Cops must ensure an arrested DUI subject is not adversely affected by her or his ambient environment while breathing properly and not holding breath
Breath testers can be very sensitive to temperature, for example, and will give false readings if not adjusted or recalibrated to account for ambient or surrounding air temperatures. The temperature of the subject is also very important.
Breathing pattern can also significantly affect breath test results. One study found that the BAC readings of subjects decreased 11 to 14% after running up one flight of stairs and 22-25% after doing so twice. Another study found a 15% decrease in BAC readings after vigorous exercise or hyperventilation. Hyperventilation for 20 seconds has been shown to lower the reading by approximately 32%. On the other hand, holding your breath for 30 seconds can increase the breath test result by about 28%.
Some breath analysis machines assume a hematocrit (cell volume of blood) of 47%. However, hematocrit values range from 42 to 52% in men and from 37 to 47% in women. A person with a lower hematocrit will have a falsely high BAC reading.
Improper maintenance & calibration can further provide a falsely elevated reading on a breath test machine
Failure of law enforcement officers to use the devices properly or of administrators to have the machines properly maintained and re-calibrated as required are particularly common sources of error. However, most states have very strict guidelines regarding officer training and instrument maintenance and calibration.
Most handheld breathalyzers use a silicon oxide sensor to determine the blood alcohol concentration. Without proper software calibration, the accuracy of these sensors degrades over time and with repeated use. The calibration process aims to focus the sensor's ability to detect an accurate reading. New advances in breathalyzer design allow some models to self-calibrate or easily replace the sensor module without the need to send the unit to a calibration lab.
Research indicates that breath tests can vary at least 15% from actual blood alcohol concentration. An estimated 23% of individuals tested will have a BAC reading higher than their true BAC.
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