Louis Albert Weaver III was working for Whitaker Furniture Company, Inc., on October 28, 1993, when he stepped down from a forklift and fell. He continued to work until his supervisor asked about his arm about four hours later. That afternoon he was sent to Dr. C. W. Koch, Jr., who determined that he had fractured his elbow. Before leaving Dr. Koch’s office, appellant gave a urine sample that was forwarded for testing. The laboratory analysis revealed the presence of cannabi-noids. A second urine sample, taken two weeks later, had no detectable level of cannabinoids.
Whitaker Furniture contended that appellant’s injury was drug-related and contested his claim for workers’ compensation benefits. The administrative law judge denied the claim, finding that the preponderance of the evidence failed to show that the claimant had sustained a compensable injury within the meaning of Act 796 of 1993. The Workers’ Compensation Commission affirmed and adopted the decision of the law judge after conducting a de novo review. On appeal Mr. Weaver contends that the Commission’s opinion is not supported by substantial evidence and is erroneous as a matter of law. We disagree and affirm.
Under our prior workers’ compensation law, there was a prima facie presumption that an injury did not result from intoxication of the injured employee while on duty. See Ark. Code Ann. § 11-9— 707(4) (1987). Act 796 of 1993, however, changed that presumption so that it now reads as follows:
The presence of alcohol, illegal drugs, or prescription drugs used in contravention of a physician’s orders shall create a rebuttable presumption that the injury or accident was substantially occasioned by the use of alcohol, illegal drugs, or prescription drugs used in contravention of physician’s orders. . . . An employee shall not be entitled to compensation unless it is proved by a preponderance of the evidence that the alcohol, illegal drugs, or prescription drugs utilized in contravention of the physician’s orders did not substantially occasion the injury or accident.
Ark. Code Ann. § ll-9-102(5)(B)(iv) (Repl. 1996).
In the case before us, the Commission’s opinion included the following discussion of appellant’s burden of proof in overcoming the rebuttable presumption that his injury was substantially occasioned by the use of illegal drugs:
[I]n determining whether the presumption has been overcome, the results of objective testing and the clear and consistent opinions of experts cannot be overlooked. Additionally, while some accidental injuries might occur with little possible relationship to intoxication, a slip and fall type injury is of the type which could be influenced by the effect of the forbidden substances. Moreover, the record does not reveal whether the other persons who allegedly did not notice intoxication possessed any special training for making such assessments.
Thus, it cannot be said that the claimant has overcome the statutory presumption and proved entitlement to benefits without impermissibly giving him the benefit of the doubt or resorting to conjecture and speculation on his behalf.
Both appellant and a co-worker testified that appellant slipped as he was stepping down from a forklift. The co-worker stated that the forklift leaked brake fluid; that he, too, had slipped on the forklift two or three times; that there was brake fluid on the concrete floor where appellant slipped; and that appellant did not appear to be “high” from drugs before or after the accident. The appellant testified that he had oil on his shoes that morning; and that although he had not used marijuana in three years, he had attended a party four days previously where marijuana smoke was heavy. It was his opinion that the oil on his shoes had caused him to slip and fall. Appellant introduced into evidence a letter from Dr. Koch, stating that there was no obvious intoxication when appellant was seen on the date of the accident.
The record also contains correspondence from two experts who evaluated appellant’s laboratory test results. Cannabinoids detected in the first urine specimen were confirmed by gas chromatography-mass spectrometry showing a level greater than 200 ng/ml carboxy acid THC, the principle metabolite of marijuana. Dr. Henry F. Simmons stated that cut-off levels used to confirm positive screening tests are 15 ng/ml for federal programs and 10 ng/ml in many private programs. He stated that a level of 200 was well above levels expected from passive exposure to marijuana smoke, was not consistent with use of marijuana two to three years before testing, and could dissipate within two weeks to a level below cut-off values. He stated that the technique used by the laboratory was a state-of-the-art method of testing with a false positive rate near zero. Stuart Bogema, Ph.D, confirmed that two weeks after initial results of 200 ng/ml, a follow-up test of an individual who was not a heavy, chronic user and had not used marijuana in the interim would most likely be negative.
Appellant contends that the Commission erred as a matter of law in that it failed to understand that when any evidence is presented, the rebuttable presumption evaporates. He contends that the Commission placed an impossible burden upon him in refusing to disregard the rebuttable presumption once he presented testimony that he had not used marijuana in three years, that he was not intoxicated the day of the accident, and that he slipped because of oil on his shoes and brake fluid on the floor.
We do not read the Commission’s decision as placing an impossible burden upon appellant, nor do we agree that the Commission erred as a matter of law. The plain language of the last sentence of section ll-9-102(5)(B)(iv) denies compensation “unless it is proved by a preponderance of the evidence that the . . . illegal drugs . . . did not substantially occasion the injury or accident.” Furthermore, section ll-7-104(c)(3) requires that all provisions of the chapter be stricdy construed. It was up to the Commission to determine whether appellant met its burden of proof in rebutting the presumption, and it did so by addressing in its decision “whether the presumption has been overcome.” Whether a rebuttable presumption is overcome by the evidence is a question of fact for the Commission to determine. See Eagle Safe Corp. v. Egan, 39 Ark. App. 79, 842 S.W.2d 438 (1992).
When reviewing a finding of fact made by the Commission, we must affirm if the Commission’s decision is supported by substantial evidence. Purolator Courier v. Chancey, 40 Ark. App. 1, 841 S.W.2d 159 (1992). Substantial evidence is such relevant evidence as a reasonable mind might accept as adequate to support a conclusion. Southern Steel & Wire v. Kahler, 54 Ark. App. 376, 927 S.W.2d 822 (1996). Furthermore, it is well established that the credibility of witnesses and the weight to be given their testimony are matters exclusively within the province of the Commission. James River Corp. v. Walters, 53 Ark. App. 59, 918 S.W.2d 211 (1996). The Commission is not required to believe the testimony of the claimant or any other witness, but may accept and translate into findings of fact only those portions of the testimony it deems worthy of belief. Jordan v. Tyson Foods, Inc., 51 Ark. App. 100, 911 S.W.2d 593 (1995).
Here, laboratory test results showed high levels of cannabinoids in appellant’s urine the day he was injured at work. Under Ark. Code Ann. § ll-9-102(5)(B)(iv) (Repl. 1996), this created a rebuttable presumption that his injury was substantially occasioned by the use of illegal drugs. The Commission weighed appellant’s evidence that he slipped because of a substance on the floor or on his shoes and that he had not used marijuana in three years, against the opinions of experts indicating that appellant had used marijuana or similar substances shortly before his accident at work and could not have attained the level detected by the testing from passive exposure to the smoke at a party a few days before the injury. We conclude that the Commission’s finding that appellant did not overcome the presumption is supported by substantial evidence.
Affirmed.
Jennings, C.J., and Pittman, Robbins, and Rogers, JJ., agree. Mayfield, J., dissents.