Even though it is still not known how the building became filled with a dangerous level of CO, ECFD Battalion Chief Bill DuBois said the building as of this morning "is perfectly clean."
"If the air monitoring has shown that there is no CO in the building, there is no reason to keep it closed," DuBois said.
GAFCU President Beverly Dawson opened the branch at 9 a.m. after inspectors from the state Department of Occupational Safety and Health Administration finished checking the building.
Dawson said the most likely explanation was the CO came from idling cars of drivers waiting to use drive-up automatic teller machines.
"Somehow something might have been sucking in that exhaust," she said.
El Centro Fire Chief Charles Beard said 16 people were transported to the emergency room of El Centro Regional. One of the 16 "went down," according to Beard.
"He threw up. He couldn't stand up anymore," Beard said.
That man was GAFCU employee Alex Medina of Holtville, according to branch Manager Joseph Ramirez.
He said Medina was decorating for the Fourth of July, hanging a banner and papier mache Liberty Bell replicas over teller windows. He was working near the vents.
"I think that's probably why he got more sick than anyone else," Ramirez said.
Nine more people transported themselves to emergency rooms in El Centro and Brawley, according to Beard.
In all, 47 employees of the GAFCU branch were evacuated along with around 25 customers in the building at the time of the exposure, according Dawson.
She said of the effect on her employees: "It started out as a headache and then people started getting nauseous."
She added: "Everyone started getting sick so we moved everyone to adjacent offices in the building next door to keep them in the cool."
People who needed to withdraw money, make payments or check on the status of their electronic deposits had to use the exterior automatic teller machines for the rest of the day.
Dawson said the building was ventilated with exhaust fans Monday afternoon.
She acknowledged that some of her employees might not be enthusiastic about re-entering the building.
"I think they'd be very apprehensive to go in there," Dawson said.
Despite that, she said she is confident employees and customers who visit the branch today will be safe.
"We've had inspectors from Cal-OSHA checking it out. They're supposed to be the experts," she said.
According to an American Lung Association "fact sheet," breathing low levels of CO can cause fatigue and increase chest pain in people with chronic heart disease.
In healthy people breathing high levels of carbon monoxide causes flu-like symptoms such as headaches, dizziness and weakness.
Carbon monoxide also causes sleepiness, nausea, vomiting, confusion and disorientation. At high levels it causes loss of consciousness and death.
Reel said there are precautions available for those worried something could happen at their workplace or home.
"There are very cheap carbon monoxide detectors for the home and workplace," he said.
Said Dawson: "I know I'm going to run down and buy one for my house and put them all over here."
Staff Writer Aaron Claverie can be reached at 337-3419.