The temperature of the subject is very important, due to homeostatic variables. Breathalysers can be very sensitive to temperature and will give false readings if not adjusted or recalibrated to account for ambient or surrounding air temperatures.

Breathing pattern can also significantly affect breath test results. One study found that the BAC readings of subjects decreased 11-14% 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 one’s breath for 30 seconds can increase the breath test result by about 28%.

The National Highway Traffic Safety Administration (NHTSA) also found that dieters and diabetics may have acetone levels hundreds or even thousand of times higher than those in others. Acetone is one of the many substances that can be falsely identified as ethyl alcohol by some breathalysers. However, fuel cell based systems are non-responsive to substances like acetone.

A study in Spain showed that metered-dose inhalers (MDIs) used in asthma treatment are also a cause of false positives in breath machines. In general, evidential breathalysers such as the Draeger 6820 are highly resistive to such issues as they are specifically designed for testing an “unwilling” subject. This why a personal, semi-conductor type breathalyser should never be used for testing anyone other than the owner, who is well aware of anything they may have taken that could affect the result.