Politicians and activists cite plenty of numbers when they debate Metrolinx’s proposed service expansion, which will include a contentious link from Union Station to Pearson International Airport; there are hundreds of trains, thousands of affected residents, and millions of dollars invested. But the claim that 30,000 students attend school near Metrolinx’s tracks stands out.

Dr. Ian Clarke lives in Brockton Village, near the tracks where Metrolinx could soon be running hundreds of diesel trains. Though he studies cancer, he spends his free time researching something else: diesel exhaust.

Diesel exhaust, he explains, is a little like a Brita filter, those contraptions that use carbon to trap impurities as tap water flows through them. Diesel engines emit small particles and other pollutants into the air. And like the carbon in a Brita filter, the airborne particles (“particulate matter” in scientific literature) in diesel exhaust attract impurities. Different substances become attached to these particles, turning them into “little sponges that bring all those chemicals deep into your lungs," as Clarke puts it.

Clarke's Brita filter analogy sounds a lot like a 2009 report from Toronto’s Medical Officer of Health [PDF] on Metrolinx’s proposed expansion. “A variety of substances can become attached to the exterior of the particles [in diesel exhaust], including air toxics and metals that are both linked to health outcomes such as cancer,” the report states.

While diesel exhaust isn’t good for anyone, researchers often describe children as particularly vulnerable. A 2009 paper from the California Environmental Protection Agency warns that particles found in diesel exhaust “likely hamper the ability of an already immature immune system to clear bacteria and other pathogens from the lung.” And a 2010 paper in the journal Environmental Research outlined the reasons for children’s vulnerability to airborne pollution: “... higher minute ventilation, mouth breathing, greater physical activity and more time outdoors.”

Malon Edwards, a Metrolinx spokesman, told OpenFile in an email that "in the next few years" they will be implementing technology that "removes most of the particulate matter from diesel exhaust"—90% of "airborne particulate emissions"—and are now "installing three air quality monitoring stations along the Georgetown South rail corridor to monitor air quality conditions well before increased train service is operational," data from which would be made available to the public. The Clean Train Coalition, formed in opposition to the rail expansion, argues that the new, promised technology will not be implemented soon enough, and won't do enough to filter out the smallest, most dangerous particulate matter, PM2.5, so named because the particles have a diameter of no greater than 2.5 micrometres.

What hasn’t been clear until now is where that "30,000 students" figure came from. Toronto District School Board officials were unable to provide any substantial information on it to OpenFile. Metrolinx declined to divulge the names of affected schools; a spokesperson suggested submitting a freedom of information request, then stopped returning phone calls. Not even school trustees Irene Atkinson, Chris Bolton, Maria Rodrigues, and Soo Wong—who complained to the school board in 2009 about the number of students who would be affected—could provide any information about the figure.

OpenFile contributing editor Patrick Cain has determined, though, that the number isn’t far off: within one kilometre of the tracks, 16,400 students attend public schools and 9,353 attend separate schools, a total of 25,753 children. (Here’s an interactive map showing the schools closest to Metrolinx’s future air-rail link.)

One kilometre is hardly a scientific measure—wind, temperature, and other factors all affect the distance that particulate material travels, and the effect it has on people's health. (PM2.5 particles can travel very, very far.)

The dangerous material in diesel exhaust is found in other sources, of course—particulate matter was a sad fact of city life long before Metrolinx came to town. But the people who commonly experience high exposure to it are usually miners, railroad workers, and firefighters, not schoolkids. With Metrolinx’s air-rail link moving forward, the schools near the tracks will likely continue to provide fuel for the Clean Train Coalition’s fight for electrification. On Monday, January 3, the group distributed an email claiming that more 1,000 children attending school near the future air-rail link have asthma. Just another figure that needs checking.