The Walters Art Museum abruptly shut its doors for three weeks starting in mid-June after about half a dozen employees became ill from inhaling fumes from chemicals used to repair the roof of one of the museum’s buildings.

But the museum never notified members of the public that it had closed for safety reasons, or that visitors to the Centre Street building might have been exposed to vapors from commonly used but potentially hazardous chemicals. Visitors were allowed inside most galleries for two weeks after staff members first told administrators June 1 that the odors were making them feel unwell, according to museum employees who are seeking to form a union.

During a June 15 inspection, an industrial hygienist hired by the Walters detected dangerously high levels of the chemicals from a sealant being used on the roof, according to Julia Marciari-Alexander, the museum’s director.

The following day, the museum announced that it would be closed to the public and staff for one week. When repairs took longer than anticipated, the shutdown was extended for two more weeks.

The notice posted on the museum’s website and Facebook page stated only that the closure was required “to complete necessary work on our buildings.”

“We were really focused,” Marciari-Alexander said, “on ensuring that our staff was safe and kept informed and that roof work was proceeding. Those were our highest priorities.”

Marciari-Alexander said the two areas where the vapors were most pervasive — the fourth-floor galleries and fifth-floor offices — were closed to all visitors and most staff members as of June 4. She said no museum guests complained to staff that they felt ill.

The Walters reopened July 7 after a second inspection determined that the levels of problem chemicals had decreased by 75% and were expected to drop further.

Tom Burke, director of the Johns Hopkins Risk Sciences and Public Policy Institute, said the sealant included xylenes, chemical compounds often used as solvents that act on the brain and nervous system.

“These kinds of materials have a neurological effect that can make you nauseous and lightheaded,” he said. “They can give you a violent headache. People who inhale a high concentration can pass out. This is all about ensuring that you have adequate ventilation and minimizing the length of exposure.”

The incident might never have become public if it hadn’t been for Walters Workers United, a group of nearly 100 employees seeking to unionize under the auspices of the American Federation of State, County and Municipal Employees.

The group held a rally Aug. 12 at the adjacent Washington Monument that attracted more than 100 people.

“Collective action benefits us all,” security guard Garrett Stralnic said at the rally. “Recently, the museum closed to staff and visitors due to unsafe organic vapor chemicals during construction work on the roof.

“Individual requests for safety information or guidance on protective measures were not addressed. Even after floors were closed, gallery officers were still posted in hazardous conditions resulting in people continuing to get sick. It was only after a letter of concern signed by nearly 50 workers was delivered to the administration that safety measures were put in place to protect workers and the public.”

Stralnic said that shortly after roof work on the Centre Street building began he began noticing “smelly vapors” that seemed especially concentrated in elevators, where ventilation was poor. Security guards assigned to “contractor duty” spent lots of time in elevators escorting workers. One Walters guard tried to position himself near stairwells, where the air seemed freshest.

Stralnic said he and a co-worker went home sick June 10 after experiencing dizziness and bouts of lightheadedness. Both began to feel better once they were outside. The employees wrote the letter the next day.

Toxicologist Joseph V. Rodricks, a founder of the environmental and health science consulting firm Environ, which was later acquired by Ramboll, said xylenes are near-ubiquitous compounds found in everything from paint thinners to the polyurethane used to refinish hardwood floors. They are found in nail salons and in the pools of gasoline that collect in parking lots.

Most adults have come into contact with xylenes, he said. Contractors working indoors generally use powerful fans to disperse vapors as the liquid solvents evaporate, thus avoiding the symptoms experienced by the Walters workers.

“Once you’re away from the solvent, the symptoms should pass pretty quickly,” he said. “There shouldn’t be any long-term effects.”

The Walters encompasses three buildings, of which the 1974 Centre Street building that houses the visitors’ entrance is the most modern. Marciari-Alexander said the same sealant was used last winter to repair the roof of the Palazzo building, which opened in 1909, long before the advent of modern building codes. There were no complaints about odors during that roofing project, she said, so Walters officials didn’t anticipate the vapors would be a problem during the Centre Street repairs.

In addition, she said, that particular sealant was chosen because it was the product with the lowest toxicity that was still effective. When the odor was first detected, staff members tried various methods to lessen it, including blocking air intakes and using air “scrubbers” to improve the air quality.

“As soon as we were aware there was a problem, we took steps immediately to remedy the situation,” Marciari-Alexander said.

But none of the methods worked. The sealant was being used to waterproof the roof and had to be spread across the entire membrane, not just applied to seams. The heating, ventilation and air-conditioning equipment located on the roof may have vented the odors into enclosed galleries, elevators and hallways.

Making matters worse, the roof work coincided with the first heat wave of the summer. For six days in a row, temperatures in the 90s baked Baltimore. For three days, they reached the mid-90s — unusual for early June.

“The speed with which this solvent evaporates depends on the temperature and humidity,” Burke said. “If the work had been done in the winter on a cool and breezy day, this probably wouldn’t have been an issue.”

The 47 employees who signed the June 11 letter to Walters administrators said they know that accidents are unavoidable. But they wish museum officials had shared information more quickly.

“We understand that things happen at work and that not everything can be prevented,” the employees wrote.

“However, communication of the hazards in the workplace to all staff could have prevented further chemical exposure and endangerment. On June 2, a worker requested the materials safety data sheet for the materials used on the roof work. On June 4, during a meeting, a worker requested that all staff be notified of symptoms they could be experiencing. These requests were met with no response.”

Data sheets for the sealant called Sikalastic-641 Lo-VOC were provided to employees June 16.

“Our health and safety is not negotiable,” the employees wrote. “This is just one more reason why we are looking forward to formally partnering with management as a union.”