(CNN) The main operating rooms at Seattle Children's Hospital are set to reopen Thursday after they were shuttered due to mold problems that killed one patient and sickened five others since last year, hospital officials said.

Some 3,000 letters also have gone out to patients who had surgery this year at the facility, informing them of air quality problems and advising them to watch for signs of infection, the hospital's chief medical officer, Dr. Mark Del Beccaro, said Wednesday.

Air filtration gaps are believed to have been key in the presence of mold, hospital spokeswoman Alyse Bernal said Tuesday. The hospital worked with outside industrial hygienists to clear rooms of Aspergillus contamination, she said.

"We have conducted extensive air testing to validate that our interventions have been and will continue to be effective," Del Beccaro said in a prepared statement. "We are taking a thoughtful and systematic approach to resuming operations in our main operating rooms."

Several operating rooms were shut down in May after Aspergillus mold was detected in the air, though the risk to patients was low, hospital officials said at the time.

Read More