New Jersey’s driving while intoxicated (DWI) statute allows law enforcement officials to obtain breath samples, for the purpose of measuring a suspect’s blood alcohol concentration (BAC), without a warrant. The Fourth Amendment to the U.S. Constitution requires police to obtain a warrant, supported by probable cause, before conducting a search. Multiple courts, however, have held that the collection of breath samples falls under an exception to the warrant requirement. One way to explain why a warrant is not required is that people do not have a reasonable expectation of privacy in the contents of their breath. This may seem a bit trite, but it makes some sense when compared to another method of measuring BAC, blood testing. Collection of a blood sample is indisputably invasive, so courts have held that a warrant is required in most cases.
Police do not have to obtain a warrant before conducting a search, according to a long line of court decisions, if “exigent circumstances” exist that risk the imminent loss or destruction of evidence. If, for example, a police officer has reason to believe that a person inside a residence is about to destroy contraband, such as by flushing it down the toilet, courts have held that they may enter the residence for the purpose of securing the contraband. The authority of police officers to search a vehicle without a warrant, known as the vehicular exception, is based on the same principle as exigent circumstances. Since a car is mobile, waiting to obtain a warrant risks the suspect fleeing the scene. An officer must still have probable cause to believe that the car contains contraband or evidence of a crime, but they do not necessarily have to have a warrant.
Drawing blood from a DWI suspect without a warrant, in order to test their BAC, has been justified by exigent circumstances in the past. The U.S. Supreme Court allowed this in its ruling in Schmerber v. California in 1966. The court significantly limited this practice, however, in a 2013 ruling, Missouri v. McNeely. Prosecutors had argued that the human body’s natural process of metabolizing alcohol, meaning that a person’s BAC steadily decreases over time, was an exigent circumstance allowing a warrantless blood draw. The Supreme Court rejected this argument. While it may still be possible to justify a warrantless blood draw based on exigent circumstances, the breakdown of alcohol in the bloodstream cannot, by itself, serve that purpose.