Driving under the influence of marijuana is illegal in New York as well as all other states. But unlike intoxication by alcohol, which can be measured by authorities at a stop, there is no reliable breath test for marijuana intoxication. That could change as some companies are working to develop breath testing devices for marijuana impairment.
According to the National Institute of Standards and Technology, measuring a person’s breath for marijuana is far more complicated than measuring breath for alcohol. In order for a breath test to work, the vapor pressure of a compound must be measured. Until recently, the vapor pressure of marijuana had never been measured. But the agency reported on July 5 that some of its scientists were able to accomplish this for the first time. Specifically, they were able to measure the primary psychoactive compound in marijuana, delta-9 tetrahydrocannabinol, or THC.
According to a NIST chemical engineer, vapor pressure involves the transition of a compound from a liquid to a gas. When a molecule of a substance leaves the blood and is exhaled, measuring the vapor pressure is necessary to measure blood levels by a breath test.
The NIST considers this breakthrough in THC vapor pressure measurement to be the groundwork for manufacturers to develop reliable marijuana breath testing devices. The agency says that more research will need to be done to determine what levels of THC indicate impairment of the ability to drive.
Roadside tests for intoxication are not proof that a driver is guilty of driving under the influence. The results of breath tests can be wrong. A charge of drunk driving or DUI can be challenged in court. Some defenses that could be presented include the contention that the machine used to conduct the test was improperly calibrated.