A Toronto councillor wants the TTC to back an independent investigation of the air quality inside the city's subway system and for the transit agency to provide a plan about what can be done to keep workers and commuters safe.

Coun. Joe Mihevc  is set to introduce a motion at Thursday's TTC board meeting asking for another look at air pollution in the transit system. In April, a study initiated by Health Canada and conducted in 2010 and 2011 concluded the rates of particulate matter in the air were far higher underground and likened it to breathing the air in China's capital, Beijing.

TTC CEO Andy Byford criticized that finding, saying it caused "unnecessary alarm," and said the agency is launching its own air quality assessment later this year.

Mihevc is now calling for the city's medical officer of health to oversee an independent study of the health impacts the subway air could have on passengers.

The councillor, who also chairs the health board, notes the data in the April report is "dated."

However, he wants answers to the questions it raises.

"We need to understand what, if any, respiratory and other health impacts may result from these particles for passengers on platforms and in trains," he said in a notice of motion.

"For the TTC there is a need to conduct a risk assessment on staff exposures … who work entire shifts underground."

Mihevc said the TTC has upgraded ventilation systems in subway cars and has purchased a specialized car for vacuuming tunnels which will start operating at the end of this year. But he still wants to know if it's doing enough to keep the air as clean as possible, and if platform barriers — which have also been called for to help prevent suicides — could help.

The so-called "rail dust" in the system is largely caused by the grinding of train wheels against the track.

Byford's latest update on how the TTC's doing found subway cleanliness is exceeding targets. However, the TTC is still slightly under its ridership target as of March.

The current customer satisfaction score, meanwhile, is 79 per cent.