An invisible killer had infiltrated Sto-Rox High School.

When workers installed a cell tower on top of the school outside Pittsburgh, no one realized the exhaust spewed by its diesel generator was being sucked into the building’s ventilation system and inhaled by everyone inside. This is stuff you really do not want in your teen’s homeroom: Diesel fumes contain particulate matter and chemicals like benzene and arsenic, which in the long term increase the risk of lung cancer and in the short term cause breathing problems and dull the mind. But lucky for the Sto-Rox students, they had Joe Krajcovic—and a Speck.

Krajcovic had installed this new device in his science classroom as a school project. The Speck measures airborne particulate pollution, which increases the risk for and exacerbates symptoms of respiratory problems like asthma. Krajcovic’s class was analyzing the data gathered by the sensor to learn about indoor air quality when they noticed spikes in particle levels every few hours. Those coincided with the generator’s daily schedule: Whenever it kicked on to power the tower’s battery, particulate pollution increased, says Speck developer Illah Nourbakhsh, a robotics researcher at nearby Carnegie Mellon University. After parsing this unnerving data, Krajcovic filed a grievance, and the tower was moved.

But that is just one school, in one corner of Pennsylvania. Right now, there’s essentially no way of knowing how many schools and homes and offices are being filled with pollutants from diesel generators on rooftops, highway overpasses down the block or some other source spitting out invisible killers in your face all year long.

Your life depends on good air. Every year, air pollution causes the premature deaths of between 5.5 million and 7 million people, making it more deadly than HIV, traffic accidents and diabetes combined. The majority of these deaths—about 4 million—are caused by indoor air pollution, primarily in developing countries. But it takes a toll in developed countries as well. In Europe, for example, air pollution shortens the average life expectancy by nearly one year. Worldwide, more than 80 percent of people living in urban areas breathe air that exceeds pollution limits advised by the World Health Organization (WHO).

Read the entire article at Newsweek Magazine.