Use of fentanyl and methamphetamine outside of a medical context has recently increased in the United States (US), resulting in increased rates of drug-related adverse outcomes such as overdoses (U.S. Department of Health & Human Services 2021; NIH 2021; Spencer et al. 2023; National Institute on Drug Abuse 2024). When used outside of a medical context, both fentanyl and methamphetamine can be consumed by swallowing a pill, nasal insufflation of a crushed pill (i.e., nasal inhalation or “snorting”), injecting a dissolved powder, or smoking powder (commonly off of a heated foil) (National Institute on Drug Abuse). This latter method of consumption has the potential to contaminate air and surfaces with drug residue.

Inadequate access to safe consumption sites and appropriate social services has led to drug use in public space such as on public transit, creating potential for second-hand exposure to airborne smoke and surface residue for transit operators and riders (Dovey et al. 2001; Kerr 2019; Collins et al. 2019; Strike and Watson 2019; Horcher 2022). For transit operators, witnessing or responding to drug use events on their vehicle could exacerbate existing job stressors, increase feelings of burnout, induce unsafe feelings, or cause feelings of panic, distress, or fear (Peters et al. 2021; Kovaleski and Vaccarelli 2023). In the US Pacific Northwest, transit operators have reported acute health effects and concerns about safely operating their vehicles due to second-hand exposure to drugs, and/or witnessing or responding to drug use on their transit vehicles (Horcher 2022; Harris 2023; Safety & Health Assessment & Research for Prevention 2023; Goodwillie 2023).

There has been limited research investigating the acute effects of second-hand occupational exposures to fentanyl and methamphetamine, but case reports among fire-fighter EMS responders (Chiu et al. 2018) and police (Chiu et al. 2019) with presumptive second-hand exposures have detailed acute health outcomes including light-headedness, palpitations, nausea, and numbness of extremities. It is not clear if these documented health outcomes were the result of second-hand drug exposure, or induced through feelings of distress or panic, due to witnessing a potentially traumatic event. As transit workers spend extended periods of time on transit vehicles, they may also experience chronic, low-dose exposures, which also have not been investigated in occupational health research or case reports. Consequently, it is a priority for both public and occupational health practitioners and researchers to investigate the potential for second-hand exposures to fentanyl and methamphetamine on transit vehicles.

Among limited prior exposure research, an occupational exposure assessment in a pharmaceutical manufacturing facility found personal airborne fentanyl levels ranging from 0.001 to 13 μg/m³ (collected outside respiratory protection), and dermal exposures on the hands of workers ranging from 0.02 to 1,090 ng/cm² (geometric mean: 3.5 ng/cm²) for production workers (workers wore gloves and practiced hand hygiene) (Van Nimmen et al. 2006). In hospital surgical suites where fentanyl is commonly used in intravenous drug delivery, research has found trace levels of fentanyl in air, particularly in exhaled breath near the patient’s mouth (Gold et al. 2006; McAuliffe et al. 2006), though other studies were unable to detect fentanyl in a similar setting (Law et al. 2010). Other exposure assessments for fentanyl have relied on worker surveys or interviews to assess exposures and impacts (Chiu et al. 2018; Chiu et al. 2019; Chiu et al. 2020), but have not directly measured second-hand drug exposures using air or surface sampling techniques.

More studies have investigated occupational and environmental levels of methamphetamine in air and on surfaces. In clandestine methamphetamine manufacturing facilities seized by police, methamphetamine concentrations in indoor air ranged from 0.2 to 7.3 μg/m³ (Martyny et al. 2007; Wright et al. 2021). A controlled smoking study found air concentrations of methamphetamine between 300 and 520 μg/m³ resulting from a simulated smoke of 100 mg of 91% pure methamphetamine, with higher levels closer to the source (Martyny et al. 2008). This same controlled smoking study measured methamphetamine surface concentrations of 0.17 to 0.31 μg/100cm² (mean: 0.23 μg/100cm²) on horizontal and vertical surfaces after a single simulated smoking event, and 0.22 to 1.50 μg/100cm² (mean: 0.68 μg/100cm²) after a second simulated smoking event, though these air and surface values don’t account for loss due to lung absorption (Martyny et al. 2008). This prior work demonstrates the potential for both air exposure and surface concentration after smoking events as exposure pathways for methamphetamine in a variety of occupational and environmental settings.

In the US, enforceable state or federal standards do not exist for fentanyl or methamphetamine exposures originating in the workplace. Table 1 summarizes the regulatory standards and recommended health-based guidelines for fentanyl and methamphetamine in the air and on surfaces. In November 2023, the American Conference of Governmental Industrial Hygienists (ACGIH) adopted an 8-hour time weighted average (8-hr TWA) Threshold Limit Value^® (TLV^®) and 15-minute short term exposure limit (STEL) for fentanyl, as well as a fentanyl surface contamination TLV^® (ACGIH 2023). No additional state or federal guidelines exist for fentanyl exposures in workplaces. While individual states have established methamphetamine surface decontamination standards, in general these are targets for cleaning property previously used for clandestine methamphetamine production, and have unknown applicability to second-hand exposures in occupational settings (United States Environmental Protection Agency 2021). No state or federal guidelines or recommendations could be found for methamphetamine in air.

Here, the potential for transit worker second-hand exposure to fentanyl and methamphetamine by collecting air and surface samples from transit vehicles in Washington state (WA) and Oregon (OR) is characterized. Currently, there is not widespread agreement on sampling or analysis methods for second-hand drug smoke in a workplace setting, though several laboratories and investigators have validated analytical methods (Van Nimmen and Veulemans 2004; Bureau Veritas 2018; Jeronimo et al. 2024). Federal agencies such as the Occupational Safety and Health Administration (OSHA) or the National Institute for Occupational Safety and Health (NIOSH) have not proposed a sampling or analytical method for fentanyl in air or on surfaces, or methamphetamine in the air, though NIOSH methods 9106 and 9109 provide methods for methamphetamine collected on surfaces via wipe samples (NIOSH 2011a; NIOSH 2011b) With limited research directly measuring second-hand exposure to fentanyl and methamphetamine in occupational settings, the work presented here is a novel exploration of how to appropriately characterize worker exposures even with a lack of guiding methods. This study addresses a major, emerging public and occupational health concern and highlights the need to protect transit workers and prioritize future research.

In collaboration with five transit agencies in the Pacific Northwest (WA, OR), researchers developed sampling strategies to collect air and surface samples on vehicles from four agencies. Details of the sampling strategies are summarized in Table 2. Routes, runs, and time of day for sampling were informed by data collected from operator and rider reports of drug use and conversation with transit agencies to maximize the potential for drug use to occur during sampling. Researchers were generally not present on transit during sampling to avoid biasing passenger behaviors and to align with transit agencies’ safety protocols, but were present during sampling periods for one agency, as required by the transit agency to ensure safety of equipment and the ability to respond to operator questions or concerns during sampling. This transit agency also requested researchers remove the pumps when trains reversed direction at the end of the line, resulting in some shorter sampling times compared to other agencies.

Researchers also collected repeat environmental air and surface samples from three locations in Seattle, WA (urban core, urban residential, low-density residential), and two locations in or near Portland, OR (urban core, suburban). These samples were collected outdoors in public spaces away from transit spaces (e.g., train tracks, bus stops, transit centers) using the same methods as those collected on buses or trains (detailed subsequently). These samples were used to assess environmental background levels of fentanyl and methamphetamine for comparison to levels measured on transit vehicles and to rule out the possibility that exposures measured on transit were a result of general background environmental contamination.

To authors’ knowledge, this is the first exposure assessment of second-hand drug smoke and residues on transit vehicles. Fentanyl was present in 25% of air samples and 46% of surface samples, while methamphetamine was found in all air samples and 98% of surface samples. Results presented here confirm the presence of fentanyl and methamphetamine both in the air and on the surfaces of transit vehicles included in this study, and the potential for second-hand exposure through both inhalation and dermal routes. In general, air and surface levels tended to be higher in passenger areas of the transit vehicle, consistent with where these drugs are being smoked. While this exposure assessment did not employ methods specifically to assess riders’ exposures, who would be on the transit vehicles for shorter periods of time than the operators, findings do show the potential of exposure for the riding public. In addition, the surface exposures present throughout the bus indicate that maintenance workers and other transit personnel could also be at risk of exposure due to their work tasks.

There is no broad scientific consensus about what levels of these drugs in air or on surfaces would pose an acute or chronic health risk of concern to transit operators, who often work 8–10 hour shifts (U.S. Department of Transportation 2022). In November 2023 the ACGIH produced health-based guidelines for fentanyl exposure in workplaces, for both chronic exposures (8-hour TWA TLV^®, 0.1 μg/m³) and acute exposures (15-minute TWA STEL, 0.2 μg/m³), though these are not regulatorily enforceable (ACGIH 2023). Similarly, using a safety factors approach, research in the pharmaceutical production industry identified a recommended occupational exposure level of 0.1 μg/m³ for fentanyl in air (Naumann et al. 1996; Binks 2003). In this study, only one measured air concentration exceeded 0.1 μg/m³, which was found in the passenger area of a train. The air sample collected in the operator cab concurrently was at about 80% of the ACGIH 8-hr TWA TLV^® (no short-term exposure samples were collected). However, these samples were collected for less than a full 8-hr shift (run time = 240 minutes) and were total dust samples as opposed to inhalable, so direct comparison to the ACGIH TLV^® should be done with caution. However, researchers assumed that conditions for the unsampled period are the same as the sampled period, given the frequency of reported events, making exposures exceeding the ACGIH 8-hr TWA TLV^® likely (Pae 2022; Harris 2023).

While the American College of Medical Toxicology (ACMT) and American Academy of Clinical Toxicology (AACT) stress in their position statement that absorption of fentanyl through the skin is negligible, and touching fentanyl or fentanyl contaminated surfaces is not believed to pose a clinically meaningful health risk (Moss et al. 2017), Van Nimmen et al. found a correlation between fentanyl hand exposure and urinary excretion, suggesting that the dermal pathway may be the primary route of fentanyl exposure for many workers (Van Nimmen et al. 2006). Similarly, the ACGIH wrote a surface level guideline for fentanyl (10 ng/cm²) with a skin notation, indicating toxicity is possible through dermal routes of exposure. No samples exceeded this guideline.

As detailed in Table 1, there are no enforceable state or federal standards for methamphetamine or fentanyl in workplace air. While no samples exceeded the WA health-based remediation surface standard for methamphetamine of 15 ng/cm² (Washington Administrative Code 246–205 2023), one sample was at about half of the standard (6.89 ng/cm²) and 29 (32.6%) exceeded the OR decontamination standard of 0.5 μg/ft² (0.54 ng/cm²) (Oregon Administrative Rules 333–040 2000). However, the OR decontamination standard is not a health-based standard and only has regulatory applicability in facilities where methamphetamine is being manufactured, not where it is being used.

This study did not assess mental or physical health impacts related to second-hand fentanyl or methamphetamine exposure but provides evidence that operators could be exposed at work, creating the potential for health impacts. Compared to research on physical health effects associated with fentanyl and methamphetamine misuse, research is limited on the mental health effects individuals may experience when they observe drug misuse or interact with individuals using drugs in their work environment. Case reports have shown firefighters and law enforcement experience mental health impacts related to opioid overdose responses, and law enforcement professionals have exhibited acute anxiety responses related to unintentional occupational exposure to opioids (Chiu et al. 2019; Persaud and Jennings 2020; Del Pozo et al. 2021); transit operators may experience similar impacts.

Given the presence of both fentanyl and methamphetamine found on public transit in this study, occupational health interventions and evaluations of these interventions, are prudent to reduce exposures and potential health impacts to operators.

Public Health has long supported smoke free workplaces, and 27 states (plus Washington, DC) have adopted workplace smoking legislation of some type, primarily focused on tobacco smoke (CDC 2024). Enacting or continuing to enforce measures to discourage or ban the smoking, including methamphetamine and fentanyl, on public transit is consistent with the public health goal of smoke free workplaces. Researchers acknowledge that this should be done without further harming individuals who use fentanyl or methamphetamine. Some transit agencies have found success employing unarmed community outreach or safety ambassadors who both discourage inappropriate and illegal behavior while also providing references and referrals for social services to those using drugs openly (TriMet N.D.). Access to safe consumption sites for drug use and other harm-reduction approaches could also be explored, though require policy actions outside of the workplace to be effective.

While elimination is the most effective control for second-hand drug exposures, engineering and administrative controls can also help to protect operators (NIOSH 2020). Upgrading air filters on transit vehicles to a minimum efficiency reporting value (MERV) of 13 (or to as a high of a MERV filter rating as feasible) will improve particulate filtration, potentially resulting in fewer particles reaching the operator or depositing on surfaces. Other methods to increase air flow, such as opening windows or bringing in more outside air through the heating/cooling system will also increase air changes per hour (ACH) though these actions are dependent on passenger and operator preferences and actions. Enhanced cleaning protocols, increased cleaning frequency, and use of solvents that are appropriate for both water soluble (e.g., fentanyl) and water insoluble (e.g., methamphetamine) contaminants could be effective for reducing drug residue on surfaces. Contaminated surfaces that may not be touched by operators or the riding public should also be cleaned, as these are accessed by and may expose transit maintenance workers. However, the real-world best practices and effectiveness of these controls still need to be studied.

Workers should be educated by their employer and/or union on the hazards of second-hand exposure to fentanyl, methamphetamine, and other hazardous substances. This can include updated or more frequent trainings on topics such as agency protocols that operators should follow when they observe a smoking event or are threatened by a passenger, real and perceived risks related to second-hand drug exposure, and how and when to deploy naloxone if comfortable doing so. Research with police officers and first responders has shown that medically accurate training about fentanyl can increase knowledge in these occupational groups, and correct misconceptions about exposure and risk of overdose (Persaud and Jennings 2020; Winograd et al. 2020; Del Pozo et al. 2021). Employers should also ensure operators have access to mental health support, as observing or responding to emergency situations involving drug use could be distressing. Including transit operators and their unions in conversations about how to best support and protect workers exposed to second-hand drugs in the workplace may increase the likelihood of acceptance and adoption of interventions.

In its guidance for emergency responders, NIOSH recommends the use of personal protective equipment (PPE), including nitrile gloves, eye protection, and respiratory protection (P100, N100 or R100) when handling paraphernalia or touching surfaces which may have drug residue or when drugs are visible (NIOSH 2022). This PPE is appropriate for trained personnel who are tasked with removal of drug paraphernalia and clean-up of visible drug residue but is unlikely to be necessary for transit operators during the normal course of their work. Given PPE relies on individual worker compliance and knowledge of its use, the authors recommend transit agencies prioritize eliminating the hazard or using engineering and administrative controls, though transit operators should be allowed to wear PPE should they choose. Any adopted control should be evaluated for effectiveness.

To the authors’ knowledge, this is the first exposure assessment to characterize air and surface concentrations of fentanyl and methamphetamine on transit vehicles and motivates future study on this and related topics. Expanding the body of research exploring the relationship between workplace exposures and acute and chronic physical and mental health outcomes (assessed either clinically or via survey) will allow for more actionable results and evidence-based regulatory guidelines to protect workers. Collecting biomarkers of exposure from transit operators and other transit workers who may be exposed, such as maintenance workers, could help inform the relationship between exposure, dose, and health outcomes. As smoking events tend to occur over short periods of time, characterizing short-term exposures would be more representative of riders and allow for exploration of potential acute impacts.

In this study, researchers only investigated methamphetamine and fentanyl, though there was evidence that other drugs were also present. Characterizing the suite of drug exposures, and how they might interact when smoked together, would allow for a greater understanding of potential health risks and how to deploy controls (such as cleaning) to best protect workers. Expanding this work to other occupations that may face second-hand exposure to drugs at work would also be beneficial for characterizing the workplace burden of these exposures.. While the assessment didn’t characterize exposures to the riding public, expanding this work to characterize exposures the public may experience would also be of scientific interest.

There were several limitations of this work which must be acknowledged. Due to the sampling strategy, researchers were generally not present with equipment, and while this likely reduced observer bias, researchers were unable to comprehensively or consistently document exposure determinants, such as whether or not smoking events occurred during sampling, where smoking events took place, event durations, the substance smoked, etc. To comply with agency requirements and ensure researchers did not disrupt operators or the riding public, sampling protocols differed for each of the agencies worked with, including sampling location, and researcher presence on the vehicles. Lastly, this was a limited-scope assessment, with data collected over a total of 28 nights from 11 buses, 19 trains, and four transit agencies. Because the vehicles and times that were chosen for sampling were those identified as having a high probability for smoking events, these data, while representing true conditions in transit vehicles in the Pacific Northwest, should not be taken to be broadly representative or generalizable.

Fentanyl and methamphetamine were frequently found both in the air and on surfaces of the trains and buses in this study, at levels exceeding those observed in environmental (background) samples collected in urban settings. While enforceable regulatory guidelines do not exist for fentanyl and methamphetamine in workplaces, protecting transit operators from second-hand exposures represents appropriate occupational health action. Following the hierarchy of controls by eliminating the hazard on transit is preferred, coupled with engineering and administrative controls such as ventilation and cleaning upgrades, and training and increased operator supports. Further research on this topic could investigate the quantitative relationship between exposure, dose, and outcome; evaluate controls for effectiveness and adoption; explore health outcomes related to acute and chronic exposures; and research short term exposure levels and impacts to the riding public.