MONTREAL -- Nine months after Montreal bus passengers were first required to wear masks, drivers also now must wear them on the job.

It's an acknowledgment of how easily the virus variants can be transmitted, said Quebec's workplace safety board, which ordered the change.

"The prevalence of new variants is worrying and workplaces are urged to be extremely cautious," a spokesperson for the board, Antoine Leclerc-Loiselle, wrote in an email to CTV News.

The change came into effect on April 12, two weeks ago this Monday -- four days after Quebec began mandating, on April 8, that all workplaces in the province require employees to wear medical masks indoors at all times.

"It has become necessary to further reduce [the variant] risk and to continuously require the wearing of the medical mask... inside all workplaces, including that of the passenger transport sector," Leclerc-Loiselle wrote. He didn't explain why there was a four-day lag time.

Passengers on Montreal transit have been required to wear masks since last July. Many have asked over the months why they kept seeing bus drivers who weren't doing the same.

The answer was that they weren't required to -- it had been decided that the plexiglass barriers around them, and the fact that bus doors usually open frequently, created enough ventilation and protection.

Many bus drivers have still chosen to wear masks voluntarily all year.

The STM transit agency and CNESST, the workplace safety board, had both signed off on the previous system, though many riders said they were uncomfortable with it.

In an online statement, the STM said the new change came at the request of the CNESST and that it complies with all public health recommendations, "which change according to the situation."

Drivers are only required to wear the procedural mask while driving, it said. The rule won't apply to metro operators or station agents in one-person lodges, who both work in spaces that are considered to be closed.

The STM has about 4,000 bus drivers. It said that no outbreak of the virus so far has been linked to Montreal buses.