Passive alcohol sensors are devices that detect the presence of alcohol by sampling the normally exhaled air in front of a person. They supplement traditional police procedures used to assess whether a driver has consumed sufficient alcohol to be impaired and are not intended to provide a quantitative evidential test. A prototype unit was developed to enable police departments to evaluate the utility of these devices for increasing the initial detection of alcohol-impaired drivers by police in a variety of patrol situations. The prototype uses a fuel cell detector specific to alcohol that minimizes interference from nonalcohol substances, such as cigarette smoke, in the air being sampled. The unit consists of a modified flashlight with an alcohol detector incorporated in the head of the flashlight. A small pump draws a sample of air into the unit, which then gives a digital readout of the amount of alcohol present in the sample. The sensitivity of the unit to the sampling distance from the subjects mouth was established together with the variability between subjects in laboratory tests with drinking subjects.

The passive alcohol sensor was tested at a series of sobriety checkpoints. The results showed that without the sensor police officers detected only 45 percent of drivers with BACs greater than 0.10 percent, whereas with the sensor the detection rate increased to nearly 70 percent. Equally important, the sensor significantly reduced the number of drivers who were unnecessarily detained (i.e., drivers thought to be impaired but who had BACs of less than 0.05 percent) from 18 percent to 8 percent.