[Copyrighted Material Omitted]
[164 Cal.Rptr.3d 523]
Charles M. Sevilla, San Diego, under appointment by the Supreme Court, for Defendant and Appellant.
Michael J. Fremont, Encinitas; Bartell & Hensel, Riverside, Donald J. Bartell and Lara J. Gressley for the California DUI Lawyers Association as Amicus Curiae on behalf of Defendant and Appellant.
Jan I. Goldsmith, City Attorney, Tricia Pummill, Assistant City Attorney, Marlea F. Dell'Anno and Angie M. Reddish-Day, Division Chiefs, and Jonathan I. Lapin, Deputy City Attorney, for Plaintiff and Respondent.
[312 P.3d 1046] In this prosecution for driving while having a proscribed alcohol concentration in blood or breath, the trial court excluded expert witness testimony challenging the reliability of breath-alcohol testing machines. Reviewing the ensuing conviction, the Court of Appeal determined that the trial court erred, and reversed for a new trial. We conclude that the trial court properly excluded the challenged expert testimony with respect to the charge of driving while having a proscribed alcohol concentration. Accordingly, we reverse the judgment of the appellate court and affirm the conviction.
I. Facts and procedure
On December 22, 2007, at approximately 2:30 a.m., Sergeant Richard W. Berg of the California Highway Patrol (CHP) observed defendant, who was then 50 years of age, driving a high-performance pickup truck more than 125 miles per hour on Highway 163 in San Diego County. Sergeant Berg followed defendant for more than five miles and eventually caught up with him, noticing that he slowed to 100 miles per hour when he approached other traffic and was not weaving outside his lane. After traveling an additional
mile and one-half, Berg activated his patrol vehicle's red lights. Defendant rapidly decelerated, pulled over, and, as directed, proceeded to a wider spot on the shoulder.
Defendant provided his license and registration and said, " I was just screwing around." Berg detected an odor of alcohol [164 Cal.Rptr.3d 524] and noted that defendant's eyes were red and watery. Defendant admitted he had consumed two glasses of wine. Berg called for backup and transferred the matter to two other CHP officers, including Gerald Guzman, who arrived at 2:58 a.m.
Officer Guzman gave defendant field sobriety tests, in which defendant exhibited few signs of impairment. Defendant told Guzman he had three glasses of wine at dinner. Guzman thought defendant smelled of alcohol and noticed that his eyes were red and glassy. Soon thereafter, at approximately 3:10 a.m., defendant consented to two in-field preliminary alcohol screening breath tests using an Intoximeter Alco-Sensor IV. Such a test is " preliminary" in the sense that it is employed— only with the driver's actual consent— prior to any arrest, in order to assist an investigating officer in determining whether to arrest the driver. Standard [312 P.3d 1047] testing protocol required that a subject be observed continuously for 15 minutes before the test was administered, in order to make sure that the subject had not during that time ingested alcohol or done anything else that might compromise the test (see Cal.Code Regs., tit. 17, § 1219.3, 3d sentence), but Officer Guzman waited and observed for only nine or 10 minutes, reasoning that defendant had generally been under earlier surveillance for many minutes by Sergeant Berg. Guzman testified that defendant displayed an alcohol concentration of 0.095 percent on the first test, and 0.086 percent on the second test which was administered two minutes later.
Based on his observations and the preliminary breath tests, Guzman believed that defendant was under the influence of alcohol, arrested him, and transported him to the county jail. There, defendant was subject to additional chemical testing under the implied consent law, Vehicle Code section 23612 (subsequent statutory citations are to this code unless otherwise indicated), which provides for testing of blood, breath, or urine. He elected breath testing, which was conducted using an Intoximeter EC/IR. The first breath test from that device, taken at 3:37 a.m., registered a level that revealed an 0.08 percent alcohol level, and the second test, taken two minutes later, produced the same result.
Finally, defendant additionally consented to a blood test, taken at 3:52 a.m. These samples, tested at a later date, showed alcohol percentage levels of 0.088 and 0.087, respectively.
The San Diego City Attorney's Office filed a misdemeanor complaint charging defendant with (1) driving under the influence of alcohol in violation of section 23152, subdivision (a) (hereafter section 23152(a))— sometimes referred to as the " generic offense" ; and (2) driving with a blood-alcohol concentration of 0.08 percent or more, in violation of section 23152, subdivision (b) (hereafter section 23152(b))— sometimes referred to as the " per se offense." 
[164 Cal.Rptr.3d 525] Prior to trial, defendant filed motions to (1) exclude evidence of the PAS tests to establish blood-alcohol content, asserting they were not conducted in accordance with applicable regulations and were unreliable; and (2) allow evidence concerning " partition ratio variability" (described post, pt. II.A.3.) with regard to the generic offense. The People filed opposing motions. The trial court deferred ruling on the PAS and partition ratio evidence issues, and the matter proceeded to trial.
A. The People's evidence
CHP Officer Brandon Garland, who was responsible for maintaining and calibrating the agency's PAS breath-testing devices, testified regarding the
requirements for successfully conducting a PAS test. Garland's [312 P.3d 1048] records showed that the particular Intoximeter Alco-Sensor IV PAS machine used to analyze defendant's breath at the scene of the vehicle stop had been tested earlier that week and had been found to be operating within acceptable limits of error. 
Marissa Ochoa, a criminologist at the San Diego County Sheriff's Regional Crime Laboratory, testified that both the particular Intoximeter EC/IR breath machine and the blood sampling device used to test defendant at the county jail were in proper working order when used to take and measure samples from defendant, and that the Intoximeter EC/IR breath-testing machine in question has an operational range within the accepted limits of error (see ante, fn. 4). Ochoa further testified regarding the effect of alcohol on the human body. She was asked to assume that a healthy male weighing 200 pounds had consumed three glasses of wine with a hearty meal between 8:00 and 9:00 p.m., was pulled over while driving at 2:45 a.m., and thereafter at the jailhouse produced a breath test result of 0.08 percent at 3:37 a.m. and a similar blood test result 15 minutes later. She concluded that under normal bodily processing of alcohol, such a " person[']s alcohol level [would] be 0.09, approximately, at the time of driving." On [164 Cal.Rptr.3d 526] cross-examination, Ochoa explained that if an average healthy male weighing 200 pounds had ingested alcohol only during a dinner between 8:00 and 9:00 p.m., he would have needed to consume approximately 11 drinks (each drink defined as a 12-ounce beer, a 4-to 6-ounce glass of wine, or a 1- to 1.25-ounce serving of hard liquor) in order to have a 0.08 percent blood-alcohol level seven and one-half hours later.
B. Defendant's evidence
1. Initial testimony by Dr. Hlastala concerning asserted unreliability of breath-testing machines in light of alcohol that is " picked up" during inhalation before reaching the deep lungs
Defendant called Dr. Michael P. Hlastala, professor of medicine, physiology, biophysics, and bioengineering at the University of Washington, to testify concerning the effects of alcohol on the respiratory system and the reliability of breath-testing machines. Hlastala explained that his field of
study focuses on " the physiology of the human body, which means I deal with the lungs, blood and the way that substances move around the body ... and one of those is alcohol, [which] I've been studying ... for about 25 years, in terms of the physiology of alcohol and also the way alcohol is measured in testing procedures." He related that he had written more than 400 articles, including approximately 170 peer-reviewed professional articles, and a textbook. He explained that he had testified in approximately 30 states, and served on committees of the National Institutes of Health. The People did not dispute Hlastala's expertise.
Defense counsel asked Dr. Hlastala to assume that both a preliminary " roadside breath test" (i.e., PAS) machine and an " EC/IR" breath-testing machine are " working perfectly, and they've been properly done. Do they provide a scientifically accurate test? " Hlastala replied, " Well, they don't. " (Italics added.)
After a sidebar conference, Dr. Hlastala continued by explaining " the way we take oxygen and put it into the body" and " the way we eliminate carbon dioxide" from the body. He used a pen to create a diagram and explained: " Starting from the throat, the windpipe is called the ‘ trachea.’ The trachea comes down ... to just about the heart, ... and that splits off into a left and right side, and that splitting goes on ... about 20 times before it gets into the air [sacs]" deep in the lungs, known as the alveolar sacs. " There are about 300 million of these [alveolar sacs] in every valve. They are ... very tiny.... There's blood vessels around that and this is where a lot of the action takes place. If we bring outside air with oxygen in it, the oxygen [312 P.3d 1049] goes into the blood, ... and then it gets ... metabolized and that provides energy for us."
Thereafter, Dr. Hlastala began to testify that even if breath-testing machines operate as designed, they do not perform a scientifically reliable test, because some alcohol present in mucus membranes and bronchial vessels in the upper airways absorbs into the breath before it finally reaches the alveolar sacs in the deep areas of the lungs. Specifically, he stated:
" Now, when the breath test was developed in the 1950s, it was understood that if there was alcohol in the blood, that some of it would get out into ... the [alveolar] air sac[ ]s and ... the idea ... of a breath test" is to take this air originating in the alveolar sacs " and breathe it out, and then measure it ... with the breath test instrument.... [¶] The concept is that this alcohol in this [sampled] air, [is] equal to what's down in here [in the alveolar sacs], [and] hence [it's] related to whatever's in the blood. We know, now, that it's not [164 Cal.Rptr.3d 527] quite that simple because alcohol is quite soluble, it goes into water quite easily. And we have, in the airway, a lot of mucus and water and that mucus lining in the airway plays an important role in protecting us from particles and things we inhale[,] goes on to this mucus, then comes out to the mouth."
Dr. Hlastala continued: " But if we have alcohol, there are little blood vessels that come along here, and these blood vessels, those are called ‘ bronchial vessels.’ And so they bring alcohol so there's a lot of alcohol if you have alcohol in your bloodstream. Now, what happens is if we inhale and we pick up alcohol from this mucus ... by the time we get down to this [alveolar] air sac[ ], it's already filled up and saturated. " (Italics added.)
In other words, Dr. Hlastala informed the jury that although breath-testing machines are designed to sample and analyze the concentration of alcohol contained in alveolar, deep lung air, they fail to do so. At this point the prosecutor objected to Dr. Hlastala's testimony. The trial court excused the jury until later in the afternoon and conducted a hearing regarding the admissibility of the initial and further proposed testimony.
2. Hearing concerning additional proposed testimony by Dr. Hlastala that other physiological factors make breath-testing machines unreliable
Outside the jury's presence, the trial court questioned whether testimony by Dr. Hlastala would constitute evidence concerning " partition ratio variability," which we had earlier held in Bransford, supra, 8 Cal.4th 885, 35 Cal.Rptr.2d 613, 884 P.2d 70, to be inadmissible in prosecutions under the per se statute, section 23152(b). Thereafter, under questioning by counsel and the court, Dr. Hlastala testified that various other factors can cause a breath-alcohol test to be scientifically inaccurate. He identified those other factors as the pattern of breathing (speed and depth of inhalation and exhalation); body and breath temperature; and hematocrit level (ratio of red blood cells to total blood volume).
On cross-examination the prosecutor asked: " Doctor, in this case, we have a result from an EC/IR [breath-testing machine] of 0.08. In your opinion, what does that 0.08 measure?" Dr. Hlastala responded: " It's measuring the alcohol concentration of the breath that is delivered to the machine." The prosecutor asked, " Is there any reason to believe, in this case, that [reading] was inaccurate?" Dr. Hlastala responded, " It's an inaccuracy in how ... the alcohol comes out of the mouth" because " from time to time, a person ... can deliver different amounts of alcohol through the mouth ... through the breath."
The prosecutor continued: " So, in your opinion, breath tests are inherently inaccurate as a measure of how much alcohol a person has in them? " Dr. Hlastala answered: " They are. And primarily because the basic
assumption that all of the manufacturers have used is that the breath that [is] measured is directly related to water in the lungs, which is directly related to what's in the blood. And in recent years, we've [312 P.3d 1050] learned that, in fact, that's not the case." (Italics added.)
The prosecutor followed up: " So that ... wouldn't mean that the machine has, in a given sample, not measured it accurately?" Dr. Hlastala responded, " No. I'm assuming that the machine is working accurately." He elaborated by stating that [164 Cal.Rptr.3d 528] breath-testing machines cannot reliably reflect the concentration of alcohol in blood because they constitute " an indirect test" of blood alcohol. He explained that the best sample of blood would be that found in the brain, and that a sample from venous blood " is the next best thing." As an alternative to such a blood sample, " the breath is the next best thing. [And then] [t]he odor of alcohol is the next best thing. They're all remote, the more remote the more variable."
At this point the court reviewed with Dr. Hlastala the other factors that, according to his proposed testimony, render breath testing scientifically unreliable. In addition to the factors that Hlastala had focused on earlier in the hearing outside the jury's presence— pattern of breathing (speed and depth of inhalation), body and breath temperature, and hematocrit level (ratio of red blood cells to total blood volume)— the witness stated that additional relevant factors include sex (explaining that " [w]omen have a smaller lung, therefore a higher breath test relative to that blood" ), and " medical condition," such as " lung diseases."
The court observed that California law, in correlating breath alcohol with blood alcohol, applies " what's known as the ‘ standard partition ratio,’ which I understand is grams of alcohol in 210 liters [of breath] equals grams of alcohol per 100 milliliters of blood" — and the court asked the witness if he believed that " the [true] ratio of one person might be different [from the ratio in] someone else?" Dr. Hlastala responded: " That's correct.... [S]ome people would be different." The court pressed, asking: " Now, ... basically what you're saying here as to why this [breath machine] test isn't scientifically reliable, [is] because it automatically applies this standard partition ratio?" Dr. Hlastala replied: " No, not really. I'm not talking about the partition ratio. I'm talking about factors that influence the breath [sample], breath-alcohol [sample], and I'm not talking about comparing it to blood."
The court continued, " [I]n this particular case, are you prepared to state that the breath sample of defendant as being 0.08 would be ... overstating or understating?" Dr. Hlastala responded that it could be either, and that he had no opinion whether defendant's breath test result of 0.08 percent was actually (or even probably) overstated or understated.
The court asked Dr. Hlastala to step outside and then entertained further argument from counsel. The prosecutor observed that section 23152 (b) criminalizes driving with a proscribed blood-alcohol level or a certain proscribed breath-alcohol level as measured by a properly calibrated and administered breath-testing machine— and he asserted that " the question of whether the breath [sample and result] accurately reflects anything else is irrelevant." The prosecutor argued that in this context a defendant should not be allowed to present expert testimony that " there was something wrong with the way that the State of California has drafted its laws." The prosecutor moved to " strike any testimony that the doctor gave with regard to the diagram" that he had drawn during his testimony before the jury, and asked " that the diagram be removed from the view of the jury.... Because that's all about how ... alcohol came from the blood and into the trachea, and how ... alcohol [is brought in breath] from the trachea and other sources."
Although the court appeared to agree with the prosecutor that " at some point" it would instruct the jury not to consider Dr. Hlastala's testimony that even if breath-testing machines are working properly, their samples and results are not scientifically reliable due to physiological factors, the court was unwilling to so instruct the jury immediately. Instead, the court said [164 Cal.Rptr.3d 529] it would " think about it" — but that " [a]t a minimum, I'm going to instruct counsel not to argue that." 
[312 P.3d 1051] Eventually the trial court ruled— over defendant's objections— that Dr. Hlastala's proposed testimony was speculative and did not materially differ from evidence concerning partition ratio variability, which we had held in Bransford, supra, 8 Cal.4th 885, 35 Cal.Rptr.2d 613, 884 P.2d 70 (discussed post, pt. II.B.2.), to be inadmissible in prosecutions under the per se statute, section 23152(b). Accordingly, the trial court ruled that although it would allow Dr. Hlastala to resume testifying before the jury, there were to be " [n]o questions to this expert, which will solicit any testimony by him" that, as a general matter, properly working breath-testing machines do not sample air as they are designed to and do not produce reliable results because of the factors mentioned in his prior (prehearing) testimony before the jury or in his subsequent testimony outside the jury's presence. The court made clear, however, that defendant remained free to argue, and present evidence, that the particular machines used in this case malfunctioned, were not properly calibrated, or that they were not properly employed. Moreover, the court reiterated, Dr. Hlastala could be questioned about the contaminating effect of
" mouth alcohol" (alcohol that stays in the mouth after ingestion, and that dissolves into mouth tissues) on the reliability of a breath test sample and result.
3. Resumed limited testimony by Dr. Hlastala
Thereafter, Dr. Hlastala resumed the stand and testified that the presence of mouth alcohol can cause a " false positive test," meaning a higher result than would be obtained otherwise. Dr. Hlastala also testified that " retrograde extrapolation" — estimating a blood-alcohol level that existed at an earlier time based on a reading at a known later time, as the People's witness, Ochoa, had undertaken— was " difficult" to do " over long periods of time." He did not ...