A bill placing tighter restrictions on people in Colorado who drive after using marijuana took another step forward today.

But the approval this morning by the state House of Representatives didn’t come without contention, including a skirmish among the bill’s two sponsors.

The bill, House Bill 1261, would create a limit of THC — the psychoactive component of marijuana — drivers could have in their blood. Anybody who tests above that amount, 5 nanograms per milliliter, would be presumed to be too high to drive, in much the same way someone with a blood-alcohol content above .08 is considered too drunk to drive.

The bill has the support of law enforcement officials, who say it brings specificity to existing laws banning stoned driving. But the proposal has been criticized by many in the medical-marijuana community, who fear the 5-nanogram level might be too low for people with high THC tolerances.

Since THC levels in the blood can spike and dip quickly and can vary based on pot potency, ingestion method and other factors, it is difficult to quantify how much marijuana people must use or how long afterward they must wait to be below the 5-nanogram limit. The bill’s detractors point out that the rise and fall of THC levels in the blood is much less predictable than that of alcohol. Furthermore, there is no way for marijuana users to easily figure out what their THC levels are, also unlike with alcohol.

But the bill’s supporters say that marijuana unquestionably impairs certain faculties critical in driving, and they argue that a number of studies conclude that people with levels above 5 nanograms are impaired. Some studies, they point out, suggest an even lower level often indicates impairment.

The fight in the House today erupted along these divisions, when Rep. Claire Levy, a Boulder Democrat who is one of the bill’s sponsors, proposed raising the limit to 8 nanograms. Levy said that, given a THC limit would be something new to Colorado law, it is best to be cautious.

“I became concerned that we were going to convict people and take their licenses away when they were not impaired,” Levy said.

But her proposal angered several lawmakers, including her co-sponsor, who said the 5-nanogram limit had been extensively discussed.

“There’s nothing to support that 8 is the right level,” said Rep. Mark Waller, a Colorado Springs Republican who is the bill’s other House sponsor.

Rep. Bob Gardner, R-Colorado Springs, was so angry he proposed lowering the limit to 2 nanograms, before backing off. The House then voted down Levy’s proposal and passed the bill with the 5-nanogram limit, but not before taking a procedural step that will make it harder in the future for lawmakers to tinker with the limit as the bill winds through the legislature.

The bill still needs one more vote in the House before moving over to the Senate, where it must survive at least three votes to make it to the governor’s desk.

John Ingold: 303-954-1068 or jingold@denverpost.com.