Heavy vog and a spike in sulfur dioxide readings at Kulani Correctional Facility on Wednesday morning had prison officials weighing whether to evacuate 23 inmates with a history of respiratory problems, but the prisoners remained where they were after the levels of the gas later dropped.

Kulani is less than 10 miles north of Halema'uma'u, and is about 15 miles northwest of Pu'u 'O'o — the two vents giving off volcanic emissions.

Department of Public Safety spokeswoman Louise Kim McCoy said one inmate was taken to the Hilo Medical Center emergency room early on April 25 because of respiratory problems associated with the volcanic emissions, but was returned to the minimum security prison later that morning.

Other than that hospital visit, the prison staff does not report any "urgent or unanticipated" visits to the prison clinic because of the emissions, she said.

Under the color-coded system developed by health officials, sulfur dioxide levels greater than three parts per million are deemed to be "extreme," and people with respiratory problems are urged to leave the area. McCoy said the readings at Kulani spiked at 11 parts per million at 10:54 a.m. Wednesday, but dropped to four parts per million about 30 minutes later before declining to lower levels considered to be "moderate" in the afternoon.

"As before, the facility continues to monitor the situation," she said. McCoy said there are two air monitors on the grounds of the prison, which houses about 150 minimum security inmates.