Police officers in Missouri and around the country use portable breath-testing devices to find out if motorists are driving under the influence of alcohol, but they currently have no tools that can effectively identify marijuana impairment. Several companies are attempting to solve this problem by developing devices that measure THC levels using a breath sample, but this kind of equipment may not provide much in the way of reliable evidence even if it functions properly.

Establishing that a driver is impaired by marijuana is a challenge because THC does not affect the body in the same way alcohol does. A blood alcohol concentration of .08% or higher is enough to intoxicate even a heavy drinker, but regular marijuana smokers develop a tolerance for the drug. This means that high levels of THC in the blood are not enough to establish intoxication beyond a reasonable doubt.

The inability to link THC levels to impairment also presents a challenge to lawmakers tasked with drafting impaired driving legislation. The results of a roadside breath test are enough to support a DUI arrest, but determining whether or not to take a motorist into custody for getting behind the wheel after consuming marijuana is a far more nebulous process. Another problem facing law enforcement and prosecutors is that THC can be detected in blood tests for up to 30 days after marijuana has been consumed.

Experienced criminal defense attorneys may seek to have DUI charges dismissed when they are not supported by compelling scientific evidence. Attorneys could also question the reliability of blood and breath-test results in certain situations. Blood tests could be challenged when a clear chain of custody is lacking and samples may have been mishandled or mixed up, and attorneys may dispute the accuracy of breath tests when the equipment used was not properly maintained or officers deviated from strict testing protocols.