Researchers at the University of Toronto have discovered a substantial reduction in pollution levels in the downtown core since the province announced measures to curb the the spread of COVID-19, including shuttering schools and non-essential businesses.

"A pandemic is not a good thing," said Greg Evans, director of the Southern Ontario Centre for Atmospheric Aerosol Research (SOCAAR).

"But seeing the change ... gives us a better understanding of the many factors that contribute to poor air quality."

Preliminary data gathered by researchers shows traffic pollution is down. (CBC)

The SOCAAR team analyzed the concentration of dozens of pollutants from air pulled directly from a duct off College Street in downtown Toronto for 24 days before March 13 and 24 days after.

The preliminary results provided to CBC News showed levels of nitrogen oxides and ultrafine particles, both indicative of vehicle pollution, have sharply decreased, a finding consistent with the reduction in traffic in the area.

"Both of them have dropped by about half," said Evans.

At this point, Evans says he isn't able to attach an exact percentage to the decrease because several factors affect the accuracy of that number, including the level of travel over March break and the onset of spring.

Evans says the data his team compiled is more specific and detailed than pollution data from satellite imagery, which gives a big-picture view.

"We can get a picture of what's going on right now at a given location."

It's too early to draw conclusions about the impact on carbon dioxide levels, according to Evans, because it's been in the air for so long — one molecule of carbon dioxide stays in the atmosphere for about 40 years, so a drop over just a few weeks "really doesn't make that much of a difference to overall levels."

As he collects more data, though, Evans said he may be able to draw more conclusions.

These ducts at the University of Toronto took in air for researchers to test. (Lisa Xing/CBC)

The findings don't come as a surprise to Evans, nor to Heather Marshall, campaigns director at the Toronto Environmental Alliance.

But Marshall says it's not necessarily good news to her.

"There's really no silver lining. We're facing a pandemic."

However, Marshall is hoping these figures will help inform government decisions and spending after the pandemic is over.

"We have to think about whether it's a bailout for the oil and gas sector, or an investment in other kinds of transportation that are zero emission or low emission."

Evans said the data he and his team continue to gather could help inform public-health policy as more research emerges about if and how air quality affects COVID-19 patients.