Colorado’s free bus and rail transit option for containing summer air pollution has ended, and Regional Transportation District officials on Wednesday blamed a shift in state funding.

Over the past two years, RTD’s “Zero Fare for Better Air” program led to reduced driving in July and August, when ground-level ozone air pollution concentrations typically surge above federal health limits. Drivers drove 9 million fewer miles during those two months (145,393 miles fewer per day), reducing air pollution emissions by 6,166,740 pounds, an RTD program evaluation found. RTD ridership increased by 12%.

That’s a tiny portion of Colorado’s air pollution, but agency evaluators concluded free transit had a positive impact. The EPA has designated Colorado as a “severe” violator of federal air quality health standards, forcing the state to sell a cleaner-burning type of gasoline starting Saturday.

“Colorado did not appropriate grant money for RTD to provide both a Zero Fare for Better Air initiative and Zero Fare for Youth program,” RTD spokeswoman Marta Sipeki said in an email. RTD directors have launched a pilot program offering free transit for teens and soon will consider making that program permanent, Sipeki said.

State lawmakers this year reduced overall funding for free transit, approving about $10 million for free-fare programs statewide. RTD officials have said the free transit during summer cost their agency more than $15.5 million in lost revenues. State funds covered $13.8 million of that cost.

The current law for free transit lets some agencies, but not RTD, apply for state funds for either a pollution reduction program or year-round free fares for riders under age 18, Sipeki said. “RTD is only eligible to apply for grant money related to offering a fare-free youth program.”

Colorado Springs and other cities are expected to use the state funds for free transit to reduce ozone pollution over the next two years.

On Wednesday, state health officials issued an Ozone Action Day alert as pollution approached potentially unhealthy levels — urging residents along the Front Range to reduce their driving.

Metro Denver pollution hit levels considered unhealthy for sensitive groups, including the elderly, people with respiratory ailments, and children, the alert said. The air quality was expected to improve to “moderate” or better later this week.