EMIT at a 50 ng/mL positive result threshold. This threshold is high enough to substantially limit the risk of false positives due to second-hand exposure and low enough to detect most actual recent marijuana use;
  2. Any positive initial screening should be retested using more accurate confirmation test techniques, such as GC/MS. A standard threshold for a positive result in a GC/MS is 15 ng/mL;
  3. A positive confirmation test would result in an individual case review to investigate the potential for cross reactivity with other medications or second-hand exposure; and,
  4. Emergency responders should be required to take measures to mitigate second-hand exposure, such as increasing room ventilation, when possible. Additionally, requiring emergency responders to document all cases of work-related exposures to marijuana smoke will aid the individual case review process and provide an opportunity to delay or reschedule drug screening as appropriate to mitigate the risk of false positives due to second-hand exposure.

Closing Thoughts

Researchers have noted that individuals who wish to avoid THC absorption should simply “avoid environments in which marijuana is combusted.” However, emergency responders often do not have this option. As such, precautions should be taken to prevent the extreme exposure conditions necessary to cause a false positive test due to second-hand exposure.

The most effective methods to reduce second-hand exposure involve reducing the concentration of THC in the air. When entering an environment where marijuana is in use, emergency responders should request that it be immediately extinguished. Where possible, room ventilation should be increased. Where very heavy marijuana smoke exists, respirators can be used to prevent passive inhalation. And finally, emergency responders should remain in the marijuana smoke filled environment no longer than necessary to accomplish their objective.

Dino A. Ross, a special districts and employment lawyer at Ireland Stapleton, Pryor & Pascoe, PC, focuses a substantial portion of his practice to public and private employment law. Mr. Ross advises and assists public and private employers, including special districts, in all aspects of employment law and employment related litigation and administrative proceedings.

Dino can be reached at 303.628.3686 or

This article is intended as general information on the topic covered and is not to be construed as legal advice. If legal advice is needed, you should consult an attorney.