Obviously I’m not a lawyer and none of this is legal advice!

Take-home message:

  • Second-hand exposure to cannabis smoke can, in certain conditions, cause some degree of subjective drug effects. Moreover, this second-hand exposure can result in the detection of cannabis in body fluids such as saliva and blood. These effects often require pretty extreme conditions (very smoky rooms for a long period of time), but it is possible.
  • Australian drug-driving laws don’t have a cut-off for drug concentrations in the body, as are used in drink-driving laws. In Australia, it is an offence to drive with any amount of THC in your system. This is a bit incoherent given the scientific knowledge about how cannabis works, but it’s the reality of the legislation.
  • Failing a drug test results in a mandatory fine of ~$1,000 and disqualification from driving for at least six months. Refusing a drug test brings an identical fine but a disqualification for at least twelve months.
  • I think the legislation in South Australia says that, rather than having an initial drug screening test or the follow-up oral drug test administered by a police officer, you can ask to have a medical practitioner or nurse administer a blood test instead and the police officer has to do everything in their power to facilitate this.
  • To me, it is pretty weird that the government can force you to submit to a somewhat invasive medical procedure (saliva test or blood test). However, this appears to be justified on philosophical grounds under the idea of “implied consent”—driving is not a natural human behaviour or right, so it is reasonable that participation in driving also involves implicitly consenting to the government’s rules.

Iglesias et al 2018, The effects and impact of second-hand cannabis smoke exposure on nurses working in the community, Australian Nursing and Midwifery Journal, link

  • compared to tobacco, “the effects of SHCS [second-hand cannabis smoke] are less well known due to cannabis being illegal until recently in most countries, which made research difficult to conduct.”
  • “SHCS from combustible cannabis has similar health risks to second-hand tobacco smoke due to the combustive organic material, which creates carcinogenic and mutagenic effects (Cone et al. 2015a). Cannabis contains many of the same toxins in tobacco, which are responsible for cardiovascular damage. In fact, many toxins such as ammonia, nitric oxide and hydrogen cyanide, are found in levels between three to 20 times higher in cannabis than in tobacco (American Chemical Society 2007). Studies in rodents (Wang et al. 2016) have demonstrated that exposure to cannabis smoke for as little as one minute impairs cardiovascular function for the following 90 minutes.”
  • “In humans, subjective drug effects of cannabis smoke inhaled passively have been found to be dose-dependent (Cone et al. 2015b).”
  • “Hours after secondary exposure to cannabis smoke, THC can be detected in screening tests of urine (Cone et al. 2015a) and oral fluids (Cone et al. 2015b), although only after exposure to high smoke concentrations.”
  • “over the last decade random roadside drug testing of drivers has become a common feature on Australian roads. Road-side drug testing determines the presence or absence of cannabis, ecstasy or amphetamines in oral fluid samples (Wilson 2012).”

Cone et al 2015, Non-smoker exposure to secondhand cannabis smoke. I. Urine screening and confirmation results, Journal of Analytical Toxicology, link

  • CONE! Hahahahahaha
  • “A systematic study was conducted of smoke exposure on drug-free participants. Six experienced cannabis users smoked cannabis cigarettes (5.3% THC in Session 1 and 11.3% THC in Sessions 2 and 3) in a sealed chamber. Six non-smokers were seated with smokers in an alternating manner.”
  • “Non-smoking participant specimens (collected 0–34 h) were analyzed with four immunoassays at different cutoff concentrations (20, 50, 75 and 100 ng/mL) and by GC-MS (LOQ = 0.75 ng/mL). No presumptive positives occurred for non-smokers at 100 and 75 ng/mL; a single positive occurred at 50 ng/mL; and multiple positives occurred at 20 ng/mL. Maximum THCCOOH concentrations by GC-MS for non-smokers ranged from 1.3 to 57.5 ng/mL.”
  • “These results demonstrate that extreme cannabis smoke exposure can produce positive urine tests at commonly utilized cutoff concentrations. However, positive tests are likely to be rare, limited to the hours immediately post-exposure, and occur only under environmental circumstances where exposure is obvious.”

Cone et al 2015, Nonsmoker exposure to secondhand cannabis smoke. III. Oral fluid and blood drug concentrations and corresponding subjective effects, Journal of Analytical Toxicology, link

  • “Subjective effect measures and amounts of THC absorbed by nonsmokers (relative to smokers) indicated that extreme secondhand cannabis smoke exposure mimicked, though to a lesser extent, active cannabis smoking.”
  • “It should be noted that conditions produced in the current study were designed to represent extreme acute exposure and could not be considered to be “unknowing” in nature.”
  • “Nonsmokers reported zero or low responses (relative to smokers) in Sessions 1 and 3 on VAS ratings for ‘drug effect’, ‘pleasant drug effect’ and ‘unpleasant drug effect’. Responses by nonsmokers in Session 2 were substantially higher for ‘drug effect’ and ‘pleasant drug effect’ and slightly elevated for ‘unpleasant drug” effect’.”
  • “Analyses of oral fluid specimens following nonsmokers’ exposure to extreme secondhand smoke indicated that inhalation of environmental cannabis smoke led to the presence of THC in oral fluid.”
  • “This combined body of data suggest that environmental exposure to cannabis smoke should be avoided by nonsmokers and potentially has implications for those who undergo drug testing and those engaged in safety-sensitive activities (e.g., driving). Extreme exposure of nonsmokers could lead to positive drug tests and drug-induced behavioral changes not unlike those produced by active cannabis smoking.”

Berthet et al 2016, A systematic review of passive exposure to cannabis, Forensic Science International, link

  • “Passive exposure to cannabis is furthermore a significant legal issue, and it may be especially so in occupational contexts and when driving vehicles. In the workplace, passive exposure should not be mistaken for active use if it might lead to sanctions against employees. With regard to driving, active and regular cannabis consumption should not be mistaken for unintended frequent passive exposure. In cases of acute exposure and a zero-tolerance driving policy, a THC-positive blood specimen may well result in the same legal and administrative consequences, regardless of whether exposure was active or passive.”
  • “concentrations exceeding the usual thresholds of positivity require very extreme environmental conditions”
  • “As a concluding remark, the experts should clearly inform persons who have to demonstrate prolonged abstinence from cannabis to avoid heavily smoky and unventilated areas.”

Drug driving and the law, Legal Services Commission South Australia, link

  • “It is a criminal offence to drive, or attempt to drive, a motor vehicle while a prescribed drug is present in your system. Roadside drug screening (saliva) tests can detect THC (cannabis), methylamphetamine (speed, ice, or crystal meth) and MDMA (ecstasy). Unlike drink driving, there is no minimum amount of drug that has to be present in your system for you to be guilty of an offence. Many drugs can remain in your system for hours or even days after they have been taken.”
  • “Any uniformed police officer can require a driver to undergo a random roadside drug screening (saliva) test, so long as an alcotest or breath analysis test has first been conducted. If the saliva test indicates a positive reading for the presence of drugs, an oral fluid sample will be taken and sent for laboratory testing. If the presence of drugs is confirmed by laboratory testing, you will be charged with driving with a ‘prescribed drug in oral fluid or blood’ or driving under the influence (DUI). This process can take several weeks. Police may issue you with an on-the-spot licence disqualification if they reasonably believe you have committed the offence.”
  • “It is an offence to refuse a request for a drug screening test, oral fluid analysis or blood test. The penalties that apply if you refuse to cooperate are more serious than those for a first offence of drug driving itself.”
  • “Police may issue an on-the-spot disqualification notice for the offences of drug driving, refuse drug screening or oral analysis test, or refuse blood test. This disqualification will usually be for the minimum period for the charge you are likely to face. A summons will be sent at a later date for you to appear in court to determine the actual penalty, including the length of the disqualification and fine.”
  • “If you test positive for THC, methylamphetamine or MDMA on a drug screening test but the police do not issue you with an on-the-spot disqualification notice, they may ask you to stop driving until the drug is no longer detectable in your system. For THC this will usually be up to 4 hours and for methylamphetamine and MDMA 24 hours. The police can direct you to leave your vehicle and surrender your keys.”
  • “Driving under the influence (DUI): It is an offence to drive, or attempt to drive, a vehicle while so much under the influence of a drug as to be incapable of exercising effective control of the vehicle. […] This offence is separate to Drug Driving and you cannot be convicted of both in relation to the same incident.”

Road Traffic Act 1961 (Version: 24.4.2025)

  • Division 5—Drink driving and drug driving
  • 47BA—Driving with prescribed drug in oral fluid or blood
  • (1) A person must not— (a) drive a motor vehicle; or (b) attempt to put a motor vehicle in motion, while a prescribed drug is present in the person’s oral fluid or blood. Penalty: (a) for a first offence—a fine of not less than $900 and not more than $1 300; (b) for a second offence—a fine of not less than $1 100 and not more than $1 600; (c) for a third or subsequent offence—a fine of not less than $1 500 and not more than $2 200.
  • it is a defence to a charge of an offence against this section if the defendant proves that the defendant did not knowingly consume the prescribed drug present in the defendant’s oral fluid or blood.
  • (4) If a court convicts a person of an offence against this section, the following provisions apply: (a) the court must order that the person be disqualified from holding or obtaining a driver’s licence— (i) in the case of a first offence—for such period, being not less than 6 months, as the court thinks fit;
  • (b) the disqualification prescribed by paragraph (a) cannot be reduced or mitigated in any way or be substituted by any other penalty or sentence unless, in the case of a first offence, the court is satisfied, by evidence given on oath, that the offence is trifling, in which case it may order a period of disqualification that is less than the prescribed minimum period but not less than 1 month;
  • 47EAA—Police may require drug screening test, oral fluid analysis and blood test
  • (1) Subject to this Act, if a person has submitted to an alcotest or breath analysis as a result of a requirement under section 47E, a police officer may require the person to submit to a drug screening test.
  • (3) A police officer may give reasonable directions for the purpose of making a requirement under this section that a person submit to a drug screening test, oral fluid analysis or blood test. (4) A person must forthwith comply with a direction under subsection (3). Maximum penalty: $2 900.
  • (9) A person required under this section to submit to a drug screening test, oral fluid analysis or blood test must not refuse or fail to comply with all reasonable directions of a police officer in relation to the requirement and, in particular, must not refuse or fail to allow a sample of oral fluid or blood to be taken in accordance with the directions of a police officer. Penalty: (a) for a first offence—a fine of not less than $900 and not more than $1 300;
  • (11) If a person of whom a requirement is made or to whom a direction is given under this section relating to a drug screening test or oral fluid analysis refuses or fails to comply with the requirement or direction by reason of some physical or medical condition of the person and forthwith makes a request of a police officer that a sample of the person’s blood be taken by a medical practitioner or registered nurse, a police officer must do all things reasonably necessary to facilitate the taking of a sample of the person’s blood— (a) by a medical practitioner or registered nurse nominated by the person; or (b) if— (i) it becomes apparent to the police officer that there is no reasonable likelihood that a medical practitioner or registered nurse nominated by the person will be available to take the sample within 1 hour of the time of the request at some place not more than 10 kilometres distant from the place of the request; or (ii) the person does not nominate a particular medical practitioner or registered nurse, by any medical practitioner or registered nurse who is available to take the sample.
  • (16) If a court convicts a person of an offence against subsection (9) or (9a), the following provisions apply: (a) the court must order that the person be disqualified from holding or obtaining a driver’s licence— (i) in the case of a first offence—for such period, being not less than 12 months, as the court thinks fit;

Zero Tolerance’ Drug Driving Laws in Australia: A Gap Between Rationale and Form?, International Journal for Crime, Justice and Social Democracy, link

  • “Whereas random breath testing tests for all forms of alcohol and is designed to determine whether there is a sufficient concentration of alcohol in the driver’s body that s/he should be deemed to be impaired, random drug testing typically tests for the presence of any quantity of only the three most widely used illicit drugs—cannabis, methamphetamine and ecstasy—in the driver’s oral fluids, without reference to what is known about the different pharmacokinetic and pharmacodynamic qualities of different drugs. This article examines this idiosyncratic approach to the criminalisation of drug driving, highlighting its weak correlation with the important road safety objective of deterring substance‐impaired driving, and the risks of both over‐ and under‐criminalisation that it creates.”
  • “RDT techniques currently employed in Australia do not test for ‘active’ drugs in a person’s ‘system’, but the presence of any quantity/residue of a drug in a person’s oral fluids.”
  • “The RDT regime in Australia (with the exception of Tasmania) is based on ‘oral fluid testing’ both for the preliminary random test and the final oral fluid test. The preliminary test is generally conducted via a drug wipe stick (such as the Securetec DrugWipe Twin or the Securetec DrugWipe II Twin).16 This involves the driver wiping his/her tongue along the testing stick. If this test is positive, the driver is taken to a roadside testing bus/van (or the police station) to provide a saliva sample, commonly tested by the Drager DrugTest 5000, or the Cozart Drug Detection System (DDS). If this test is positive a direction is normally given to the driver prohibiting him/her from driving (driver directions are discussed below), and this sample is then sent to a laboratory and tested by an analyst to confirm the presence of the drug(s). An analyst’s certificate confirming presence of the relevant drug(s) is admissible as evidence in the prosecution against the driver.”
  • “While the Australian RDT regime relies on oral fluid sampling, the ‘gold standard’ for drug detection is said to be blood sample analysis (Wolff 2013: 57). This is because: ‘oral fluid tests cannot be used to give a precise prediction of the concentration of a drug in blood (or plasma or serum) for confirmation testing and therefore prediction of possible drug effects’ (Wolff 2013: 57). In other words, oral fluid testing is a relatively poor mechanism for assessing whether a person is drug impaired. While oral fluid testing may be appropriate for preliminary testing, it is not well‐ suited to confirmatory testing (Wolff 2013: 57, 129).”
  • “A positive test for THC may result, therefore, from the detection of residual deposits of THC in the mouth. However, the drug may no longer be present in the driver’s bloodstream and is, therefore, unlikely to have an adverse effect on driving ability (Wolff 2013: 65).”

McKinstry 1970, Administrative Law–Kentucky’s” Implied Consent” Statute–Revocation of Motor Vehicle Operator’s License for Refusal to Take Blood Alcohol Test, *Kentucky Law Journal, link

  • “Under the theory of implied consent, any person who operates a motor vehicle on the state’s highways 5 is deemed to have given his consent to a chemical test of his blood, breath, urine, or saliva for the purpose of determining the alcoholic content of his blood, whenever he is arrested for any affense involving the operation of a motor vehicle while under the influence of intoxicating liquor.”
  • “The underlying rationale is: ‘[T]o operate a motor vehicle on a public road is not a natural or unrestricted right but rather a privilege granted by the state and subject to reasonable regulation under the police power of the state. Such a license is issued and accepted under the terms and conditions of the statute.’”
  • “The United States Supreme Court in upholding the validity of the Massachusetts non-resident motorist statute, a form of implied consent, ruled that “in advance of the operation of a motor vehicle on its highways … the state may require” the operator to consent to reasonable conditions to the exercise of this privilege. There have been few challenges to implied consent legislation on the basis of the state’s power to regulate the use of the highways and none have been successful. The courts have looked favorably on these statutes and have consistently rejected constitutional challenges.”