Two more employees have reported possible exposure to chemical vapors on the Hanford Nuclear Reservation — even as workers engaged in a job action to demand better protection from such fumes.

The two workers were given medical evaluations Tuesday and cleared to return to work, according to Washington River Protection Solutions, a private contractor that manages a section of the reservation where nuclear waste is buried in underground tanks.

It brought the number of workers receiving medical checks for possible chemical vapor exposure to about 55 in recent months.

Some of the workers have reported respiratory problems, and others smelled a chemical odor. All were cleared to return to work, the contractor said.

The latest reports of possible exposure came just a day after the Hanford Atomic Metal Trades Council issued a stop work order on Monday to demand greater protections for workers who deal with underground nuclear waste storage tanks. The council is comprised of unions that represent Hanford workers.

The order required a halt to any tasks within the boundaries of Hanford tank farms if workers were not being supplied with mandatory air respirators.

Washington River Protection Solutions complied by giving employees the bottled air on Tuesday and work resumed.

Hanford, located near Richland, for decades made plutonium for nuclear weapons, and waste from that work are stored in 177 giant underground tanks.

The government is now engaged in cleaning up the site, a process that will take decades and cost tens of billions of dollars.

The two workers evaluated Tuesday were outside the tank farm boundaries when they reported smelling chemical vapors, the contractor said, noting that samples showed air quality was compliant with safety standards.

Officials at the trade council contend that workers have been sickened by the vapors, reporting symptoms like headaches and bloody noses. The group contends the problems go back for two decades.

The vapors aren't radioactive and are invisible to the eye. They are suspected of escaping some tanks when the contents are disturbed during cleanup activities.

Last month, the council demanded that air respirators be required for work in all tank farms. The contractor denied the demand, saying the equipment was not needed for routine work among newer double-walled tanks that do not vent passively into the environment.