HALIFAX -- Despite a reduced schedule and far fewer passengers, a growing list of possible exposures involving Halifax Transit buses has the driver's union deeply concerned.

"I would call it a crisis," Amalgamated Transit Union Local 508 President Ken Wilson told CTV News Tuesday morning.

Monday's exposure list released by Nova Scotia Health included 19 potential exposure routes on buses.

Wilson says he's been told there have been four positive case among drivers, with about a hundred others self-isolating, awaiting test results.

Transit has reduced service on some routes, but Wilson balks at suggestions it's because of staffing shortages.

"So, let's be honest, let's say what it is. It's not that they don't want to come to work -- they can't come to work. Public health is telling them to stay home, get a test, and self-isolate until you get the results."

Halifax Transit says ridership remains down about 70 per cent from what it was prior to the pandemic.

The union says it's been asking for on-site testing for drivers, who were instead directed to a junior high school in Dartmouth last weekend.

Beyond ongoing mandatory-masking and daily sanitizing of high-touch surfaces, transit re-introduced seat-signage to encourage distancing between passengers last week.

"Obviously, you never want to see that many exposures for the sake of our passengers, but at least we do know that we've taken those steps to safeguard our operators," Halifax Transit Executive Director Dave Reage told CTV News, adding transit continues to take direction from Nova Scotia Public Health.

A news release issued by HRM last Thursday said Nova Scotia's Chief Medical Officer of Health Dr. Robert Strang, had written to operators the day before.

"Public Health does not recommend testing and self-isolation for Operators on PSA routes because appropriate control measures have been implemented (e.g., masks, polycarbonate barriers, daily sanitizing of high-touch areas on buses, blocking of the first seat behind the driver on Nova Buses). These control measures significantly help reduce the risk of COVID-19 exposure to transit operators," the release said.

"We appreciate Dr. Strang's guidance and direction and greatly value the dedication of our Halifax Transit bus and ferry operators during these challenging and stressful times.  The courage and dedication of all front-line workers is remarkable and the municipality is taking every reasonable measure to ensure the safety of its workers and work places."

But the union says the efforts fall short, with some temporary barriers falling down, and passengers standing on some routes where all designated seats were filled.

"I'm hearing from my members every day of the week, multiple times a day, that they're scared. They're afraid going to work. They feel like the employer is not putting the protocols in place to keep them safe, and their clients and their passengers safe," Wilson said.