With the city once again blanketed in wildfire smoke, experts are warning that long-term exposure to air pollution can lead to higher risk of heart failure.

Environment Canada issued a special air quality statement on Sunday night due to wildfire smoke from northern Ontario pushing Toronto’s Air Quality Health Index to 10+, classified as “very high risk.” Poor air quality was expected to persist Monday and possibly into Tuesday.

The effects of air pollution on the heart are underscored in a recent study, which found that long-term exposure to air pollution, in particular fine particulate matter, was linked to higher levels of diffuse myocardial fibrosis, a type of heart scarring that can lead to higher risk of heart failure.

According to air quality tracker IQAir, Toronto had the second-worst air quality in the world on Monday morning, with PM2.5 (a measurement of fine particulate matter) concentration at 17.3 times the World Health Organization annual guideline value. It later improved to sixth-worst.

The study, published in the medical journal Radiology and led by University Health Network clinician-scientist Dr. Kate Hanneman, used an advanced imaging technique called cardiac MRI on 694 participants in Ontario who were exposed to fine particulate matter over one year.

Heart damage was observed even when levels were below global air quality guidelines, and there is no safe exposure limit for air pollution, according to the research.