Wildfires are no longer local affairs. Smoke from California has colored sunsets in New York, Australian bushfires have polluted Santiago, Chile, and people in Vancouver have inhaled the burnt remnants of Siberian forests. As a byproduct of these megafires, the world is watching a natural experiment in the effects of exposing a significant portion of the global population to wildfire smoke.

Researchers are starting to see just how much damage this pollution is doing. A team of more than 70 scientists from all around the world tallied up the death toll in a first-of-its-kind study published Wednesday in the journal Lancet Planet Health. Their estimate? Smoke from the world’s worsening wildfires is now killing 33,510 people every year.

It’s important to know this number because if societies don’t react, it’s likely to grow worse, said Yuming Guo, a biostatician at Australia’s Monash University and one of the authors of the paper. “Wildfire smoke is predicted to increase in the future because of climate change,” said Guo. “We should understand the relationship between smoke and human health.”

The scientists collected data on wildfire smoke exposure and deaths in 749 cities between 2000 and 2016. Their estimate of deaths is lower than the likely total number because the study only counted mortality in the three days immediately following blanketing smoke — not the more insidious damage that accumulates from long-term exposure.

Read the full article about wildfire smoke by Nathanael Johnson at Grist.