A breath analyzer system for determining an estimate of blood alcohol concentration of a person, said system comprising: a light source configured to project light into a sampling region where breath from the person is expected; a first light detector configured to detect a first light intensity of light emanating from the sampling region at an ethanol gas absorption wavelength, said first light detector further configured to output an ethanol gas signal indicative of an ethanol gas concentration in the sampling region; a second light detector configured to detect a second light intensity of light emanating from the sampling region at a carbon dioxide absorption wavelength, said second light detector further configured to output a trace gas signal indicative of a trace gas concentration in the sampling region; and a controller configured to receive a first ethanol gas signal and a first trace gas signal sampled at substantially a first time, receive a second ethanol gas signal and a second trace gas signal sampled at substantially a second time distinct from the first time, and determine an estimate of the blood alcohol concentration of the person based on the first ethanol gas signal, the second ethanol gas signal, the first trace gas signal, and the second trace gas signal, wherein the controller is further configured to determine an ethanol gas signal change (ΔVE) based on a difference of the first ethanol gas signal and the second ethanol gas signal, determine a trace gas signal change ΔVT based on a difference of the first trace gas signal and the second trace gas signal, and determine an estimate of the blood alcohol concentration of the person based on a ratio of the ethanol gas signal change (ΔVE) to the trace gas signal change (ΔVT), characterized in that, the trace gas is carbon dioxide and said second light detector is configured to output a carbon dioxide gas signal 46 indicative of a carbon dioxide gas concentration in the sampling region, whereby the first trace gas signal is a first carbon dioxide signal and the second trace gas signal is a second carbon dioxide signal, said system further comprising a third light detector configured to detect a third light intensity of light emanating from the sampling region at a water vapor absorption wavelength and output a water vapor signal indicative of a water vapor concentration in the sampling region, and wherein the controller is further configured to receive a first water vapor signal substantially at the first time, receive a second water vapor signal substantially at the second time, and determine an estimate of the blood alcohol concentration of the person based on the first ethanol gas signal, the first carbon dioxide gas signal, the first water vapor signal, the second ethanol gas signal, the second carbon dioxide gas signal, and the second water vapor signal. a light source configured to project light into a sampling region where breath from the person is expected; a first light detector configured to detect a first light intensity of light emanating from the sampling region at an ethanol gas absorption wavelength, said first light detector further configured to output an ethanol gas signal indicative of an ethanol gas concentration in the sampling region; a second light detector configured to detect a second light intensity of light emanating from the sampling region at a carbon dioxide absorption wavelength, said second light detector further configured to output a trace gas signal indicative of a trace gas concentration in the sampling region; and a controller configured to receive a first ethanol gas signal and a first trace gas signal sampled at substantially a first time, receive a second ethanol gas signal and a second trace gas signal sampled at substantially a second time distinct from the first time, and determine an estimate of the blood alcohol concentration of the person based on the first ethanol gas signal, the second ethanol gas signal, the first trace gas signal, and the second trace gas signal, wherein the controller is further configured to determine an ethanol gas signal change (ΔVE) based on a difference of the first ethanol gas signal and the second ethanol gas signal, determine a trace gas signal change ΔVT based on a difference of the first trace gas signal and the second trace gas signal, and determine an estimate of the blood alcohol concentration of the person based on a ratio of the ethanol gas signal change (ΔVE) to the trace gas signal change (ΔVT), characterized in that, the trace gas is carbon dioxide and said second light detector is configured to output a carbon dioxide gas signal 46 indicative of a carbon dioxide gas concentration in the sampling region, whereby the first trace gas signal is a first carbon dioxide signal and the second trace gas signal is a second carbon dioxide signal, said system further comprising a third light detector configured to detect a third light intensity of light emanating from the sampling region at a water vapor absorption wavelength and output a water vapor signal indicative of a water vapor concentration in the sampling region, and wherein the controller is further configured to receive a first water vapor signal substantially at the first time, receive a second water vapor signal substantially at the second time, and determine an estimate of the blood alcohol concentration of the person based on the first ethanol gas signal, the first carbon dioxide gas signal, the first water vapor signal, the second ethanol gas signal, the second carbon dioxide gas signal, and the second water vapor signal.