Researchers with McMaster University say they have identified the cause of a mysterious black grime that has blanketed houses and yards near the city’s industrial area for years.

The team behind the ‘What’s The Grime’ study imaged and processed samples, and discovered that coal combustion is the likely source for the dust.

The study also found that the composition of the soot is quite complex, meaning it is coming from multiple sources.

Grace Yao is a PhD student at McMaster and the co-lead of the study. She says the grime is of most concern for small children and pets, who may ingest it by accident.

Yao added that there is growing frustration among city residents over the soot.

“There’s not much publicly available information on how to keep yourself safe from this stuff, or how to clean it properly,” Yao said.

She says residents are also concerned about their gardens, and if food that has come into contact with the soot is still safe to eat.

Her team plans to continue their investigation into the causes and the makeup of the grime with a second study aimed at addressing community concerns.