PER CURIAM.

Plaintiff Calvert Sims appeals from a September 29, 2009 Law Division order that granted the motion of defendant C & D Technologies, Inc. for a directed verdict. On appeal, plaintiff maintains that the judge erred by concluding that: his proposed liability expert, Mark J. Keith, P.E., lacked sufficient expertise to offer an expert opinion on the cause of the battery failure and ensuing emission of sulfuric acid; and Keith's opinion was an inadmissible net opinion. We agree with the latter conclusion and therefore do not reach the other basis upon which the judge relied when she excluded Keith's testimony. We affirm the order under review.

I.

For approximately four years, plaintiff was employed by Ceres Terminals, Inc., which operated a facility for marine and rail transportation. Plaintiff worked at Ceres's Weehawken office, which included a dedicated air-conditioned room where the company's computer equipment was located. The computer equipment was connected to an uninterruptible power supply (UPS) unit, which provided emergency back-up power in the event that Ceres's computers lost power. The UPS incorporated a group of twenty rechargeable valve-regulated, lead-acid batteries (VRLA), all of which were manufactured by defendant C & D Technologies, Inc. (C & D).

On March 29, 2005, one or more of the batteries that comprised the UPS emitted sulfuric acid fumes as part of a "thermal runaway," a term used to describe the overheating of batteries and the resulting release of noxious battery fumes. Plaintiff alleged that he suffered serious health problems as a result of the exposure to the sulfuric acid during the thermal runaway. The manufacturer of the UPS, Eaton Corporation, was dismissed from the case on summary judgment, leaving C & D as the only remaining defendant. A different judge later granted C & D's motion for partial summary judgment, limiting plaintiff's cause of action against C & D to a claim that the VRLA batteries were defectively manufactured.

During the pretrial discovery period, plaintiff furnished an expert report from Keith, who was plaintiff's sole liability expert. Before describing Keith's September 3, 2008 report, we pause to describe the trial testimony concerning the service calls that Ceres initiated in the two months preceding the March 29, 2005 thermal runaway. In January 2005, Eaton technician David Thornton responded to a service call at Ceres regarding an unpleasant odor emanating from the batteries. Thornton concluded that the UPS was not malfunctioning, but the batteries "needed to be replaced." Thornton did not elaborate on his reason for so concluding. The actual replacement of the twenty batteries was completed by a company known as PowerServe. PowerServe technician John Lichtenstein replaced all twenty of the batteries and was satisfied that all of the newly-installed batteries were free of any defects and were functioning properly.

A few weeks later, on February 2, 2005, Thornton responded to another service call at Ceres because the UPS unit had shut down. Thornton was able to restart the UPS unit, which meant, according to Thornton, that the batteries had not failed because "[i]f it had been the batteries, the [UPS] unit wouldn't have restarted."

On the day in question, March 29, 2005, Thornton was again called to the Ceres facility in response to a strong "rotten egg" smell that was especially strong in the computer room where the UPS unit was located. By checking the "charging levels" of the UPS unit, Thornton was able to determine that the unit was not running properly. He shut down the UPS unit by placing it in bypass mode and proceeded to open the cabinet door to check the batteries. When he opened the cabinet door, he noted that the batteries were "very hot," some of them were leaking, and a strong "rotten egg" smell was present. Because the batteries were leaking and emitting the smell, neither of which was a normal condition for the batteries, Thornton arranged for the replacement of all twenty batteries, commenting that it was "unusual" to replace a full set of batteries after only seventy days of operation. Once the batteries were replaced, he was able to restart the UPS unit with no difficulty.

Thornton explained that VRLA batteries are designed with a "vent system" to reduce the internal pressure of the battery and release "the gas" if the battery begins to "overcharge" or develop excess pressure. He explained that because there was sulfuric acid gas coming out of the batteries on March 29, he ordered the replacement of the entire battery bank "to protect the customer," Ceres. The batteries that were removed were discarded and not preserved.

With those events as background, we turn to the opinions expressed by Keith. In his cover letter enclosing his September 3, 2008 report, Keith observed that a manufacturing defect in the batteries was one of three possible causes of the thermal runaway that had resulted in the release of sulfuric acid fumes on March 29, 2005. Keith was not, however, able to determine the precise role any such manufacturing defect would have played in the events of that day. He wrote:

In his eight-page report, Keith noted that he had not been able to examine the actual batteries as they had been discarded. He did, however, review deposition transcripts and exhibits, numerous scholarly articles and the service records provided by Thornton and Lichtenstein. In his report, Keith again concluded that plaintiff's injuries resulted from a "product defect or negligence," but he was unable to specify which product — the UPS unit or the batteries — contained such a defect. He opined:

Keith additionally stated in his report:

During a hearing held outside the presence of the jury, Keith testified consistently with his report, stating that the cause of the March 29, 2005, incident was "undetermined because we did not have the facts. We did not have the batteries and the UPS was not — we didn't have an inspection on the UPS."

When asked to describe his qualifications, Keith conceded at the hearing that he is "not knowledgeable of battery technology, the chemical reactions that are the battery and the, the components that are — that happens in the chemical reaction." Similarly, in his deposition, he admitted that he was not "a UPS expert or a battery expert."

In terms of practical experience, Keith testified that he had approximately one year of hands-on experience working for Exide, where he examined malfunctioning batteries and tested them to determine the source of the malfunctions. However, he admitted that he had not "worked in the field [i]n an extended period of time in [his] career focused on battery technology."

At the conclusion of the N.J.R.E. 104(a) hearing, the judge granted C & D's motion to bar Keith's testimony, finding that: 1) Keith did not have sufficient expertise to offer an opinion regarding whether the batteries were defectively manufactured and whether any such defect caused, or contributed to, plaintiff's toxic chemical exposure; and 2) Keith's opinion amounted to a net opinion. After the judge barred Keith's testimony, plaintiff's counsel acknowledged that without Keith's testimony he could not prove liability on the part of defendant; the judge then entered a directed verdict in favor of C & D and dismissed the jury.

II.

When we review a trial judge's decision excluding expert testimony, our scope of review is narrow. Hisenaj v. Kuehner, 194 N.J. 6, 12 (2008). We are "limited to examining the decision for [an] abuse of discretion." Ibid. We may not sift through the record on which the trial court's admissibility determination was based "to create anew the record." Ibid.

When conducting a N.J.R.E. 104(a) hearing, the trial judge's role is "to act as [a] gatekeeper[]" to ensure that the threshold for the admission of expert testimony is satisfied. Id. at 16. That threshold is derived from N.J.R.E. 702, which provides:

In addition to determining whether a witness is qualified to testify as an expert, the trial court must also decide whether the expert's opinion is, as required by N.J.R.E. 703, based on "facts or data." An opinion of an expert that does not provide the "why and wherefore" of the expert's opinion, and provides only a "mere conclusion" will be deemed an inadmissible net opinion. Id. at 24. "The net opinion rule is a prohibition against speculative testimony." Grzanka v. Pfeifer, 301 N.J.Super. 563, 580 (App. Div.), certif. denied, 154 N.J. 607 (1997). "Under this doctrine, expert testimony is excluded if it is based merely on unfounded speculation and unquantified possibilities." Vuocolo v. Diamond Shamrock Chem. Co., 240 N.J.Super. 289, 300 (App. Div.), certif. denied, 122 N.J. 333 (1990). The rule requires an expert "to give the why and wherefore" of his or her opinion rather than a mere conclusion. Jimenez v. GNOC, Corp., 286 N.J.Super. 533, 540 (App. Div.), cert. denied, 145 N.J. 374 (1996).

Moreover, when an expert opinion is unsupported by factual evidence, and lacks a foundation in the record, it is inadmissible as a net opinion. Ibid. See also Johnson v. Salem Corp., 97 N.J. 78, 91 (1984).

Applying the standard established in Johnson, Grzanka, Jimenez and Vuocolo, we are satisfied that Judge Tolentino did not abuse her discretion when she concluded that Keith's opinion was an inadmissible net opinion. We reach that conclusion for two reasons. First, Keith opined that the only way a manufacturing defect in the batteries could have caused plaintiff's chemical exposure was "[i]f there was a fracture of the UPS battery casing," yet nothing in the record supports the conclusion that any of the batteries were cracked. In his report, Keith concluded that because Thornton observed battery leakage, the battery's casing must have been cracked, but this conclusion is not supported by Thornton's testimony, in which he was asked if he had seen any "cracked cases on any of these batteries," and he answered "no." Moreover Lichtenstein, who actually removed and replaced the batteries on behalf of PowerServ, testified that the batteries, prior to their removal, "looked normal." Although Lichtenstein was not specifically asked whether he had observed any cracks in the casing, his testimony that the batteries "looked normal" would presumably exclude the possibility that he had observed a crack.

Thus, because Keith's opinion of a manufacturing defect is premised upon the fact that the battery casing was cracked1 — a conclusion that finds no support in the record — his opinion constitutes a net opinion. Johnson, supra, 97 N.J. at 91. As in Johnson, where the record contained no underlying facts to support the expert's opinion that the machine in question was safe, ibid., here too the record is devoid of any underlying fact to support Keith's opinion that the batteries were cracked or that any such cracks, if they existed at all, were the result of a manufacturing defect.

Second, because Keith admittedly did not have all of the necessary facts to prove a single hypothesis on the cause of the incident in question, he was required to first rule in all plausible causes, and then systematically rule out all but the most likely cause, Creanga v. Jardal, 185 N.J. 345, 356 (2005), but did not do so. As we have noted, Keith acknowledged in his report that any one of three causes could have been the "individual" cause of the thermal runaway: a "defectively manufactured battery," or "improper maintenance of the UPS system," or "negligent substitution of UPS batteries." Alternatively, according to Keith, those three causes when "combined" could have led to plaintiff's exposure to battery fumes on March 29, 2005.

Ultimately, Keith was only able to conclude that a "product defect or negligence" was the cause of the chemical emissions. He was unable to determine which one, acknowledging at the N.J.R.E. 104(a) hearing that the cause of the March 29, 2005 incident was "undetermined" because he had been unable to inspect either the batteries or the UPS unit.

As defendant argues, the evidence in the record, namely the testimony of Thornton, establishes that VRLA batteries are designed with a valve that is intended to open and release gases — thereby preventing an explosion — when the batteries are overheated. Here, the batteries performed as designed when the valves opened, and released sulfuric acid gas, to reduce the internal pressure in the batteries. What Keith never explained was whether the valves opened due to a manufacturing defect in the batteries that caused the batteries to overheat, or whether, instead, the overheating of the batteries and the ensuing opening of the valves, was the result of a failure in the UPS unit that caused the batteries to overheat. This latter possibility was far more than a remote possibility in light of the February 2005 incident, during which Thornton observed that the UPS unit had "shut [itself] down" but there was nothing wrong with the batteries.

Thus, Keith could not, as Creanga requires, systematically rule out the other possible causes, improper maintenance of the UPS system and negligent substitution of UPS batteries. In the end, all Keith was able to say was that the batteries might have been a partial or sole cause of plaintiff's injuries, but it was equally likely that other factors were the sole cause. Under such circumstances, in light of Creanga, his opinion was inadmissible.

We thus conclude that for both reasons, absence of underlying facts to support his opinion and inability to exclude other possible causes, Keith's opinion was an inadmissible net opinion and Judge Tolentino correctly barred it. In light of our determination that the judge correctly excluded Keith's opinion as a net opinion, we need not address the judge's additional finding that Keith lacked expertise in battery manufacture and battery functioning and his opinion should be excluded on that ground as well.

Affirmed.