Pulmonologist Martin Tobin returned to the stand in the murder trial of former Minneapolis police officer Derek Chauvin on Thursday, testifying against the claims made by a defense expert about George Floyd’s carbon monoxide levels, concluding that they were “simply wrong.”

Tobin, a witness for the prosecution, refuted the argument made by former Maryland chief medical examiner David Fowler, who testified on Wednesday that carbon monoxide poisoning could have contributed to Floyd’s death. 

Floyd was killed during a 2020 arrest after Chauvin kneeled on his neck while pinning him to the street for about nine minutes.

Chauvin faces murder charges over Floyd’s death, and his defense has sought to argue that there were other causes for Floyd’s death.

The prosecution argues Chauvin is responsible for cutting off Floyd’s air while pinning him to the street.

Tobin testified that Floyd’s oxygen saturation levels were 98 percent when he died, prompting prosecutor Jerry Blackwell to ask “does that tell us anything whatsoever about what the carbon monoxide content could have been at a maximum?”

“Yes it does, it tells us that if the hemoglobin is saturated at 98 percent, it tells you all there was for everything else is 2 percent, and so the maximum amount of carbon, carbon monoxide, would be 2 percent,” Tobin said.

“It tells you the maximum amount of carboxyhemoglobin, that was what was mentioned yesterday, the maximum amount is 2 percent. It doesn’t even tell you that it is 2 percent, it could be something else, but 2 percent of carboxyhemoglobin is within the normal range,” Tobin continued.

In testimony on Wednesday, Fowler said carbon monoxide poisoning could have been one factor that led to Floyd’s death, while acknowledging that he did not review data or test results that showed Floyd had an injury from the gas. He testified, however, the carbon monoxide levels in Floyd’s blood could have increased by 10 to 18 percent.

Tobin told Blackwell that he did not agree with that proposition, calling it “simply wrong.”

“In other words, as to the statement that his carboxyhemoglobin could have increased by 10 to 18 percent, in your view, that’s not possible?” Blackwell asked Tobin.

“It’s simply wrong,” Tobin responded.

Tobin was recalled to the stand as a rebuttal witness on the 14th day of the Chauvin trial. Minutes before, the defense rested its case after calling seven witnesses to the stand.

On Tuesday morning, the prosecution rested its case after hearing testimony from nearly 40 witnesses over 11 days.

A key point of contention throughout the trial has been Floyd’s cause of death. In June 2020, the Hennepin County medical examiner ruled Floyd’s death a homicide, writing that he experienced cardiopulmonary arrest while being restrained by law enforcement.

The defense, however, has argued that Floyd died from a combination of the drugs in his system and his underlying heart disease.