Three workers at the Hanford Site in eastern Washington were taken to an on-site medical clinic on Monday after reporting that they had possibly been exposed to toxic chemicals.

Part of the AP tank farm at Hanford was evacuated after the two workers reported the possible toxic vapor exposure. That brings to 26 the number of tank farm workers who have been sent to a medical clinic since last Thursday, according to Washington River Protection Solutions, the private contractor in charge of managing the tank farms at Hanford.

Three workers who reported being exposed to toxic vapors last week also sought a medical evaluation on Monday.

Plutonium was produced at Hanford for more than 40 years, leaving behind millions of gallons of highly radioactive and chemically active waste stored in underground tanks. The tanks can vent excess gas unpredictably. While radioactive particles are filtered, many toxic chemicals escape into the atmosphere and pose a threat to workers.

A KING 5 series in 2014 reported that vapor exposure events date back decades at Hanford, which officials there often dismissing their significance and threat to worker health. KING 5 interviewed workers who were left ill and disabled after breathing in the harsh chemicals.

Measures to minimize exposure -- such as requiring the use of suits with supplied air -- have been expanded and curtailed over the years. In the wake of KING 5's series, state Attorney General Bob Ferguson filed suit against the Department of Energy for failing to make worker safety a priority.

The workers who went to the on-site clinic on Monday were wearing respirators, instead of supplied-air suits, at the time of the possible exposure.

The clean-up at Hanford, which started in the late 1980s, is decades away from completion. Budget projections show costs related to the effort stretching into the 2080s and costing taxpayers $100 billion.

Full statement from WRPS:

Three Hanford workers sought medical attention today after experiencing odors in the AP tank farm this morning. Three others requested an evaluation related to an odor event at the worksite last week.

Twenty-six workers have been evaluated at the on-site medical facility since Thursday. Nineteen reported symptoms, while seven went for precautionary reasons; all have been released to return to work.

Today, workers reported odors in the AP tank farm. Access to the farm is restricted while samples are gathered and analyzed. Workers were not wearing respiratory protection, nor were they required to, since recent sampling of the area has shown chemical vapor levels consistently below occupational standards.

Last Thursday, odors were reported in several areas in and around the A, AX and AP Tank Farms, near where waste was being transferred from double-shell tank AY-102 to a tank in AP Farm. Industrial hygiene workers gathered air samples and analyzed data, which found chemical concentrations well below regulatory standards.

Chemical vapors are generated in the radioactive and chemical waste stored in underground tanks at Hanford. Washington River Protection Solutions (WRPS) has taken a number of steps to manage the hazards posed by chemical vapors. These steps include engineered controls, setting up restricted areas where known vapor sources exist, training employees to strict procedures and supplying workers with protective equipment, up to use of a self-contained breathing apparatus supplied with bottled air.

An independent assessment led by a national laboratory provided a road map for enhancing the tank farms vapor protection program. WRPS began rolling out the detailed plan about 15 months ago. Implementing the plan will result in enhanced protection for Hanford’s tank farm workers.