Prior to administering a breath test to you, Washington law requires that the officer check in your mouth to confirm that you have nothing (other than fixed dental work) there. This would include a penny, mouth jewelry, or any other foreign object. Regardless of whether or not a penny can effect the actual breath test results, a breath test administered while you had a penny in your mouth would not be legally admissible.
There is zero credible information available indicating that this would have any effect on alcohol breath test. Also know that If you put something in your mouth with intent to get a lower reading, particularly if that item is porous or has cavities in it that can collect mouth alcohol, there exists a possibility that your mouth will retain alcohol to a higher degree than if you had nothing in your mouth at all. Tooth cavities and poor dental work can also impact a breath test. Experts have testified that this can theoretically raise the numerical reading or cause the breath alcohol machine to give an error message.
Because of the 15 minute observation period under Washington State Law, and the mouth check that preceeds it - the liklihood that you'd be able to keep a penny under your tongue for a BAC is low. But even so - any foreign object in your mouth would indicate no mouth check was done, so the test would be invalid, regardless of the results.
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