Which of the following factors can influence the accuracy of breath alcohol measurements?

The choice regarding the presence of other substances influencing the accuracy of breath alcohol measurements is correct because various substances can interfere with the test results produced by the Intoxilyzer 9000. For example, compounds present in the mouth, such as mouthwash, certain medications, or even residual alcohol from previous consumption, can lead to false readings because the breath sample may contain trace amounts of these substances that the device is unable to differentiate from alcohol.

In contrast, while factors like the time of day or air temperature can affect physiological conditions or sensor performance, they are not direct influencers of the measurement accuracy in the same way substances present in the mouth are. The type of mouthpiece used may impact hygiene or sample collection but does not directly alter the measurement accuracy of breath alcohol concentration. Therefore, the presence of other substances is a critical factor in ensuring accurate breath alcohol measurement outcomes.